Magnetic-Field Induced Quantum Critical Point in YbRh2_2Si2_2

We report low-temperature calorimetric, magnetic and resistivity measurements on the antiferromagnetic (AF) heavy-fermion metal YbRh$_2$Si$_2$ (${T_N =}$ 70 mK) as a function of magnetic field $B$. While for fields exceeding the critical value ${B_{c0}}$ at which ${T_N\to0}$ the low temperature resistivity shows an ${AT^2}$ dependence, a ${1/(B-B_{c0})}$ divergence of ${A(B)}$ upon reducing $B$ to ${B_{c0}}$ suggests singular scattering at the whole Fermi surface and a divergence of the heavy quasiparticle mass. The observations are interpreted in terms of a new type of quantum critical point separating a weakly AF ordered from a weakly polarized heavy Landau-Fermi liquid state.Comment: accepted for publication in Phys. Rev. Let

Spin dynamics in a structurally ordered non-Fermi liquid compound: YbRh_2Si_2

Muon spin relaxation (muSR) experiments have been carried out at low temperatures in the non-Fermi-liquid heavy-fermion compound YbRh_2Si_2. The longitudinal-field muSR relaxation function is exponential, indicative that the dynamic spin fluctuations are homogeneous. The relaxation rate 1/T_1 varies with applied field as H^{-y}, y = 1.0 \pm 0.1, which implies a scaling law of the form \chi''(\omega) \propto \omega^{-y} f(\omega/T), \lim_{x\to0} f(x) = x for the dynamic spin susceptibility.Comment: 5 pages, 2 figures. To be published in proceedings of musr2002 (Physica B

Coexistence of Superconductivity and Antiferromagnetism in Heavy-Fermion Superconductor CeCu_{2}(Si_{1-x}Ge_{x})_{2} Probed by Cu-NQR --A Test Case for the SO(5) Theory--

We report on the basis of Cu-NQR measurements that superconductivity (SC) and antiferromagnetism (AF) coexist on a microscopic level in CeCu_{2}(Si_{1-x}Ge_{x})_{2}, once a tiny amount of 1%Ge (x = 0.01) is substituted for Si. This coexistence arises because Ge substitution expands the unit-cell volume in nearly homogeneous CeCu2Si2 where the SC coexists with slowly fluctuating magnetic waves. We propose that the underlying exotic phases of SC and AF in either nearly homogeneous or slightly Ge substituted CeCu2Si2 are accountable based on the SO(5) theory that unifies the SC and AF. We suggest that the mechanism of the SC and AF is common in CeCu2Si2.Comment: 7 pages with 6 figures embedded in the text. To be published in J. Phys. Condens. Matter, 200

The break up of heavy electrons at a quantum critical point

The point at absolute zero where matter becomes unstable to new forms of order is called a quantum critical point (QCP). The quantum fluctuations between order and disorder that develop at this point induce profound transformations in the finite temperature electronic properties of the material. Magnetic fields are ideal for tuning a material as close as possible to a QCP, where the most intense effects of criticality can be studied. A previous study on theheavy-electron material $YbRh_2Si_2$ found that near a field-induced quantum critical point electrons move ever more slowly and scatter off one-another with ever increasing probability, as indicated by a divergence to infinity of the electron effective mass and cross-section. These studies could not shed light on whether these properties were an artifact of the applied field, or a more general feature of field-free QCPs. Here we report that when Germanium-doped $YbRh_2Si_2$ is tuned away from a chemically induced quantum critical point by magnetic fields there is a universal behavior in the temperature dependence of the specific heat and resistivity: the characteristic kinetic energy of electrons is directly proportional to the strength of the applied field. We infer that all ballistic motion of electrons vanishes at a QCP, forming a new class of conductor in which individual electrons decay into collective current carrying motions of the electron fluid.Comment: Pdf files of article available at http://www.physics.rutgers.edu/~coleman/online/breakup.pdf, pdf file of news and views article available at http://www.physics.rutgers.edu/~coleman/online/nvbreakup.pd

Hall-effect evolution across a heavy-fermion quantum critical point

A quantum critical point (QCP) develops in a material at absolute zero when a new form of order smoothly emerges in its ground state. QCPs are of great current interest because of their singular ability to influence the finite temperature properties of materials. Recently, heavy-fermion metals have played a key role in the study of antiferromagnetic QCPs. To accommodate the heavy electrons, the Fermi surface of the heavy-fermion paramagnet is larger than that of an antiferromagnet. An important unsolved question concerns whether the Fermi surface transformation at the QCP develops gradually, as expected if the magnetism is of spin density wave (SDW) type, or suddenly as expected if the heavy electrons are abruptly localized by magnetism. Here we report measurements of the low-temperature Hall coefficient ($R_H$) - a measure of the Fermi surface volume - in the heavy-fermion metal YbRh2Si2 upon field-tuning it from an antiferromagnetic to a paramagnetic state. $R_H$ undergoes an increasingly rapid change near the QCP as the temperature is lowered, extrapolating to a sudden jump in the zero temperature limit. We interpret these results in terms of a collapse of the large Fermi surface and of the heavy-fermion state itself precisely at the QCP.Comment: 20 pages, 3 figures; to appear in Natur

Thermoelectric Behaviour Near Magnetic Quantum Critical Point

We use the coupled 2d-spin-3d-fermion model proposed by Rosch {\sl et. al.} (Phys. Rev. Lett. {\bf 79}, 159 (1997)) to study the thermoelectric behaviour of a heavy fermion compound when it is close to an antiferromagnetic quantum critical point. When the low energy spin fluctuations are quasi two dimensional, as has been observed in ${\rm YbRh}_2{\rm Si}_2$ and ${\rm CeCu}_{6-x}{\rm Au}_x$, with a typical 2d ordering wavevector and 3d Fermi surface, the ``hot'' regions on the Fermi surface have a finite area. Due to enhanced scattering with the nearly critical spin fluctuations, the electrons in the hot region are strongly renormalized. We argue that there is an intermediate energy scale where the qualitative aspects of the renormalized hot electrons are captured by a weak-coupling perturbative calculation. Our examination of the electron self energy shows that the entropy carried by the hot electrons is larger than usual. This accounts for the anomalous logarithmic temperature dependence of specific heat observed in these materials. We show that the same mechanism produces logarithmic temperature dependence in thermopower. This has been observed in ${\rm CeCu}_{6-x}{\rm Au}_x$. We expect to see the same behaviour from future experiments on ${\rm YbRh}_2{\rm Si}_2$.Comment: RevTex, two-column, 7 pages, 2 figure

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.Comment: 5pages, 1 Figur

Local quantum critical point and non-Fermi liquid properties

Quantum criticality provides a means to understand the apparent non-Fermi liquid phenomena in correlated electron systems. How to properly describe quantum critical points in electronic systems has however been poorly understood. The issues have become particularly well-defined due to recent experiments in heavy fermion metals, in which quantum critical points have been explicitly identified. In this paper, I summarize some recent theoretical work on the subject, with an emphasis on the notion of ``local quantum criticality''. I describe the microscopic work based on an extended dynamical mean field theory, as well as Ginzburg-Landau arguments for the robustness of the local quantum critical point beyond the microscopics. I also present the consequences of this picture on the inelastic neutron scattering, NMR, Fermi surface properties and Hall coefficient, and compare them with the available experiments. Some analogies with the Mott transition phenomena are also noted.Comment: 7 pages, 1 figure; references updated according to the published versio

Temperature dependence of the Kondo resonance and its satellites in CeCu_2Si_2

We present high-resolution photoemission spectroscopy studies on the Kondo resonance of the strongly-correlated Ce system CeCu$_2$Si$_2$. Exploiting the thermal broadening of the Fermi edge we analyze position, spectral weight, and temperature dependence of the low-energy 4f spectral features, whose major weight lies above the Fermi level $E_F$. We also present theoretical predictions based on the single-impurity Anderson model using an extended non-crossing approximation (NCA), including all spin-orbit and crystal field splittings of the 4f states. The excellent agreement between theory and experiment provides strong evidence that the spectral properties of CeCu$_2$Si$_2$ can be described by single-impurity Kondo physics down to $T \approx 5$ K.Comment: 4 pages, 3 figure