arXiv subjects

F. Steglich

Publications and source records attributed to F. Steglich.

At least 91 records · Page 5Linked to original sources

arXiv2009

Quantum Griffiths phase in CePd(1-x)Rh(x) with x ~ 0.8

The magnetic field dependence of the magnetisation ($M$) and the temperature dependence of the ac susceptibility ($χ' = dM/dH$) of CePd(1-x)Rh(x) single crystals with $0.80 \leq x \leq 0.86$ are analysed within the frame of the quantum Griffiths phase scenario, which predicts $M \propto H^λ$ and $χ' \propto T^{λ-1}$ with $0 \leq λ\leq 1$. All $M$ vs $H$ and $χ'$ vs $T$ data follow the predicted power-law behaviour. The parameter $λ$, extracted from $χ'(T)$, is very sensitive to the Rh content $x$ and varies systematically with $x$ from -0.1 to 0.4. The value of $λ$, derived from $M(H)$ measurements on a \cpr single crystal, seems to be rather constant, $λ\approx 0.2$, in a broad range of temperatures between 0.05 and 2 K and fields up to about 10 T. All observed signatures and the $λ$ values are thus compatible with the quantum Griffiths scenario.

cond-mat.str-el↗

arXiv2009

Development of the critical exponent at the antiferromagnetic phase transition of YbRh2Si2 under chemical pressure

We present specific-heat measurements in the vicinity of the antiferromagnetic phase transition on single crystals of the alloy Yb(Rh_{1-x}Co_x)2Si2 for x<= 0.38. This study was motivated by the violation of critical universality in the undoped YbRh2Si2 (Krellner et al., Phys. Rev. Lett. 102, 196402) where we have found a large critical exponent a=0.38. For Co-doped samples we observe a drastic change of the critical fluctuations resulting in a negative a, explainable within the universality classes of phase transitions. The development of a under chemical pressure gives strong indication that the violation of critical universality in YbRh2Si2 is due to the nearby quantum critical point.

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arXiv2009

Quantum criticality in layered CeRhIn_{5-x}Sn_x compared with cubic CeIn$_{3-x}Sn_x

We report low-temperature thermal-expansion measurements on single crystals of the {\it layered} heavy fermion system \cri ($0.3\leq x \leq 0.6$) and compare it with a previous study on the related {\it cubic} system \ci [R. Küchler {\it et al.}, Phys. Rev. Lett. {\bf 96}, 256403 (2006)]. Both systems display a quantum critical point as proven by a divergent Grüneisen ratio. Most remarkably, the three-dimensional itinerant model explains quantum criticality in {\it both} systems, suggesting that the crystalline anisotropy in \cri is unimportant. This is ascribed to the effect of weak disorder in these doped systems.

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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.

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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.

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arXiv2009

Electrical resistivity ofYb(Rh1-xCox)2Si2 single crystals at low temperatures

We report low-temperature measurements of the electrical resistivity of Yb(Rh1-xCox)2Si2 single crystals with 0 <= x <= 0.12. The isoelectronic substitution of Co on the Rh site leads to a decrease of the unit cell volume which stabilizes the antiferromagnetism. Consequently, the antiferromagnetic transition temperature increases upon Co substitution. For x = 0.07 Co content a subsequent low-temperature transition is observed in agreement with susceptibility measurements and results on YbRh2Si2 under hydrostatic pressure. Above the Neel transition the resistivity follows a non-Fermi liquid behavior similar to that of YbRh2Si2.

cond-mat.str-el↗

arXiv2009

Magnetization study on the field-induced quantum critical point in YbRh_2Si_2

We study the field-induced quantum critical point (QCP) in YbRh$_2$Si$_2$ by low-temperature magnetization, $M(T)$, and magnetic Grüneisen ratio, $Γ_{\rm mag}$, measurements and compare the results with previous thermal expansion, $β(T)$, and critical Grüneisen ratio, $Γ^{cr}(T)$, data on YbRh$_2$(Si$_{0.95}$Ge$_{0.05}$)$_2$. In the latter case, a slightly negative chemical pressure has been used to tune the system towards its zero-field QCP. The magnetization derivative $-dM/dT$ is far more singular than thermal expansion, reflecting a strongly temperature dependent pressure derivative of the field at constant entropy, $(dH/dP)_S=V_mβ/(dM/dT)$ ($V_m$: molar volume), which saturates at $(0.15\pm 0.04)$ T/GPa for $T\to 0$. The line $T^\star(H)$, previously observed in Hall- and thermodynamic measurements, separates regimes in $T$-$H$ phase space of stronger $(ε>1$) and weaker $(ε<1$) divergent $Γ_{\rm mag}(T)\propto T^{-ε}$.

cond-mat.str-el↗

arXiv2009

Kondo-Cluster-Glass State near a Ferromagnetic Quantum Phase Transition

We report on a comprehensive study of CePd$_{1-x}$Rh$_x$ $(0.6 \leq x \leq 0.95)$ poly- and single crystals close to the ferromagnetic instability by means of low-temperature ac susceptibility, magnetization and volume thermal expansion. The signature of ferromagnetism in this heavy-fermion system can be traced from 6.6 K in CePd down to 25 mK for $x=0.87$. Despite pronounced non-Fermi-liquid (NFL) effects in both, specific heat and thermal expansion, the Grüneisen ratio {\it does not} diverge as $T\to 0$, providing evidence for the absence of a quantum critical point. Instead, a peculiar "Kondo-cluster-glass" state is found for $x\geq 0.65$, and the NFL effects in the specific heat, ac susceptibility and magnetization are compatible with the quantum Griffiths phase scenario.

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arXiv2009

Violation of critical universality at the antiferromagnetic phase transition of YbRh2Si2

We report on precise low-temperature specific-heat measurements, C(T), of YbRh2Si2 in the vicinity of the antiferromagnetic phase transition on a single crystal of superior quality (RRR 150). We observe a very sharp peak at T_N=72mK with absolute values as high as C/T=8J/molK^2. A detailed analysis of the critical exponent αaround T_N reveals α=0.38 which differs significantly from those of the conventional universality classes in the Ginzburg-Landau theory, where α<0.11. Thermal-expansion measurements corroborate this large positive critical exponent. These results provide insight into the nature of the critical magnetic fluctuations at a temperature-driven phase transition close to a quantum critical point.

cond-mat.str-el↗

arXiv2009

Extreme vortex pinning in the non-centrosymmetric superconductor CePt$_{3}$Si

We report on the vortex dynamics of a single crystal of the non-centrosymmetric heavy-fermion superconductor CePt$_{3}$Si. Decays of the remnant magnetization display a clean logarithmic time dependence with rates that follow the temperature dependence expected from the Kim-Anderson theory. The creep rates are lower than observed in any other centrosymmetric superconductor and are not caused by high critical currents. On the contrary, the critical current in CePt$_{3}$Si is considerably lower than in other superconductors with strong vortex pinning indicating that an alternative impediment on the flux line motion might be at work in this superconductor.

cond-mat.supr-con↗

arXiv2008

Divergence of the Magnetic Grüneisen Ratio at the Field-Induced Quantum Critical Point in YbRh$_2$Si$_2$

The heavy fermion compound YbRh$_2$Si$_2$ is studied by low-temperature magnetization $M(T)$ and specific-heat $C(T)$ measurements at magnetic fields close to the quantum critical point ($H_c=0.06$ T, $H\perp c$). Upon approaching the instability, $dM/dT$ is more singular than $C(T)$, leading to a divergence of the magnetic Grüneisen ratio $Γ_{\rm mag}=-(dM/dT)/C$. Within the Fermi liquid regime, $Γ_{\rm mag}=-G_r(H-H_c^{fit})$ with $G_r=-0.30\pm 0.01$ and $H_c^{fit}=(0.065\pm 0.005)$ T which is consistent with scaling behavior of the specific-heat coefficient in YbRh$_2$(Si$_{0.95}$Ge$_{0.05}$)$_2$. The field-dependence of $dM/dT$ indicates an inflection point of the entropy as a function of magnetic field upon passing the line $T^\star(H)$ previously observed in Hall- and thermodynamic measurements.

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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

Effect of pressure and Ir substitution in YbRh2Si2

In this article we present a study of the electrical resistivity of Yb(Rh-xIrx)2Si2, x=0.06, under high pressure and in magnetic field. Ir substitution is expanding the unit cell and leads to a suppression of the antiferromagnetic transition temperature to zero, where eventually a quantum critical point (QCP) exists. We applied hydrostatic pressure to reverse the effect of substitution. Our results indicate that Yb(Rh0.94Ir0.06)2Si2 is situated in the immediate proximity to a volume controlled QCP, but still on the magnetically ordered side of the phase diagram. The temperature - pressure phase diagram of Yb(Rh0.94Ir0.06)2Si2 resembles that of the pure compound. Substitution acts mainly as chemical pressure. Disorder introduced by substitution has only minor effects.

cond-mat.str-el↗

arXiv2008

Analysis of the normal state magnetotransport in CeIrIn${_5}$

We present an analysis of the normal state magnetotransport in the heavy fermion superconductor CeIrIn${_5}$. The Hall effect and the transverse magnetoresistance in this material do not appear to be uniquely correlated, as inferred from the field dependence of the current ratio ($R_σ = σ_{xy} / σ_{xx}^2 H$). The Hall coefficient is seen to satisfy a scaling equation of the form $R_H = f [H / (a + b T^c)]$. These results are compared to those observed earlier in CeCoIn$_5$, and are discussed in terms of the contrasting phase diagram which the CeIrIn$_5$ system exhibits in relation to its Co counterpart.

cond-mat.str-el↗

arXiv2008

Magnetotransport in the CeIrIn${_5}$ system: The influence of antiferromagnetic fluctuations

We present an overview of magnetotransport measurements on the heavy-fermion superconductor CeIrIn$_5$. Sensitive measurements of the Hall effect and magnetoresistance (MR) are used to elucidate the low temperature phase diagram of this system. The normal-state magnetotransport is highly anomalous, and experimental signatures of a pseudogap-like precursor state to superconductivity as well as evidence for two distinct scattering times governing the Hall effect and the MR are observed. Our observations point out the influence of antiferromagnetic fluctuations on the magnetotransport in this class of materials. The implications of these findings, both in the context of unconventional superconductivity in heavy-fermion systems as well as in relation to the high temperature superconducting cuprates are discussed.

cond-mat.str-el↗

arXiv2008

CeFePO: A Heavy Fermion Metal with Ferromagnetic Correlations

The ground state properties of CeFePO, a homologue of the new high temperature superconductors RFePnO(1-x)Fx, were studied by means of susceptibility, specific heat, resistivity, and NMR measurements on polycrystals. All the results demonstrate that this compound is a magnetically non-ordered heavy Fermion metal with a Kondo temperature TK~10K, a Sommerfeld coefficient gamma=700mJ/molK2 and a mass enhancement factor of the order of 200. The absence of a Fe-contribution to the effective moment at high temperatures indicates that the magnetism in CeFePO is completely dominated by the effect of Ce. Thus the strong electronic correlation effects originate from the Ce-4f electrons rather than from the Fe-3d electrons. An enhanced Sommerfeld-Wilson ratio R=5.5 as well as a Korringa product S0/T1TK2~0.065 well below 1 indicate the presence of ferromagnetic correlations. Therefore, CeFePO appears to be on the non-magnetic side of a ferromagnetic instability.

cond-mat.str-el↗

arXiv2008

The precursor state to superconductivity in CeIrIn${_5}$: Unusual scaling of magnetotransport

We present an analysis of the normal-state Hall effect and magnetoresistance in the heavy fermion superconductor CeIrIn${_5}$. It is demonstrated that the modified Kohler's scaling--which relates the magnetoresistance to the Hall angle--breaks down prior to the onset of superconductivity due to the presence of a precursor state to superconductivity in this system. A model-independent, single-parameter scaling of the Hall angle governed solely by this precursor state is observed. Neither the Hall coefficient nor the resistivity exhibit this scaling implying that this precursor state preferentially influences the Hall channel.

cond-mat.str-el↗

arXiv2008

Vortex avalanches in the non-centrosymmetric superconductor Li2Pt3B

We investigated the vortex dynamics in the non-centrosymmetric superconductor Li_2Pt_3B in the temperature range 0.1 K - 2.8 K. Two different logarithmic creep regimes in the decay of the remanent magnetization from the Bean critical state have been observed. In the first regime, the creep rate is extraordinarily small, indicating the existence of a new, very effective pinning mechanism. At a certain time a vortex avalanche occurs that increases the logarithmic creep rate by a factor of about 5 to 10 depending on the temperature. This may indicate that certain barriers against flux motion are present and they can be opened under increased pressure exerted by the vortices. A possible mechanism based on the barrier effect of twin boundaries is briefly discussed.

cond-mat.supr-con↗