Bulletin of the American Physical Society, Mar 3, 2014
Submitted for the MAR14 Meeting of The American Physical Society Magnetotransport study of the te... more Submitted for the MAR14 Meeting of The American Physical Society Magnetotransport study of the ternary topological insulator (Bi 0.5 Sb 0.5) 2 Te 3 via in situ low temperature deposition of Cr LIUQI YU, JORGE BARREDA, LONGQIAN HU, P. XIONG, Florida State University, TONG GUAN, XIAOYUE HE, K. WU, Y. LI, Institute of Physics, Chinese Academy of Sciences-The robustness of the surface state of three dimensional topological insulators against local magnetic perturbation is still under debate, since a precise and well-controlled electrical characterization of the effects of the ferromagnetic dopant and their evolution with doping density are exceedingly difficult. Here we report results of magnetotransport measurements on epitaxial thin films of the (Bi 0.5 Sb 0.5) 2 Te 3 in the presence of electrostatic gating and magnetic impurity. Magnetoresistance (MR) and Hall effect measurements have been performed in various back gate voltages. Ambipolar field effect has been observed, enabling effective tuning of the Fermi level across the band gap and identification of the surface transport in the topological transport regime. Taking advantage of the unique capability of in situ deposition of Cr atoms in a customized dilution refrigerator, magnetic impurities were incrementally quench-condensed onto the sample surface. Our results show the deposition of Cr effectively yields electron doping. The weak antilocalization (WAL) effect was found to be surprisingly insensitive to the magnetic impurity; the cusplike negative magnetoconductivity remains even at the highest Cr concentration and no apparent weak localization was observed as expected from a gap opening at the Dirac point. WAL effect has the largest modification at a back gate voltage of-200 V where the Fermi level is considered relatively close to the Dirac point.
Nanomaterials made from binary metal oxides are of increasing interest because of their versatili... more Nanomaterials made from binary metal oxides are of increasing interest because of their versatility in applications from flexible electronics to portable chemical and biological sensors. Controlling the electrical properties of these materials is the first step in device implementation. Tin dioxide (SnO2) nanobelts (NB) synthesized by the vapor-liquid-solid mechanism have shown much promise in this regard. We explore the modification of devices prepared with single crystalline NBs by thermal annealing in vacuum and oxygen, resulting in a viable field-effect transistor (FET) for numerous applications at ambient temperature. An oxygen annealing step initially increases the device conductance by up to a factor of 10(5), likely through the modification of the surface defects of the NB, leading to Schottky barrier limited devices. A multi-step annealing procedure leads to further increase of the conductance by approximately 350% and optimization of the electronic properties. The effects of each step is investigated systematically on a single NB. The optimization of the electrical properties of the NBs makes possible the consistent production of channel-limited FETs and control of the device performance. Understanding these improvements on the electrical properties over the as-grown materials provides a pathway to enhance and tailor the functionalities of tin oxide nanostructures for a wide variety of optical, electronic, optoelectronic, and sensing applications that operate at room temperature.
Bulletin of the American Physical Society, Mar 18, 2013
Submitted for the MAR13 Meeting of The American Physical Society Magneto-transport properties of ... more Submitted for the MAR13 Meeting of The American Physical Society Magneto-transport properties of the ternary topological insulator (Bi 0.5 Sb 0.5) 2 Te 3 in the presence of electrostatic gating and magnetic impurity LIUQI YU, JORGE BARREDA, LONGQIAN HU, P. XIONG, Department of Physics, Florida State University, USA, TONG GUAN, XIAOYUE HE, K. WU, Y. LI, Institute of Physics, Chinese Academy of Sciences, China-A three-dimensional topological insulator, (Bi 0.5 Sb 0.5) 2 Te 3 , is used to characterize the electronic properties of the spin helical conducting surface state. Epitaxial films are grown via MBE on (111) SrTiO 3 substrate, which serves as the gate dielectric. Magnetoresistance (MR) and Hall effect measurements have been performed at various back gate voltages. Ambipolar field effect has been observed, enabling effective tuning of the Fermi level across the band gap. Weak antilocalization effect is identified and used to differentiate the surface state. The Hikami-Larkin-Nagaoka (HLN) equation is used to analyze the MR data and the results show the top and bottom surfaces become decoupled when the Fermi level is in the bulk band gap. We also examine the effects of paramagnetic impurity (MI), which introduces time reversal symmetry breaking scattering, on the TI surface states. Taking advantage of the unique capability of in situ deposition in a customized dilution refrigerator, paramagnetic Cr atoms were incrementally quench-condensed onto the sample surface and transport measurements were performed at each MI density. The procedure eliminates any sample-to-sample variation and complications from air exposure. Pronounced changes in the weak antilocalization effect and the sample carrier density with increasing MI concentration were observed. Possible origins of these observations will be discussed.
High quality HgCr2Se4 single crystals have been investigated by magnetization, electron transport... more High quality HgCr2Se4 single crystals have been investigated by magnetization, electron transport and Andreev reflection spectroscopy. In the ferromagnetic ground state, the saturation magnetic moment of each unit cell corresponds to an integer number of electron spins (3 µB/Cr 3+), and the Hall effect measurements suggest n-type charge carriers. Spin polarizations as high as 97% were obtained from fits of the differential conductance spectra of HgCr2Se4/Pb junctions with the modified Blonder-Tinkham-Klapwijk theory. The temperature and bias-voltage dependencies of the sub-gap conductance are consistent with recent theoretical calculations based on spin active scatterings at a superconductor/half metal interface. Our results suggest that n-HgCr2Se4 is a half metal, in agreement with theoretical calculations that also predict undoped HgCr2Se4 is a magnetic Weyl semimetal.
Bulletin of the American Physical Society, Mar 14, 2017
Submitted for the MAR17 Meeting of The American Physical Society Systematic study of spin transpo... more Submitted for the MAR17 Meeting of The American Physical Society Systematic study of spin transport in Si nanowires with axial doping gradient KONSTANTINOS KOUNTOURIOTIS, JORGE BARREDA, TIM KEIPER, MEI ZHANG, PENG XIONG, Florida State Univ-Spin transport experiments have been widely employed to study spin relaxation mechanisms in bulk and two-dimensional semiconductors. Quasi 1D systems such as semiconductor nanowires (NWs) could offer some advantages as spin transport channels; it was predicted that quantum confinement can lead to significant enhancement in spin lifetime and diffusion lengths. We have performed systematic spin transport, including local 2T, 3T, and nonlocal 4T spin valve measurements, in phosphorusdoped Si NWs exhibiting a pronounced doping gradient along the axial direction. The doping gradient enables the formation of Ohmic contacts and Schottky barriers of different widths and heights between a series of ferromagnetic electrodes and a single NW. This facilitates a methodical study of the dependence of the spin signal on interfacial resistance. Spin injection/extraction is effective within a window of interfacial resistance, which in our devices corresponds to zero-bias 2T resistances between 100 kΩ and 2 MΩ, corresponding to the estimated carrier densities between 6.810 17 cm −3 and 4.510 18 cm −3. Within this region we measured nonlocal spin signals of magnitudes between 1 µV and 50 µV (at I=20 nA). Also, the spin signals are observed to increase when the spins are injected from a more resistive interface. Comparison of the local 2T and nonlocal 4T signals and the effects of interchanging the injector and detector electrodes for the same transport channel will be presented.
Bulletin of the American Physical Society, Mar 18, 2013
Electrical spin injection/detection experiments have been performed on Si:Al 0.3 Ga 0.7 As, a per... more Electrical spin injection/detection experiments have been performed on Si:Al 0.3 Ga 0.7 As, a persistent photoconductor. The carrier density of this material can be tuned in situ via photo excitation across the insulator-metal transition (IMT) [1], which enables spin accumulation and transport measurements in one and the same sample over orders of magnitude variation in carrier density, thus circumventing the difficulties of making many replicas to realize different doping levels. Fe/AlGaAs heterostructures were grown by MBE, in which AlGaAs and GaAs graded Schottky junctions were tested for optimum spin injection. Spin transport devices, suitable for 3-terminal and non-local 4-terminal Hanle-type measurements and on-chip determination of the carrier density, were fabricated from the wafers. The spin lifetimes, determined from fits of the Hanle curves to a Lorentzian or the spin drift-diffusion model, range from 0.5 ns to 2.8 ns and exhibit a nonmonotonic carrier density dependence possibly peaked at the IMT.
Bulletin of the American Physical Society, Nov 17, 2012
Electrical Hanle-type measurements have been performed to determine spin lifetimes at various car... more Electrical Hanle-type measurements have been performed to determine spin lifetimes at various carrier densities in Si:Al 0.3 Ga 0.7 As, a persistent photoconductor (PPC). The carrier density of this material can be tuned, changing it from insulating to metallic in situ via photo excitation. Utilizing this PPC effect, we conduct electrical measurements of spin accumulation and transport under the same experimental conditions without the necessity of making replicas to realize different doping levels. We report the carrier density dependence of the spin lifetime derived from Hanle measurements with spin devices formed on wafers which have different graded junctions and Si doping levels. Carrier densities ranged from 3.5x10 16 to 2.4x10 17 cm −3 and from 7.2x10 16 to 6.5x10 17 cm −3 in two different samples. The spin lifetimes (determined using Lorentzian fits to the Hanle curves) ranged from 0.5 to 2.8 ns. From optical studies, the spin lifetime at zero bias and at low temperature in n-GaAs was reported to be larger than 100 ns on the insulating side and ∼80 ns on the metallic side. Based on our measurements in Si:Al 0.3 Ga 0.7 As, the extrapolated spin lifetime at zero bias and at 5 K is found to be only ∼2.3 ns on the insulating side and decreases with increasing bias current.
Bulletin of the American Physical Society, Mar 14, 2016
Institute of Semiconductors, Chinese Academy of Sciences-3-terminal (3T) and nonlocal 4-terminal ... more Institute of Semiconductors, Chinese Academy of Sciences-3-terminal (3T) and nonlocal 4-terminal (4T) Hanle measurements have been performed on a spin injection/detection device with patterned Fe electrodes and Al 0.3 Ga 0.7 As:Si, a persistent photoconductor, as the channel. The persistent photoconductivity facilitates in situ incremental photo-doping of the AlGaAs channel, which enables direct comparisons of the 3T and 4T Hanle results on the same device over a broad range of carrier densities across the insulator-metal transition. Although their magnitudes differ by about an order of magnitude, the 3T and 4T Hanle signals exhibit broad similarities in their dependencies on the injection current and carrier density, as well as the resulting spin lifetimes. Specifically, at each bias current, the magnitudes of both the 3T and 4T Hanle signals are observed to decrease exponentially with increasing carrier density of the AlGaAs deep into the metallic state. The spin lifetimes extracted from the 3T and 4T Hanle curves, both via the FWHM of the Lorentzian fit and the 1D spin drift-diffusion model analysis, show similar values and evolution with the carrier density.
Bulletin of the American Physical Society, Mar 5, 2015
Hall effect measurements on EuB 6 have revealed manifestations of the microscopic electronic phas... more Hall effect measurements on EuB 6 have revealed manifestations of the microscopic electronic phase separation and resulting percolative phase transition in a macroscopic magnetotransport property of this semimetallic ferromagnet [1]: the Hall resistivity as a function of applied field in the paramagnetic phase exhibits two distinct linear regions with a transition point at a single critical magnetization in a broad temperature range, which was interpreted as the percolation point for the more conducting phase. To further understand this phenomenon, magnetotransport measurements were performed on EuB 6 under high pressure. Hydrostatic pressure is known to substantially modify the magnetic state of EuB 6 [2]. EuB 6 single crystals were inserted in a high-pressure cell filled with silicone oil and measurements were taken at different pressures up to 1.8 GPa. Increasing hydrostatic pressure caused a decrease in resistivity and an increase in T θ , while the ferromagnetic ordering temperature stayed approximately constant. The Hall resistivity in the paramagnetic phase developed an intermediate region between the two previously observed regions. The transition fields between the low-field and intermediate regions depend linearly on temperature and their intercepts increase with pressure similar to the variation of T θ indicated by the resistivity peak.
Bulletin of the American Physical Society, Feb 28, 2012
The spin lifetime in GaAs varies strongly with carrier density near the insulator to metal transi... more The spin lifetime in GaAs varies strongly with carrier density near the insulator to metal transition (IMT), possibly peaking at the transition [1]. However, determining the optimal spin lifetime in this material is challenging because many replica samples need to be fabricated and measured. This difficulty can be circumvented by employing Si:Al 0.3 Ga 0.7 As, a persistent photoconductor, as the spin transport medium. This material has been characterized and has an effective carrier density which can be tuned in situ via photo-excitation from 10 14 to 10 18 cm −3 and a critical carrier density for the IMT of 9.0 x 10 16 cm −3 at 5K [2]. Heterostructures have been grown by MBE, consisting of 2 µm Si:AlGaAs, a thin epitaxial Fe layer, and an AlGaAs graded junction to create Schottky tunnel barrier contacts. Non-local spin devices have been fabricated and measured. Based on non-local 4 terminal (NL 4T) and local and NL 3T Hanle effect measurements, the initial electrical spin transport and accumulation measurements in this material are reported. The spin lifetimes range from 600 ps to 2.8 ns for multiple carrier densities, ranging from 3.5 x 10 16 to 2.4 x 10 17 cm −3. [1] J. M.
Bulletin of the American Physical Society, Mar 22, 2011
Institute of Semiconductors, Chinese Academy of Sciences-The spin lifetime in GaAs is known to va... more Institute of Semiconductors, Chinese Academy of Sciences-The spin lifetime in GaAs is known to vary strongly with carrier density near the metal to insulator transition [1]. However, a detailed study to optimize this lifetime is complicated because many replica samples need to be made and measured. This difficulty can be circumvented by employing Si:Al 0.3 Ga 0.7 As, a persistent photoconductor, as the spin transport medium. This material, which is structurally similar to GaAs, has been characterized and shown to have an effective carrier density which can be tuned in situ via photo-excitation from 10 14 to 10 18 cm −3 [2]. Heterostructures (2-µm Si:Al 0.3 Ga 0.7 As, a thin epitxial Fe layer, and a GaAs graded junction to create linear contacts between them) have been grown by MBE and non-local spin devices have been patterned by photolithography and wet etching. Magnetic measurements on Fe micro-patterns demonstrated the possibility of controlling the coercivity of the Fe electrodes [3]. Electrical characterization of the devices will be presented. [1] J.
The nature of the superconducting state in kagome metals AV 3 Sb 5 is a key issue in need of expe... more The nature of the superconducting state in kagome metals AV 3 Sb 5 is a key issue in need of experimental clarification. Here, we report on a study of the superconducting order parameter in the kagome superconductor CsV 3 Sb 5 through simultaneous "soft" point-contact spectroscopy and resistivity measurements under both ambient and a hydrostatic pressure. Signatures of two-gap superconductivity are resolved in the soft point-contact spectra, accompanied by an asymmetric excess conductance above T c. Quantitative analysis based on the two-dimensional Blonder-Tinkham-Klapwijk model reveals an (s + s)-wave superconducting gap with 2 0 /k B T c 7.2, placing CsV 3 Sb 5 in the strong-coupling regime. The strong-coupling two-gap feature indicates a high electronic density of states (DOS) and possible existence of flat-band-driven multiple van Hove singularities (VHSs) at the Fermi level. The presence of asymmetric excess spectral conductance above T c hints at a modest electronic correlation in CsV 3 Sb 5. Under a hydrostatic pressure of 2.1 kbar, the nodeless multigap nature of the superconducting state remains, whereas both the larger gap and the excess spectral conductance are greatly suppressed, accompanied by an enhanced T c. An estimate of the spectral-weighted gap ratio reveals a weakened coupling strength, indicative of a reduced total superconducting DOS upon pressure. Our results point to key roles of both flat-band-associated VHSs and electronic correlation in the onset of kagome superconductivity and shed some light on the interplay between charge-density-wave order and superconductivity in vanadium-based kagome superconductors.
Professor Stephan von Molnár (Figs. 1 and 2), known for his groundbreaking work on magnetic semic... more Professor Stephan von Molnár (Figs. 1 and 2), known for his groundbreaking work on magnetic semiconductors and spintronics, passed away peacefully on 17 November 2020 in Tallahassee, Florida, USA. Stephan was born in Leipzig, eastern Germany in 1935. After a tumultuous childhood through World War II, he immigrated to the United States in 1947. He received his Ph.D. at University of California, Riverside in 1965. He then joined the IBM T.J. Watson Research Center as a Research Staff, and later held positions as manager and senior manager. Upon arriving at IBM, he commenced a highly influential and consequential study of magnetic semiconductors, which would last for nearly six decades. The effort was broadly motivated by the goal of combining magnetism with semiconductors, the two fundamental pillars of IBM’s business interests in magnetic information storage and semiconductor electronics. The research foreshadowed the emergence of the field of spinbased electronics, long before the word “spintronics” was minted in 1990s. In 1994, he joined the Florida State University (FSU). In addition to leading a research group focusing on spin related physics and bionanotechnology, he served as the director of the Center for Materials Research and Technology (MARTECH) for nearly 14 years. Stephan was a Distinguished Research Professor of FSU, a fellow of the American Physical Society, and a recipient of the Alexander von Humboldt Senior U.S. Scientist Award. In early 2000s, he chaired a panel sponsored by several US government agencies, which surveyed the worldwide research efforts in spintronics and provided important guidance in its subsequent development. Beginning with his Ph.D. work on the magnetic resonance studies of EuS, a model system for the first generation of magnetic semiconductors, Stephan was intimately involved in the investigation of all three generations of magnetic semiconductors. He contributed to the inception of the field and heralded its spectacular growth and development into the mainstream of materials science and condensed matter physics. His work resulted in 170 scientific papers and garnered at least 24 000 citations. Many of Stephan’s seminal contributions to the field of magnetic semiconductors have found far-reaching impact in other areas; it is informative to briefly review a few of them here: (1) Magnetic tunnel junction. In 1967, working with Phil Stiles and Leo Esaki, Stephan fabricated metal/EuSe heterostructures by thin film deposition method. Deftly utilizing the spin-splitting of the Schottky barrier height as the magnetic semiconductor EuSe enters the ferromagnetic state, they demonstrated that decreasing temperature or applying a magnetic field could greatly modify the field emission current (Fowler-Nordheim tunneling). This was widely regarded as the first conceptual demonstration of a spintronic device, for the realization of spin-polarized tunneling current and the control of the charge current by way of the magnetic state of the semiconductor. (2) Giant negative magnetoresistance. In 1967, in transport measurements of Gd-doped magnetic semiconductor EuSe, Stephan and coworkers observed an apparent metal– insulator phase transition around the Curie temperature. Around the Curie temperature, the resistance of the sample could be reduced by more than 10 000-fold by applying a mag-
China-Anisotropic transport measurements have been performed on LCMO films grown on NdGaO 3 (001)... more China-Anisotropic transport measurements have been performed on LCMO films grown on NdGaO 3 (001) substrates. Three samples from a film 48 nm thick were post-annealed for 1.5h, 5h and 20h to produce increasing degrees of anisotropic strain, which promotes electronic phase separation (PS). As demonstrated previously, the presence and growth of antiferromagnetic insulating (AFI) regions in the samples can be controlled by the strain, resulting in a state of coexisting ferromagnetic metallic (FMM) and AFI domains. To study the effects of the strain anisotropy on the PS and formation of the AFI states, we carried out simultaneous magnetotransport measurements along the two orthogonal in-plane directions using an L-bar geometry. Substantial anisotropy in the temperature and magnetic field dependent resistivity between the two directions was observed, implying the formation of the AFI states has an orientation preference under the anisotropic strain. These differences are dramatically enhanced with increasing strain. Furthermore, accompanying the emergence of the AFI states, a glass-like behavior signified by time relaxation was observed in the field-dependent resistivity, which provides new insight into the dynamics of the phase-separated AFI and FMM domains.
Bulletin of the American Physical Society, Mar 3, 2014
Submitted for the MAR14 Meeting of The American Physical Society Magnetotransport study of the te... more Submitted for the MAR14 Meeting of The American Physical Society Magnetotransport study of the ternary topological insulator (Bi 0.5 Sb 0.5) 2 Te 3 via in situ low temperature deposition of Cr LIUQI YU, JORGE BARREDA, LONGQIAN HU, P. XIONG, Florida State University, TONG GUAN, XIAOYUE HE, K. WU, Y. LI, Institute of Physics, Chinese Academy of Sciences-The robustness of the surface state of three dimensional topological insulators against local magnetic perturbation is still under debate, since a precise and well-controlled electrical characterization of the effects of the ferromagnetic dopant and their evolution with doping density are exceedingly difficult. Here we report results of magnetotransport measurements on epitaxial thin films of the (Bi 0.5 Sb 0.5) 2 Te 3 in the presence of electrostatic gating and magnetic impurity. Magnetoresistance (MR) and Hall effect measurements have been performed in various back gate voltages. Ambipolar field effect has been observed, enabling effective tuning of the Fermi level across the band gap and identification of the surface transport in the topological transport regime. Taking advantage of the unique capability of in situ deposition of Cr atoms in a customized dilution refrigerator, magnetic impurities were incrementally quench-condensed onto the sample surface. Our results show the deposition of Cr effectively yields electron doping. The weak antilocalization (WAL) effect was found to be surprisingly insensitive to the magnetic impurity; the cusplike negative magnetoconductivity remains even at the highest Cr concentration and no apparent weak localization was observed as expected from a gap opening at the Dirac point. WAL effect has the largest modification at a back gate voltage of-200 V where the Fermi level is considered relatively close to the Dirac point.
Nanomaterials made from binary metal oxides are of increasing interest because of their versatili... more Nanomaterials made from binary metal oxides are of increasing interest because of their versatility in applications from flexible electronics to portable chemical and biological sensors. Controlling the electrical properties of these materials is the first step in device implementation. Tin dioxide (SnO2) nanobelts (NB) synthesized by the vapor-liquid-solid mechanism have shown much promise in this regard. We explore the modification of devices prepared with single crystalline NBs by thermal annealing in vacuum and oxygen, resulting in a viable field-effect transistor (FET) for numerous applications at ambient temperature. An oxygen annealing step initially increases the device conductance by up to a factor of 10(5), likely through the modification of the surface defects of the NB, leading to Schottky barrier limited devices. A multi-step annealing procedure leads to further increase of the conductance by approximately 350% and optimization of the electronic properties. The effects of each step is investigated systematically on a single NB. The optimization of the electrical properties of the NBs makes possible the consistent production of channel-limited FETs and control of the device performance. Understanding these improvements on the electrical properties over the as-grown materials provides a pathway to enhance and tailor the functionalities of tin oxide nanostructures for a wide variety of optical, electronic, optoelectronic, and sensing applications that operate at room temperature.
Bulletin of the American Physical Society, Mar 18, 2013
Submitted for the MAR13 Meeting of The American Physical Society Magneto-transport properties of ... more Submitted for the MAR13 Meeting of The American Physical Society Magneto-transport properties of the ternary topological insulator (Bi 0.5 Sb 0.5) 2 Te 3 in the presence of electrostatic gating and magnetic impurity LIUQI YU, JORGE BARREDA, LONGQIAN HU, P. XIONG, Department of Physics, Florida State University, USA, TONG GUAN, XIAOYUE HE, K. WU, Y. LI, Institute of Physics, Chinese Academy of Sciences, China-A three-dimensional topological insulator, (Bi 0.5 Sb 0.5) 2 Te 3 , is used to characterize the electronic properties of the spin helical conducting surface state. Epitaxial films are grown via MBE on (111) SrTiO 3 substrate, which serves as the gate dielectric. Magnetoresistance (MR) and Hall effect measurements have been performed at various back gate voltages. Ambipolar field effect has been observed, enabling effective tuning of the Fermi level across the band gap. Weak antilocalization effect is identified and used to differentiate the surface state. The Hikami-Larkin-Nagaoka (HLN) equation is used to analyze the MR data and the results show the top and bottom surfaces become decoupled when the Fermi level is in the bulk band gap. We also examine the effects of paramagnetic impurity (MI), which introduces time reversal symmetry breaking scattering, on the TI surface states. Taking advantage of the unique capability of in situ deposition in a customized dilution refrigerator, paramagnetic Cr atoms were incrementally quench-condensed onto the sample surface and transport measurements were performed at each MI density. The procedure eliminates any sample-to-sample variation and complications from air exposure. Pronounced changes in the weak antilocalization effect and the sample carrier density with increasing MI concentration were observed. Possible origins of these observations will be discussed.
High quality HgCr2Se4 single crystals have been investigated by magnetization, electron transport... more High quality HgCr2Se4 single crystals have been investigated by magnetization, electron transport and Andreev reflection spectroscopy. In the ferromagnetic ground state, the saturation magnetic moment of each unit cell corresponds to an integer number of electron spins (3 µB/Cr 3+), and the Hall effect measurements suggest n-type charge carriers. Spin polarizations as high as 97% were obtained from fits of the differential conductance spectra of HgCr2Se4/Pb junctions with the modified Blonder-Tinkham-Klapwijk theory. The temperature and bias-voltage dependencies of the sub-gap conductance are consistent with recent theoretical calculations based on spin active scatterings at a superconductor/half metal interface. Our results suggest that n-HgCr2Se4 is a half metal, in agreement with theoretical calculations that also predict undoped HgCr2Se4 is a magnetic Weyl semimetal.
Bulletin of the American Physical Society, Mar 14, 2017
Submitted for the MAR17 Meeting of The American Physical Society Systematic study of spin transpo... more Submitted for the MAR17 Meeting of The American Physical Society Systematic study of spin transport in Si nanowires with axial doping gradient KONSTANTINOS KOUNTOURIOTIS, JORGE BARREDA, TIM KEIPER, MEI ZHANG, PENG XIONG, Florida State Univ-Spin transport experiments have been widely employed to study spin relaxation mechanisms in bulk and two-dimensional semiconductors. Quasi 1D systems such as semiconductor nanowires (NWs) could offer some advantages as spin transport channels; it was predicted that quantum confinement can lead to significant enhancement in spin lifetime and diffusion lengths. We have performed systematic spin transport, including local 2T, 3T, and nonlocal 4T spin valve measurements, in phosphorusdoped Si NWs exhibiting a pronounced doping gradient along the axial direction. The doping gradient enables the formation of Ohmic contacts and Schottky barriers of different widths and heights between a series of ferromagnetic electrodes and a single NW. This facilitates a methodical study of the dependence of the spin signal on interfacial resistance. Spin injection/extraction is effective within a window of interfacial resistance, which in our devices corresponds to zero-bias 2T resistances between 100 kΩ and 2 MΩ, corresponding to the estimated carrier densities between 6.810 17 cm −3 and 4.510 18 cm −3. Within this region we measured nonlocal spin signals of magnitudes between 1 µV and 50 µV (at I=20 nA). Also, the spin signals are observed to increase when the spins are injected from a more resistive interface. Comparison of the local 2T and nonlocal 4T signals and the effects of interchanging the injector and detector electrodes for the same transport channel will be presented.
Bulletin of the American Physical Society, Mar 18, 2013
Electrical spin injection/detection experiments have been performed on Si:Al 0.3 Ga 0.7 As, a per... more Electrical spin injection/detection experiments have been performed on Si:Al 0.3 Ga 0.7 As, a persistent photoconductor. The carrier density of this material can be tuned in situ via photo excitation across the insulator-metal transition (IMT) [1], which enables spin accumulation and transport measurements in one and the same sample over orders of magnitude variation in carrier density, thus circumventing the difficulties of making many replicas to realize different doping levels. Fe/AlGaAs heterostructures were grown by MBE, in which AlGaAs and GaAs graded Schottky junctions were tested for optimum spin injection. Spin transport devices, suitable for 3-terminal and non-local 4-terminal Hanle-type measurements and on-chip determination of the carrier density, were fabricated from the wafers. The spin lifetimes, determined from fits of the Hanle curves to a Lorentzian or the spin drift-diffusion model, range from 0.5 ns to 2.8 ns and exhibit a nonmonotonic carrier density dependence possibly peaked at the IMT.
Bulletin of the American Physical Society, Nov 17, 2012
Electrical Hanle-type measurements have been performed to determine spin lifetimes at various car... more Electrical Hanle-type measurements have been performed to determine spin lifetimes at various carrier densities in Si:Al 0.3 Ga 0.7 As, a persistent photoconductor (PPC). The carrier density of this material can be tuned, changing it from insulating to metallic in situ via photo excitation. Utilizing this PPC effect, we conduct electrical measurements of spin accumulation and transport under the same experimental conditions without the necessity of making replicas to realize different doping levels. We report the carrier density dependence of the spin lifetime derived from Hanle measurements with spin devices formed on wafers which have different graded junctions and Si doping levels. Carrier densities ranged from 3.5x10 16 to 2.4x10 17 cm −3 and from 7.2x10 16 to 6.5x10 17 cm −3 in two different samples. The spin lifetimes (determined using Lorentzian fits to the Hanle curves) ranged from 0.5 to 2.8 ns. From optical studies, the spin lifetime at zero bias and at low temperature in n-GaAs was reported to be larger than 100 ns on the insulating side and ∼80 ns on the metallic side. Based on our measurements in Si:Al 0.3 Ga 0.7 As, the extrapolated spin lifetime at zero bias and at 5 K is found to be only ∼2.3 ns on the insulating side and decreases with increasing bias current.
Bulletin of the American Physical Society, Mar 14, 2016
Institute of Semiconductors, Chinese Academy of Sciences-3-terminal (3T) and nonlocal 4-terminal ... more Institute of Semiconductors, Chinese Academy of Sciences-3-terminal (3T) and nonlocal 4-terminal (4T) Hanle measurements have been performed on a spin injection/detection device with patterned Fe electrodes and Al 0.3 Ga 0.7 As:Si, a persistent photoconductor, as the channel. The persistent photoconductivity facilitates in situ incremental photo-doping of the AlGaAs channel, which enables direct comparisons of the 3T and 4T Hanle results on the same device over a broad range of carrier densities across the insulator-metal transition. Although their magnitudes differ by about an order of magnitude, the 3T and 4T Hanle signals exhibit broad similarities in their dependencies on the injection current and carrier density, as well as the resulting spin lifetimes. Specifically, at each bias current, the magnitudes of both the 3T and 4T Hanle signals are observed to decrease exponentially with increasing carrier density of the AlGaAs deep into the metallic state. The spin lifetimes extracted from the 3T and 4T Hanle curves, both via the FWHM of the Lorentzian fit and the 1D spin drift-diffusion model analysis, show similar values and evolution with the carrier density.
Bulletin of the American Physical Society, Mar 5, 2015
Hall effect measurements on EuB 6 have revealed manifestations of the microscopic electronic phas... more Hall effect measurements on EuB 6 have revealed manifestations of the microscopic electronic phase separation and resulting percolative phase transition in a macroscopic magnetotransport property of this semimetallic ferromagnet [1]: the Hall resistivity as a function of applied field in the paramagnetic phase exhibits two distinct linear regions with a transition point at a single critical magnetization in a broad temperature range, which was interpreted as the percolation point for the more conducting phase. To further understand this phenomenon, magnetotransport measurements were performed on EuB 6 under high pressure. Hydrostatic pressure is known to substantially modify the magnetic state of EuB 6 [2]. EuB 6 single crystals were inserted in a high-pressure cell filled with silicone oil and measurements were taken at different pressures up to 1.8 GPa. Increasing hydrostatic pressure caused a decrease in resistivity and an increase in T θ , while the ferromagnetic ordering temperature stayed approximately constant. The Hall resistivity in the paramagnetic phase developed an intermediate region between the two previously observed regions. The transition fields between the low-field and intermediate regions depend linearly on temperature and their intercepts increase with pressure similar to the variation of T θ indicated by the resistivity peak.
Bulletin of the American Physical Society, Feb 28, 2012
The spin lifetime in GaAs varies strongly with carrier density near the insulator to metal transi... more The spin lifetime in GaAs varies strongly with carrier density near the insulator to metal transition (IMT), possibly peaking at the transition [1]. However, determining the optimal spin lifetime in this material is challenging because many replica samples need to be fabricated and measured. This difficulty can be circumvented by employing Si:Al 0.3 Ga 0.7 As, a persistent photoconductor, as the spin transport medium. This material has been characterized and has an effective carrier density which can be tuned in situ via photo-excitation from 10 14 to 10 18 cm −3 and a critical carrier density for the IMT of 9.0 x 10 16 cm −3 at 5K [2]. Heterostructures have been grown by MBE, consisting of 2 µm Si:AlGaAs, a thin epitaxial Fe layer, and an AlGaAs graded junction to create Schottky tunnel barrier contacts. Non-local spin devices have been fabricated and measured. Based on non-local 4 terminal (NL 4T) and local and NL 3T Hanle effect measurements, the initial electrical spin transport and accumulation measurements in this material are reported. The spin lifetimes range from 600 ps to 2.8 ns for multiple carrier densities, ranging from 3.5 x 10 16 to 2.4 x 10 17 cm −3. [1] J. M.
Bulletin of the American Physical Society, Mar 22, 2011
Institute of Semiconductors, Chinese Academy of Sciences-The spin lifetime in GaAs is known to va... more Institute of Semiconductors, Chinese Academy of Sciences-The spin lifetime in GaAs is known to vary strongly with carrier density near the metal to insulator transition [1]. However, a detailed study to optimize this lifetime is complicated because many replica samples need to be made and measured. This difficulty can be circumvented by employing Si:Al 0.3 Ga 0.7 As, a persistent photoconductor, as the spin transport medium. This material, which is structurally similar to GaAs, has been characterized and shown to have an effective carrier density which can be tuned in situ via photo-excitation from 10 14 to 10 18 cm −3 [2]. Heterostructures (2-µm Si:Al 0.3 Ga 0.7 As, a thin epitxial Fe layer, and a GaAs graded junction to create linear contacts between them) have been grown by MBE and non-local spin devices have been patterned by photolithography and wet etching. Magnetic measurements on Fe micro-patterns demonstrated the possibility of controlling the coercivity of the Fe electrodes [3]. Electrical characterization of the devices will be presented. [1] J.
The nature of the superconducting state in kagome metals AV 3 Sb 5 is a key issue in need of expe... more The nature of the superconducting state in kagome metals AV 3 Sb 5 is a key issue in need of experimental clarification. Here, we report on a study of the superconducting order parameter in the kagome superconductor CsV 3 Sb 5 through simultaneous "soft" point-contact spectroscopy and resistivity measurements under both ambient and a hydrostatic pressure. Signatures of two-gap superconductivity are resolved in the soft point-contact spectra, accompanied by an asymmetric excess conductance above T c. Quantitative analysis based on the two-dimensional Blonder-Tinkham-Klapwijk model reveals an (s + s)-wave superconducting gap with 2 0 /k B T c 7.2, placing CsV 3 Sb 5 in the strong-coupling regime. The strong-coupling two-gap feature indicates a high electronic density of states (DOS) and possible existence of flat-band-driven multiple van Hove singularities (VHSs) at the Fermi level. The presence of asymmetric excess spectral conductance above T c hints at a modest electronic correlation in CsV 3 Sb 5. Under a hydrostatic pressure of 2.1 kbar, the nodeless multigap nature of the superconducting state remains, whereas both the larger gap and the excess spectral conductance are greatly suppressed, accompanied by an enhanced T c. An estimate of the spectral-weighted gap ratio reveals a weakened coupling strength, indicative of a reduced total superconducting DOS upon pressure. Our results point to key roles of both flat-band-associated VHSs and electronic correlation in the onset of kagome superconductivity and shed some light on the interplay between charge-density-wave order and superconductivity in vanadium-based kagome superconductors.
Professor Stephan von Molnár (Figs. 1 and 2), known for his groundbreaking work on magnetic semic... more Professor Stephan von Molnár (Figs. 1 and 2), known for his groundbreaking work on magnetic semiconductors and spintronics, passed away peacefully on 17 November 2020 in Tallahassee, Florida, USA. Stephan was born in Leipzig, eastern Germany in 1935. After a tumultuous childhood through World War II, he immigrated to the United States in 1947. He received his Ph.D. at University of California, Riverside in 1965. He then joined the IBM T.J. Watson Research Center as a Research Staff, and later held positions as manager and senior manager. Upon arriving at IBM, he commenced a highly influential and consequential study of magnetic semiconductors, which would last for nearly six decades. The effort was broadly motivated by the goal of combining magnetism with semiconductors, the two fundamental pillars of IBM’s business interests in magnetic information storage and semiconductor electronics. The research foreshadowed the emergence of the field of spinbased electronics, long before the word “spintronics” was minted in 1990s. In 1994, he joined the Florida State University (FSU). In addition to leading a research group focusing on spin related physics and bionanotechnology, he served as the director of the Center for Materials Research and Technology (MARTECH) for nearly 14 years. Stephan was a Distinguished Research Professor of FSU, a fellow of the American Physical Society, and a recipient of the Alexander von Humboldt Senior U.S. Scientist Award. In early 2000s, he chaired a panel sponsored by several US government agencies, which surveyed the worldwide research efforts in spintronics and provided important guidance in its subsequent development. Beginning with his Ph.D. work on the magnetic resonance studies of EuS, a model system for the first generation of magnetic semiconductors, Stephan was intimately involved in the investigation of all three generations of magnetic semiconductors. He contributed to the inception of the field and heralded its spectacular growth and development into the mainstream of materials science and condensed matter physics. His work resulted in 170 scientific papers and garnered at least 24 000 citations. Many of Stephan’s seminal contributions to the field of magnetic semiconductors have found far-reaching impact in other areas; it is informative to briefly review a few of them here: (1) Magnetic tunnel junction. In 1967, working with Phil Stiles and Leo Esaki, Stephan fabricated metal/EuSe heterostructures by thin film deposition method. Deftly utilizing the spin-splitting of the Schottky barrier height as the magnetic semiconductor EuSe enters the ferromagnetic state, they demonstrated that decreasing temperature or applying a magnetic field could greatly modify the field emission current (Fowler-Nordheim tunneling). This was widely regarded as the first conceptual demonstration of a spintronic device, for the realization of spin-polarized tunneling current and the control of the charge current by way of the magnetic state of the semiconductor. (2) Giant negative magnetoresistance. In 1967, in transport measurements of Gd-doped magnetic semiconductor EuSe, Stephan and coworkers observed an apparent metal– insulator phase transition around the Curie temperature. Around the Curie temperature, the resistance of the sample could be reduced by more than 10 000-fold by applying a mag-
China-Anisotropic transport measurements have been performed on LCMO films grown on NdGaO 3 (001)... more China-Anisotropic transport measurements have been performed on LCMO films grown on NdGaO 3 (001) substrates. Three samples from a film 48 nm thick were post-annealed for 1.5h, 5h and 20h to produce increasing degrees of anisotropic strain, which promotes electronic phase separation (PS). As demonstrated previously, the presence and growth of antiferromagnetic insulating (AFI) regions in the samples can be controlled by the strain, resulting in a state of coexisting ferromagnetic metallic (FMM) and AFI domains. To study the effects of the strain anisotropy on the PS and formation of the AFI states, we carried out simultaneous magnetotransport measurements along the two orthogonal in-plane directions using an L-bar geometry. Substantial anisotropy in the temperature and magnetic field dependent resistivity between the two directions was observed, implying the formation of the AFI states has an orientation preference under the anisotropic strain. These differences are dramatically enhanced with increasing strain. Furthermore, accompanying the emergence of the AFI states, a glass-like behavior signified by time relaxation was observed in the field-dependent resistivity, which provides new insight into the dynamics of the phase-separated AFI and FMM domains.
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