arXiv subjects

J. Sichelschmidt

Publications and source records attributed to J. Sichelschmidt.

At least 55 records · Page 3Linked to original sources

arXiv2012

Spin dynamics of S-state ions in the filled skutterudites La1-xRx$Pt4Ge12 (R= Gd, Eu)

A detailed study of the spin dynamics of the S-state ions Gd3+ and Eu2+ in the filled skutterudites La1-xRxPt4Ge12 (R= Gd, Eu) is reported. The spin dynamics is investigated directly by means of Gd3+ and Eu2+ electron spin resonance (ESR), performed at X-band (~ 9.4 GHz) and Q-band (~ 34 GHz) frequencies in the temperature intervals 8<T<300 K and 1.5<T<300 K respectively. The ESR parameters provide direct evidence for the vibrational behavior of the Gd3+ ions but not for the Eu2+ ions. These results are interpreted in the light of recent discussions about the spin relaxation in cage systems. In particular, the Gd3+ spin relaxation in La0.9Gd0.1Pt4Ge12 provide evidence for the existence of an extra phonon mode with an Einstein temperature (thetaE) ~ 24 K in this system. The work suggests that the so-called "rattling" modes have a general and important role that should be taken into account in the study of spin dynamics in cage systems.

cond-mat.str-el↗

arXiv2012

Electron Spin Resonance of the Yb 4f moment in Yb(Rh1-xCox)2Si2

[published in Phys. Rev. B 85, 035119 (2012)] The evolution of spin dynamics from the quantum critical system YbRh2Si2 to the stable trivalent Yb system YbCo2Si2 was investigated by Electron Spin Resonance (ESR) spectroscopy. While the Kondo temperature changes by one order of magnitude, all compositions of the single crystalline series Yb(Rh1-xCox)2Si2 show well defined ESR spectra with a clear Yb3+ character for temperatures below \approx 20 K. With increasing Co-content the ESR g-factor along the c-direction strongly increases indicating a continuous change of the ground state wave function and, thus, a continuous change of the crystal electric field. The linewidth presents a complex dependence on the Co-content and is discussed in terms of the Co-doping dependence of the Kondo interaction, the magnetic anisotropy and the influence of ferromagnetic correlations between the 4f states. The results provide evidence that, for low Co-doping, the Kondo interaction allows narrow ESR spectra despite the presence of a large magnetic anisotropy, whereas at high Co-concentrations, the linewidth is controlled by ferromagnetic correlations. A pronounced broadening due to critical correlations at low temperatures is only observed at the highest Co-content. This might be related to the presence of incommensurate magnetic fluctuations.

cond-mat.str-el↗

arXiv2011

Electron spin resonance study of anisotropic interactions in a two-dimensional spin gap magnet PHCC

Fine details of the excitation spectrum of the two-dimensional spin-gap magnet PHCC are revealed by electron spin resonance investigations. The values of anisotropy parameters and the orientations of the anisotropy axes are determined by accurate measurements of the angular, frequency-field and temperature dependences of the resonance absorption. The properties of a spin-gap magnet in the vicinity of critical field are discussed in terms of sublevel splittings and g-factor anisotropy.

cond-mat.str-el↗

arXiv2011

Peculiar long-range superexchange in Cu2A2O7 (A = P, As, V) as a key element of the microscopic magnetic model

A microscopic magnetic model for alpha-Cu2P2O7 is evaluated in a combined theoretical and experimental study. Despite a dominant intradimer coupling J1, sizable interdimer couplings enforce long-range magnetic ordering at T_N=27 K. The spin model for alpha-Cu2P2O7 is compared to the models of the isostructural beta-Cu2V2O7 and alpha-Cu2As2O7 systems. As a surprise, coupled dimers in alpha-Cu2P2O7 and alternating chains in alpha-Cu2As2O7 contrast with a honeycomb lattice in beta-Cu2V2O7. We find that the qualitative difference in the coupling regime of these isostructural compounds is governed by the nature of AO4 side groups: d-elements (A = V) hybridize with nearby O atoms forming a Cu-O-A-O-Cu superexchange path, while for p-elements (A = P, As) the superexchange is realized via O-O edges of the tetrahedron. Implications for a broad range of systems are discussed.

cond-mat.str-el↗

arXiv2010

Strong electron correlations in FeSb2: An optical investigation and comparison with RuSb2

We report investigations of the optical properties of the narrow gap semiconductor FeSb2 in comparison with the structural homolog RuSb2. In the infrared region the latter shows insulating behavior in whole investigated temperature range (10 - 300 K) whereas the optical reflectivity of FeSb2 shows typical semiconductor behavior upon decreasing the temperature. The conduction electron contribution to the reflectivity is suppressed and the opening of a direct and an indirect charge excitation gap in the far-infrared energy region is observed. Those gap openings are characterized by a redistribution of spectral weight of the optical conductivity in an energy region much larger than the gap energies indicating that strong electron-electron correlations are involved in the formation of the charge gap. Calculations of the optical conductivity from the band structure also provided evidence for the presence of strong electronic correlations. Analyzing the spectra with a fundamental absorption across the gap of parabolic bands yields a direct gap at 130 meV and two indirect gaps at 6 and 31 meV. The strong reduction in the free-carrier concentration at low energies and low temperatures is also reflected in a change in the asymmetry of the phonon absorption which indicates a change in the phonon-conduction-electron interaction.

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arXiv2010

Temperature- and Magnetic-Field-Dependent Optical Properties of Heavy Quasiparticles in YbIr2Si2

We report the temperature- and magnetic-field-dependent optical conductivity spectra of the heavy electron metal YbIr$_2$Si$_2$. Upon cooling below the Kondo temperature ($T_{\rm K}$), we observed a typical charge dynamics that is expected for a formation of a coherent heavy quasiparticle state. We obtained a good fitting of the Drude weight of the heavy quasiparticles by applying a modified Drude formula with a photon energy dependence of the quasiparticle scattering rate that shows a similar power-law behavior as the temperature dependence of the electrical resistivity. By applying a magnetic field of 6T below $T_{\rm K}$, we found a weakening of the effective dynamical mass enhancement by about 12% in agreement with the expected decrease of the $4f$-conduction electron hybridization on magnetic field.

cond-mat.str-el↗

arXiv2010

Electron Spin Resonance of the ferromagnetic Kondo lattice CeRuPO

The spin dynamics of the ferromagnetic Kondo lattice CeRuPO is investigated by Electron Spin Resonance (ESR) at microwave frequencies of 1, 9.4, and 34~GHz. The measured resonance can be ascribed to a rarely observed bulk Ce3+ resonance in a metallic Ce compound and can be followed below the ferromagnetic transition temperature Tc=14 K. At T>Tc the interplay between the RKKY-exchange interaction and the crystal electric field anisotropy determines the ESR parameters. Near Tc the spin relaxation rate is influenced by the critical fluctuations of the order parameter.

cond-mat.str-el↗

arXiv2010

Effect of pressure on the Electron Spin Resonance of a Heavy-Fermion metal

We investigate the electron-spin resonance (ESR) phenomenon in the heavy-fermion metal YbRh2Si2 by applying hydrostatic pressure up to 3 GPa and by inducing internal pressure on the Yb site chemically in Yb(Rh1-xCox)2Si2 samples. We found that the increase in pressure, reducing the hybridization between 4f and conduction electrons, leads to a remarkable change in the temperature dependence of the g factor and broadens the ESR line. We relate the differences between the effect of internal and external pressure on the low temperature ESR parameters to the disorder induced by Co doping. The pressure effects on the Yb3+ related resonance in YbRh2Si2 again manifest its local character when compared with the resonance of diluted Gd in heavy-fermion metals.

cond-mat.str-el↗

arXiv2010

Anisotropic electron spin resonance of YbIr2Si2

A series of electron spin resonance (ESR) experiments were performed on a single crystal of the heavy fermion metal YbIr2Si2 to map out the anisotropy of the ESR-intensity I_ESR which is governed by the microwave field component of the g-factor. The temperature dependencies of I_ESR(T) and g(T) were measured for different orientations and compared within the range 2.6K \le T \le 16K. The analysis of the intensity dependence on the crystal orientation with respect to both the direction of the microwave field and the static magnetic field revealed remarkable features: The intensity variation with respect to the direction of the microwave field was found to be one order of magnitude smaller than expected from the g-factor anisotropy. Furthermore, we observed a weak basal plane anisotropy of the ESR parameters which we interpret to be an intrinsic sample property.

cond-mat.str-el↗

arXiv2009

Low temperature properties of the Electron Spin Resonance in YbRh2Si2

We present the field and temperature behavior of the narrow Electron Spin Resonance (ESR) response in YbRh2Si2 well below the single ion Kondo temperature. The ESR g factor reflects a Kondo-like field and temperature evolution of the Yb3+ magnetism. Measurements towards low temperatures (>0.5K) have shown distinct crossover anomalies of the ESR parameters upon approaching the regime of a well defined heavy Fermi liquid. Comparison with the field dependence of specific heat and electrical resistivity reveal that the ESR parameters can be related to quasiparticle mass and cross section and, hence, contain inherent heavy electron properties.

cond-mat.str-el↗

arXiv2009

Why could Electron Spin Resonance be observed in a heavy fermion Kondo lattice?

We develop a theoretical basis for understanding the spin relaxation processes in Kondo lattice systems with heavy fermions as experimentally observed by electron spin resonance (ESR). The Kondo effect leads to a common energy scale that regulates a logarithmic divergence of different spin kinetic coefficients and supports a collective spin motion of the Kondo ions with conduction electrons. We find that the relaxation rate of a collective spin mode is greatly reduced due to a mutual cancelation of all the divergent contributions even in the case of the strongly anisotropic Kondo interaction. The contribution to the ESR linewidth caused by the local magnetic field distribution is subject to motional narrowing supported by ferromagnetic correlations. The developed theoretical model successfully explains the ESR data of YbRh2Si2 in terms of their dependence on temperature and magnetic field.

cond-mat.str-el↗

arXiv2008

Evolution of the Kondo state of YbRh2Si2 probed by high field ESR

An electron spin resonance (ESR) study of the heavy fermion compound YbRh2Si2 for fields up to ~ 8 T reveals a strongly anisotropic signal below the single ion Kondo temperature T_K ~ 25 K. A remarkable similarity between the T-dependence of the ESR parameters and that of the specific heat and the 29Si nuclear magnetic resonance data gives evidence that the ESR response is given by heavy fermions which are formed below T_K and that ESR properties are determined by their field dependent mass and lifetime. The signal anisotropy, otherwise typical for Yb{3+} ions, suggests that, owing to a strong hybridization with conduction electrons at T < T_K, the magnetic anisotropy of the 4f states is absorbed in the ESR of heavy quasiparticles. Tuning the Kondo effect on the 4f states with magnetic fields ~ 2 - 8 T and temperature 2 - 25 K yields a gradual change of the ESR g-factor and linewidth which reflects the evolution of the Kondo state in this Kondo lattice system.

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arXiv2008

Magnetic susceptibility of YbRh2Si2 and YbIr2Si2 on the basis of a localized 4f electron approach

We consider the local properties of the Yb3+ ion in the crystal electric field in the Kondo lattice compounds YbRh2Si2 and YbIr2Si2. On this basis we have calculated the magnetic susceptibility taking into account the Kondo interaction in the simplest molecular field approximation. The resulting Curie-Weiss law and Van Vleck susceptibilities could be excellently fitted to experimental results in a wide temperature interval where thermodynamic and transport properties show non-Fermi-liquid behaviour for these materials.

cond-mat.str-el↗

arXiv2007

ESR of YbRh2Si2 and 174YbRh2Si2 : local and itinerant properties

Below the Kondo temperature the heavy Fermion compound YbRh$_{2}$Si$_{2}$ shows a well defined Electron Spin Resonance (ESR) with local Yb$^{3+}$ properties. We report a detailed analysis of the ESR intensity which gives information on the number of ESR active centers relative to the ESR of well localized Yb$^{3+}$ in YPd$_3$:Yb. The ESR lineshape is investigated regarding contributions from itinerant centers. From the ESR of monoisotopic $^{174}$YbRh$_{2}$Si$_{2}$ we could exclude unresolved hyperfine contributions to the lineshape.

cond-mat.str-el↗

arXiv2006

Spin dynamics of $Yb Rh_2 Si_2$ observed by Electron Spin Resonance

Below the Kondo temperature $T_{\rm K}$ electron spin resonance (ESR) usually is not observable from the Kondo-ion itself because the characteristic spin fluctuation energy results in a huge width of the ESR line. The heavy fermion metal YbRh$_{2}$Si$_{2}$ seems to be an exceptional case where definite ESR spectra show characteristic properties of the Kondo-ion Yb$^{3+}$ well \textit{below} $T_{\rm K}$. We found that the spin dynamics of YbRh$_{2}$Si$_{2}$, as determined by its ESR relaxation, is spatially characterized by an anisotropy of the zero temperature residual relaxation only.

cond-mat.str-el↗

arXiv2006

Observation of an optical non-Fermi-liquid behavior in the heavy fermion state of YbRh$_{2}$Si$_{2}$

We report far-infrared optical properties of YbRh$_{2}$Si$_{2}$ for photon energies down to 2 meV and temperatures 0.4 -- 300 K. In the coherent heavy quasiparticle state, a linear dependence of the low-energy scattering rate on both temperature and photon energy was found. We relate this distinct dynamical behavior different from that of Fermi liquid materials to the non-Fermi liquid nature of YbRh$_{2}$Si$_{2}$ which is due to its close vicinity to an antiferromagnetic quantum critical point.

cond-mat.str-el↗

arXiv2002

Sr2V3O9 and Ba2V3O9: quasi one-dimensional spin-systems with an anomalous low temperature susceptibility

The magnetic behaviour of the low-dimensional Vanadium-oxides Sr2V3O9 and Ba2V3O9 was investigated by means of magnetic susceptibility and specific heat measurements. In both compounds, the results can be very well described by an S=1/2 Heisenberg antiferromagnetic chain with an intrachain exchange of J = 82 K and J = 94 K in Sr2V3O9 and Ba2V3O9, respectively. In Sr2V3O9, antiferromagnetic ordering at T_N = 5.3 K indicate a weak interchain exchange of the order of J_perp ~ 2 K. In contrast, no evidence for magnetic order was found in Ba2V3O9 down to 0.5 K, pointing to an even smaller interchain coupling. In both compounds, we observe a pronounced Curie-like increase of the susceptibility below 30 K, which we tentatively attribute to a staggered field effect induced by the applied magnetic field. Results of LDA calculations support the quasi one-dimensional character and indicate that in Sr2V3O9, the magnetic chain is perpendicular to the structural one with the magnetic exchange being transferred through VO4 tetrahedra.

cond-mat↗

arXiv2000

Quasi-one-dimensional spin chains in CuSiO3: an EPR study

Temperature dependent EPR studies were performed on a single crystal of CuSiO3.This recently discovered compound is isostructural with the spin-Peierls compound CuGeO3. The EPR signals show different characteristics than those of CuGeO3 and aredue to Cu2+ spins located along quasi one-dimensional chains. For T > 8.2K the spin susceptibility closely follows the predictions of a S=1/2 one-dimensional Heisenberg antiferromagnet with J/kB = 21K. Below T = 8.2K the spin susceptibility immediately drops to zero indicating long range magnetic order.

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