HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific r... more HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013), 2014
We present surface resistance measurements of the archetypical heavy-fermion compound CeCu 6 for ... more We present surface resistance measurements of the archetypical heavy-fermion compound CeCu 6 for frequencies between 3.7 and 18 GHz and temperatures from 1.2 to 6 K. The measurements were performed with superconducting stripline resonators that allow simultaneous measurements at multiple frequencies. The surface resistance of CeCu 6 exhibits a pronounced decrease below 3 K, in consistence with dc resistivity. The low-temperature frequency dependence of the surface resistance follows a power law with exponent 2/3. While for conventional metals this would be consistent with the anomalous skin effect, we discuss the present situation of a heavy-fermion metal, where this frequency dependence might instead stem from the influence of electronic correlations.
We report on high-resolution specific-heat and magnetocaloric-effect measurements of the triangul... more We report on high-resolution specific-heat and magnetocaloric-effect measurements of the triangular-lattice antiferromagnets CsMn(BrxI1-x)3 with different x. The evolution of the magnetic phase diagrams from the easy-axis system for x = 0 to the easy-plane system for x = 1 was studied in detail. The specific-heat critical exponent α of the almost isotropic x = 0.19 system agrees with the value predicted for a chiral Heisenberg scenario. In an applied magnetic field (B = 6 T) a crossover to a weak first-order transition is detected.
We report measurements of the specific heat C (0.08 K & T (7 K) of Xe films condensed at low temp... more We report measurements of the specific heat C (0.08 K & T (7 K) of Xe films condensed at low temperatures (=6 K). Low-T x-ray dilfraction indicates that the films are crystalline but strongly disordered, with very small crystallites. The specific heat is very large compared to bulk Xe, and the excess specific heat can be attributed to surface modes and to tunneling states. Disordered solids exhibit a large variety of low- temperature behavior. Often these features can be traced
A quantum phase transition (QPT) in a heavy-fermion (HF) compound may destroy the Fermi liquid gr... more A quantum phase transition (QPT) in a heavy-fermion (HF) compound may destroy the Fermi liquid groundstate. However, the conditions for this breakdown have remained obscure. We report the first direct investigation of heavy quasiparticle formation and breakdown in the canonical system CeCu6-xAux by ultraviolet photoemission spectroscopy at elevated temperatures without the complications of lattice coherence. Surprisingly, the single-ion Kondo energy scale TK exhibits an abrupt step near the quantum critical Au concentration of xc = 0.1. We show theoretically that this step is expected from a highly non-linear renormalization of the local spin coupling at each Ce site, induced by spin fluctuations on neighboring sites. It provides a general high-temperature indicator for HF quasiparticle breakdown at a QPT.
The quasi-two-dimensional organic superconductor b 00 -͑BEDT-TTF͒ 2 SF 5 CH 2 CF 2 SO 3 ͑T c ഠ 4.... more The quasi-two-dimensional organic superconductor b 00 -͑BEDT-TTF͒ 2 SF 5 CH 2 CF 2 SO 3 ͑T c ഠ 4.4 K͒ shows very strong Shubnikov -de Haas (SdH) oscillations which are superimposed on a highly anomalous steady background magnetoresistance, R b . Comparison with de Haas-van Alphen oscillations allows a reliable estimate of R b which is crucial for the correct extraction of the SdH signal. At low temperatures and high magnetic fields insulating behavior evolves. The magnetoresistance data violate Kohler's rule, i.e., cannot be described within the framework of semiclassical transport theory, but converge onto a universal curve appropriate for dynamical scaling at a metal-insulator transition.
Far-infrared reflection measurements on Si:P both on the metallic and insulating side of the meta... more Far-infrared reflection measurements on Si:P both on the metallic and insulating side of the metal-insulator transition (MIT) were performed at low temperatures. In the metallic regime free- carrier absorption and, additionally, absorption peaks due to interband transitions from the impurity band to the conduction band and to transitions between the broadened valley-orbit split ls states are observed. This gives clear evidence that the impurity band is formed by the overlap of the Is(A&) ground states and is well separated from the conduction band when the MIT occurs. Only far beyond the metallic limit the impurity band merges completely with the conduction band.
The Neel temperature of the antiferromagnetic heavy-fermion alloy CeCu57Auo3 (T~= 0.49 K for p = ... more The Neel temperature of the antiferromagnetic heavy-fermion alloy CeCu57Auo3 (T~= 0.49 K for p = 0) can be continuously tuned to zero with increasing hydrostatic pressure p. At p = p, where TN = 0 is reached, the specific heat C varies as C/T --ln(T/To) between 0.1 and 2 K, indicative of non-Fermi-liquid behavior. The observed linear TN(p) dependence puts rigorous constraints on theories for a quantum phase transition at T = 0 between magnetic and nonmagnetic ground states.
We report measurements of the specific heat of the one-dimensional ionic conductor hollandite for... more We report measurements of the specific heat of the one-dimensional ionic conductor hollandite for temperatures bebveen 0.07 and 3 K. An anomalous contribution is ob- served which exhibits a pronounced peak unlike most disordered systems. The peak height strongly depends on the concentration of the diffusirg ions. A theoretical interpretation is given.
The low-temperature thermal expansion of CeCoIn5 single crystals measured parallel and perpendicu... more The low-temperature thermal expansion of CeCoIn5 single crystals measured parallel and perpendicular to magnetic fields B oriented along the c axis yields the volume thermal-expansion coefficient β. Considerable deviations of β(T ) from Fermi-liquid behavior occur already within the superconducting region of the (B, T ) phase diagram and become maximal at the upper critical field B 0 c2 . However, β(T ) and the Grüneisen parameter Γ are incompatible with a quantum critical point (QCP) at B 0 c2 , but allow for a QCP shielded by superconductivity and extending to negative pressures for B < B 0 c2 . We construct a tentative (p, B, T ) phase diagram of CeCoIn5 suggesting a quantum critical line in the (p, B) plane.
We report on the low-temperature properties of the ternary europium pnictide EuZn 2 Sb 2 . Bulk p... more We report on the low-temperature properties of the ternary europium pnictide EuZn 2 Sb 2 . Bulk properties were characterized in terms of the electrical resistivity, magnetoresistance, Hall effect, magnetization, specific heat, and thermal expansion. The data are consistent with an antiferromagnetic order of divalent europium below T N = 13.3 K. The phase transition at T N is clearly visible in all properties measured. The magnetic field-temperature phase diagram was derived from the magnetization, specific heat, and resistivity. For the field parallel and perpendicular to the trigonal c axis the antiferromagnetism is suppressed above B ʈ Ϸ 4.7 T and B Ќ Ϸ 3.5 T, respectively. The critical exponent ␣ observed in the specific heat near T N is consistent with both the predictions of the three-dimensional ͑3D͒ XY as well as the 3D-Heisenberg model.
Scanning tunneling spectroscopy is used to study the electronic properties of the Si͑111͒-2 ϫ 1 s... more Scanning tunneling spectroscopy is used to study the electronic properties of the Si͑111͒-2 ϫ 1 surface at 8 K. Wave-function mapping reveals highly anisotropic scattering of electrons with one-dimensional ͑1D͒ confinement on the -bonded chains. The differential tunneling conductance is strongly reduced at the Fermi energy E F and exhibits a Coulomb gap. The gap width W scales with the inverse length L of the chain segments defined by the distance between two adjacent scatterers such as surface steps or dopant atoms within a given chain. A model of electronic transport through 1D chain segments of finite length describes the observed behavior.
The magnetization of superconductor and/or normal-metal ͑Nb/ Ag͒ double layers is investigated in... more The magnetization of superconductor and/or normal-metal ͑Nb/ Ag͒ double layers is investigated in dependence on temperature T and magnetic field B. Screening currents in the normal-metal induced by the proximity effect give rise to a diamagnetic transition in a weak magnetic field at a temperature T b . The phase transition is suppressed when Fe is in direct contact with Ag. Surprisingly, the diamagnetic signal of Ag is recovered for small Ag film thickness d Ag . These findings are qualitatively explained by the competition in the Ag layer between proximity-induced superconductivity by the Nb layer and pair breaking by the ferromagnetic Fe layer.
We report on non-equilibrium electronic transport through normal-metal (Cu) nanobridges coupled t... more We report on non-equilibrium electronic transport through normal-metal (Cu) nanobridges coupled to large reservoirs at low temperatures. We observe a logarithmic temperature dependence of the zero-bias conductance, as well as a universal scaling behavior of the differential conductance. Our results are explained by electron-electron interactions in diffusive metals in the zero-dimensional limit.
We report on ac-susceptibility and heat-capacity measurements of the superconductor PrOs 4 Sb 12 ... more We report on ac-susceptibility and heat-capacity measurements of the superconductor PrOs 4 Sb 12 in magnetic fields. The resulting phase diagram reveals two distinguishable superconducting phases with transitions at the upper critical field B c2 and a slightly lower field B c2 * . Between B c2 * and 0.7ϫ B c2 * the ac-susceptibility data shows a region with enhanced pinning properties characterized by an extended peak effect. The heat-capacity data reveal an extremely strongly coupled superconductivity with a considerable contribution of heavy quasiparticles. This unusual strong-coupling behavior originates in a sharp increase of the superfluid density at T c . The decrease of the discontinuity of the specific heat at T c and the corresponding pronounced increase of the Ginzburg-Landau-Maki parameter 2 indicate that the superconductivity is most probably not Pauli limited in a large temperature range.
We present an X-ray photoemission study of the heavy fermion system CeCu6-xAux across the magneti... more We present an X-ray photoemission study of the heavy fermion system CeCu6-xAux across the magnetic quantum phase transition of this compound at temperatures above the single-ion Kondo temperature TK. In dependence of the Au concentration x we observe a sudden change of the f -occupation number n f and the core-hole potential U df at the critical concentration xc = 0.1. We interpret these findings in the framework of the single-impurity Anderson model. Our results are in excellent agreement with findings from earlier UPS measurements and provide further information about the precursors of quantum criticality at elevated temperatures.
In this paper, we report on measurements of the specific heat C of single-crystalline EuxSrl_xTe ... more In this paper, we report on measurements of the specific heat C of single-crystalline EuxSrl_xTe at temperatures between 60 mK and 15 K and in magnetic fields up to 6 T. Pure antiferromagnetic EuTe shows unusual critical behavior in the vicinity of the N6el temperature T N = 9.8 K with a positive critical exponent instead of the 3d-Heisenberg exponent c~---0.12. Possible reasons for this discrepancy between theory and experiment include magnetic anisotropy effects due to magnetic dipole-dipole interactions, which may give rise to a cross-over of the critical behavior very close to T N. This anisotropy is also seen in the specific heat below 1 K where an exponential decay of C is observed, and in the dependence of the magnetic susceptibility on the direction of the applied field. With increasing dilution of EuTe with nonmagnetic Sr, the critical behavior changes: ~ becomes negative and decreases continuously towards ~ ~-1 at x~x c. This concentration dependence of e was previously observed in the diluted ferromagnetic system EuxSr ~ _xS. Our data thus support that the apparent change in the critical behavior depends on the degree of disorder. Samples with concentration x lower than the critical concentration x c reveal spin-glass behavior in the specific heat. In addition, the dependence of T N on magnetic fields is discussed. The data yield a normalized magnetic phase boundary Bc(T)/Bc(T=O ) vs. TN(B)/TN(B=O ) which is independent of concentration.
HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific r... more HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L'archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
Proceedings of the International Conference on Strongly Correlated Electron Systems (SCES2013), 2014
We present surface resistance measurements of the archetypical heavy-fermion compound CeCu 6 for ... more We present surface resistance measurements of the archetypical heavy-fermion compound CeCu 6 for frequencies between 3.7 and 18 GHz and temperatures from 1.2 to 6 K. The measurements were performed with superconducting stripline resonators that allow simultaneous measurements at multiple frequencies. The surface resistance of CeCu 6 exhibits a pronounced decrease below 3 K, in consistence with dc resistivity. The low-temperature frequency dependence of the surface resistance follows a power law with exponent 2/3. While for conventional metals this would be consistent with the anomalous skin effect, we discuss the present situation of a heavy-fermion metal, where this frequency dependence might instead stem from the influence of electronic correlations.
We report on high-resolution specific-heat and magnetocaloric-effect measurements of the triangul... more We report on high-resolution specific-heat and magnetocaloric-effect measurements of the triangular-lattice antiferromagnets CsMn(BrxI1-x)3 with different x. The evolution of the magnetic phase diagrams from the easy-axis system for x = 0 to the easy-plane system for x = 1 was studied in detail. The specific-heat critical exponent α of the almost isotropic x = 0.19 system agrees with the value predicted for a chiral Heisenberg scenario. In an applied magnetic field (B = 6 T) a crossover to a weak first-order transition is detected.
We report measurements of the specific heat C (0.08 K & T (7 K) of Xe films condensed at low temp... more We report measurements of the specific heat C (0.08 K & T (7 K) of Xe films condensed at low temperatures (=6 K). Low-T x-ray dilfraction indicates that the films are crystalline but strongly disordered, with very small crystallites. The specific heat is very large compared to bulk Xe, and the excess specific heat can be attributed to surface modes and to tunneling states. Disordered solids exhibit a large variety of low- temperature behavior. Often these features can be traced
A quantum phase transition (QPT) in a heavy-fermion (HF) compound may destroy the Fermi liquid gr... more A quantum phase transition (QPT) in a heavy-fermion (HF) compound may destroy the Fermi liquid groundstate. However, the conditions for this breakdown have remained obscure. We report the first direct investigation of heavy quasiparticle formation and breakdown in the canonical system CeCu6-xAux by ultraviolet photoemission spectroscopy at elevated temperatures without the complications of lattice coherence. Surprisingly, the single-ion Kondo energy scale TK exhibits an abrupt step near the quantum critical Au concentration of xc = 0.1. We show theoretically that this step is expected from a highly non-linear renormalization of the local spin coupling at each Ce site, induced by spin fluctuations on neighboring sites. It provides a general high-temperature indicator for HF quasiparticle breakdown at a QPT.
The quasi-two-dimensional organic superconductor b 00 -͑BEDT-TTF͒ 2 SF 5 CH 2 CF 2 SO 3 ͑T c ഠ 4.... more The quasi-two-dimensional organic superconductor b 00 -͑BEDT-TTF͒ 2 SF 5 CH 2 CF 2 SO 3 ͑T c ഠ 4.4 K͒ shows very strong Shubnikov -de Haas (SdH) oscillations which are superimposed on a highly anomalous steady background magnetoresistance, R b . Comparison with de Haas-van Alphen oscillations allows a reliable estimate of R b which is crucial for the correct extraction of the SdH signal. At low temperatures and high magnetic fields insulating behavior evolves. The magnetoresistance data violate Kohler's rule, i.e., cannot be described within the framework of semiclassical transport theory, but converge onto a universal curve appropriate for dynamical scaling at a metal-insulator transition.
Far-infrared reflection measurements on Si:P both on the metallic and insulating side of the meta... more Far-infrared reflection measurements on Si:P both on the metallic and insulating side of the metal-insulator transition (MIT) were performed at low temperatures. In the metallic regime free- carrier absorption and, additionally, absorption peaks due to interband transitions from the impurity band to the conduction band and to transitions between the broadened valley-orbit split ls states are observed. This gives clear evidence that the impurity band is formed by the overlap of the Is(A&) ground states and is well separated from the conduction band when the MIT occurs. Only far beyond the metallic limit the impurity band merges completely with the conduction band.
The Neel temperature of the antiferromagnetic heavy-fermion alloy CeCu57Auo3 (T~= 0.49 K for p = ... more The Neel temperature of the antiferromagnetic heavy-fermion alloy CeCu57Auo3 (T~= 0.49 K for p = 0) can be continuously tuned to zero with increasing hydrostatic pressure p. At p = p, where TN = 0 is reached, the specific heat C varies as C/T --ln(T/To) between 0.1 and 2 K, indicative of non-Fermi-liquid behavior. The observed linear TN(p) dependence puts rigorous constraints on theories for a quantum phase transition at T = 0 between magnetic and nonmagnetic ground states.
We report measurements of the specific heat of the one-dimensional ionic conductor hollandite for... more We report measurements of the specific heat of the one-dimensional ionic conductor hollandite for temperatures bebveen 0.07 and 3 K. An anomalous contribution is ob- served which exhibits a pronounced peak unlike most disordered systems. The peak height strongly depends on the concentration of the diffusirg ions. A theoretical interpretation is given.
The low-temperature thermal expansion of CeCoIn5 single crystals measured parallel and perpendicu... more The low-temperature thermal expansion of CeCoIn5 single crystals measured parallel and perpendicular to magnetic fields B oriented along the c axis yields the volume thermal-expansion coefficient β. Considerable deviations of β(T ) from Fermi-liquid behavior occur already within the superconducting region of the (B, T ) phase diagram and become maximal at the upper critical field B 0 c2 . However, β(T ) and the Grüneisen parameter Γ are incompatible with a quantum critical point (QCP) at B 0 c2 , but allow for a QCP shielded by superconductivity and extending to negative pressures for B < B 0 c2 . We construct a tentative (p, B, T ) phase diagram of CeCoIn5 suggesting a quantum critical line in the (p, B) plane.
We report on the low-temperature properties of the ternary europium pnictide EuZn 2 Sb 2 . Bulk p... more We report on the low-temperature properties of the ternary europium pnictide EuZn 2 Sb 2 . Bulk properties were characterized in terms of the electrical resistivity, magnetoresistance, Hall effect, magnetization, specific heat, and thermal expansion. The data are consistent with an antiferromagnetic order of divalent europium below T N = 13.3 K. The phase transition at T N is clearly visible in all properties measured. The magnetic field-temperature phase diagram was derived from the magnetization, specific heat, and resistivity. For the field parallel and perpendicular to the trigonal c axis the antiferromagnetism is suppressed above B ʈ Ϸ 4.7 T and B Ќ Ϸ 3.5 T, respectively. The critical exponent ␣ observed in the specific heat near T N is consistent with both the predictions of the three-dimensional ͑3D͒ XY as well as the 3D-Heisenberg model.
Scanning tunneling spectroscopy is used to study the electronic properties of the Si͑111͒-2 ϫ 1 s... more Scanning tunneling spectroscopy is used to study the electronic properties of the Si͑111͒-2 ϫ 1 surface at 8 K. Wave-function mapping reveals highly anisotropic scattering of electrons with one-dimensional ͑1D͒ confinement on the -bonded chains. The differential tunneling conductance is strongly reduced at the Fermi energy E F and exhibits a Coulomb gap. The gap width W scales with the inverse length L of the chain segments defined by the distance between two adjacent scatterers such as surface steps or dopant atoms within a given chain. A model of electronic transport through 1D chain segments of finite length describes the observed behavior.
The magnetization of superconductor and/or normal-metal ͑Nb/ Ag͒ double layers is investigated in... more The magnetization of superconductor and/or normal-metal ͑Nb/ Ag͒ double layers is investigated in dependence on temperature T and magnetic field B. Screening currents in the normal-metal induced by the proximity effect give rise to a diamagnetic transition in a weak magnetic field at a temperature T b . The phase transition is suppressed when Fe is in direct contact with Ag. Surprisingly, the diamagnetic signal of Ag is recovered for small Ag film thickness d Ag . These findings are qualitatively explained by the competition in the Ag layer between proximity-induced superconductivity by the Nb layer and pair breaking by the ferromagnetic Fe layer.
We report on non-equilibrium electronic transport through normal-metal (Cu) nanobridges coupled t... more We report on non-equilibrium electronic transport through normal-metal (Cu) nanobridges coupled to large reservoirs at low temperatures. We observe a logarithmic temperature dependence of the zero-bias conductance, as well as a universal scaling behavior of the differential conductance. Our results are explained by electron-electron interactions in diffusive metals in the zero-dimensional limit.
We report on ac-susceptibility and heat-capacity measurements of the superconductor PrOs 4 Sb 12 ... more We report on ac-susceptibility and heat-capacity measurements of the superconductor PrOs 4 Sb 12 in magnetic fields. The resulting phase diagram reveals two distinguishable superconducting phases with transitions at the upper critical field B c2 and a slightly lower field B c2 * . Between B c2 * and 0.7ϫ B c2 * the ac-susceptibility data shows a region with enhanced pinning properties characterized by an extended peak effect. The heat-capacity data reveal an extremely strongly coupled superconductivity with a considerable contribution of heavy quasiparticles. This unusual strong-coupling behavior originates in a sharp increase of the superfluid density at T c . The decrease of the discontinuity of the specific heat at T c and the corresponding pronounced increase of the Ginzburg-Landau-Maki parameter 2 indicate that the superconductivity is most probably not Pauli limited in a large temperature range.
We present an X-ray photoemission study of the heavy fermion system CeCu6-xAux across the magneti... more We present an X-ray photoemission study of the heavy fermion system CeCu6-xAux across the magnetic quantum phase transition of this compound at temperatures above the single-ion Kondo temperature TK. In dependence of the Au concentration x we observe a sudden change of the f -occupation number n f and the core-hole potential U df at the critical concentration xc = 0.1. We interpret these findings in the framework of the single-impurity Anderson model. Our results are in excellent agreement with findings from earlier UPS measurements and provide further information about the precursors of quantum criticality at elevated temperatures.
In this paper, we report on measurements of the specific heat C of single-crystalline EuxSrl_xTe ... more In this paper, we report on measurements of the specific heat C of single-crystalline EuxSrl_xTe at temperatures between 60 mK and 15 K and in magnetic fields up to 6 T. Pure antiferromagnetic EuTe shows unusual critical behavior in the vicinity of the N6el temperature T N = 9.8 K with a positive critical exponent instead of the 3d-Heisenberg exponent c~---0.12. Possible reasons for this discrepancy between theory and experiment include magnetic anisotropy effects due to magnetic dipole-dipole interactions, which may give rise to a cross-over of the critical behavior very close to T N. This anisotropy is also seen in the specific heat below 1 K where an exponential decay of C is observed, and in the dependence of the magnetic susceptibility on the direction of the applied field. With increasing dilution of EuTe with nonmagnetic Sr, the critical behavior changes: ~ becomes negative and decreases continuously towards ~ ~-1 at x~x c. This concentration dependence of e was previously observed in the diluted ferromagnetic system EuxSr ~ _xS. Our data thus support that the apparent change in the critical behavior depends on the degree of disorder. Samples with concentration x lower than the critical concentration x c reveal spin-glass behavior in the specific heat. In addition, the dependence of T N on magnetic fields is discussed. The data yield a normalized magnetic phase boundary Bc(T)/Bc(T=O ) vs. TN(B)/TN(B=O ) which is independent of concentration.
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Papers by H. Löhneysen