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Lijun Zhu

Publications and source records attributed to Lijun Zhu.

116 records · Page 7Linked to original sources
arXiv2006

Helicity Order: Hidden Order Parameter in URu$_2$Si$_2$

We propose that the "hidden order parameter" in URu$_2$Si$_2$ is a helicity order which must arise, if the Pomeranchuk criteria for the spin-antisymmetric Landau parameters with respect to the stability of a Fermi liquid state are violated. In a simple model, we calculate the specific heat, linear and nonlinear magnetic susceptibilities and the change of transition temperature in a magnetic field with such an order parameter, and obtain quantitative agreement with experiments in terms of two parameters extracted from the data. The peculiar temperature dependence of the NMR linewidth and the nature of the loss of excitations in the ordered phase seen by neutron scattering are also explained and experiments are suggested to directly confirm the proposed order parameter.

cond-mat.str-el↗
arXiv2005

Quantum Criticality in Ferromagnetic Single-Electron Transistors

Considerable evidence exists for the failure of the traditional theory of quantum critical points (QCPs), pointing to the need to incorporate novel excitations. The destruction of Kondo entanglement and the concomitant critical Kondo effect may underlie these emergent excitations in heavy fermion metals -- a prototype system for quantum criticality -- but the effect remains poorly understood. Here, we show how ferromagnetic single-electron transistors can be used to study this effect. We theoretically demonstrate a gate-voltage induced quantum phase transition. The critical Kondo effect is manifested in a fractional-power-law dependence of the conductance on temperature ($T$). The AC conductance and thermal noise spectrum have related power-law dependences on frequency ($ω$) and, in addition, show an $ω/T$ scaling. Our results imply that the ferromagnetic nanostructure constitutes a realistic model system to elucidate magnetic quantum criticality that is central to the heavy fermions and other bulk materials with non-Fermi liquid behavior.

cond-mat.str-el↗
arXiv2005

Quantum critical properties of the Bose-Fermi Kondo Model in a large-N limit

Studies of non-Fermi liquid properties in heavy fermions have led to the current interest in the Bose-Fermi Kondo model. Here we use a dynamical large-N approach to analyze an SU(N)xSU($κN$) generalization of the model. We establish the existence in this limit of an unstable fixed point when the bosonic bath has a sub-ohmic spectrum ($|ω|^{1-ε} \sgn ω$, with $0<ε<1$). At the quantum critical point, the Kondo scale vanishes and the local spin susceptibility (which is finite on the Kondo side for κ<1) diverges. We also find an ω/T scaling for an extended range (15 decades) of ω/T. This scaling violates (for $ε\ge 1/2$) the expectation of a naive mapping to certain classical models in an extra dimension; it reflects the inherent quantum nature of the critical point.

cond-mat.str-el↗
arXiv2004

Destruction of the Kondo effect in a multi-channel Bose-Fermi Kondo model

We consider the SU(N) x SU(kappa N) generalization of the spin-isotropic Bose-Fermi Kondo model in the limit of large N. There are three fixed points corresponding to a multi-channel non-Fermi liquid phase, a local spin-liquid phase, and a Kondo-destroying quantum critical point (QCP). We show that the QCP has strong similarities with its counterpart in the single-channel model, even though the Kondo phase is very different from the latter. We also discuss the evolution of the dynamical scaling properties away from the QCP.

cond-mat.str-el↗
arXiv2003

Universally diverging Grueneisen parameter and the magnetocaloric effect close to quantum critical points

At a generic quantum critical point, the thermal expansion $α$ is more singular than the specific heat $c_p$. Consequently, the "Grüneisen ratio'', $\GE=α/c_p$, diverges. When scaling applies, $\GE \sim T^{-1/(νz)}$ at the critical pressure $p=p_c$, providing a means to measure the scaling dimension of the most relevant operator that pressure couples to; in the alternative limit $T\to0$ and $p \ne p_c$, $\GE \sim \frac{1}{p-p_c}$ with a prefactor that is, up to the molar volume, a simple {\it universal} combination of critical exponents. For a magnetic-field driven transition, similar relations hold for the magnetocaloric effect $(1/T)\partial T/\partial H|_S$. Finally, we determine the corrections to scaling in a class of metallic quantum critical points.

cond-mat.str-el↗
arXiv2003

Divergence of the Grueneisen Ratio at Quantum Critical Points in Heavy Fermion Metals

We present low-temperature volume thermal expansion, $β$, and specific heat, $C$, measurements on high-quality single crystals of CeNi2Ge2 and YbRh2(Si$_{0.95}$Ge$_{0.05}$)$_2$ which are located very near to quantum critical points. For both systems, $β$ shows a more singular temperature dependence than $C$, and thus the Grueneisen ratio ${Γ\propto β/C}$ diverges as T --> 0. For CeNi2Ge2, our results are in accordance with the spin-density wave (SDW) scenario for three-dimensional critical spin-fluctuations. By contrast, the observed singularity in YbRh2$(Si$_{0.95}$Ge$_{0.05}$)$_2$ cannot be explained by the itinerant SDW theory but is qualitatively consistent with a locally quantum critical picture.

cond-mat.str-el↗
arXiv2002

Critical local moment fluctuations in the Bose-Fermi Kondo model

We consider the critical properties of the Bose-Fermi Kondo model, which describes a local moment simultaneously coupled to a conduction electron band and a fluctuating magnetic field, i.e., a dissipative bath of vector bosons. We carry out an $ε-$expansion to higher than linear orders. (Here $ε$ is defined in terms of the power-law exponent of the bosonic-bath spectral function.) An unstable fixed point is identified not only in the spin-isotropic case but also in the presence of anisotropy. It marks the point where the weight of the Kondo resonance has just gone to zero, and the local moment fluctuations are critical. The exponent for the local spin susceptibility at this critical point is found to be equal to $ε$ in all cases. Our results imply that a quantum phase transition of the ``locally critical'' type is a robust microscopic solution to Kondo lattices.

cond-mat.str-el↗