arXiv subjects

Qimiao Si

Publications and source records attributed to Qimiao Si.

At least 253 records · Page 14Linked to original sources

arXiv2004

Orthogonality Catastrophe in Bose-Einstein Condensates

Orthogonality catastrophe in fermionic systems is well known: in the thermodynamic limit, the overlap between the ground state wavefunctions with and without a single local scattering potential approaches zero algebraically as a function of the particle number $N$. Here we examine the analogous problem for bosonic systems. In the homogeneous case, we find that ideal bosons display an orthogonality stronger than algebraic: the wavefunction overlap behaves as ${\rm exp}[-λN^{1/3}]$ in three dimensions and as ${\rm exp}[-λN/\ln ^2 N]$ in two dimensions. With interactions, the overlap becomes finite but is still (stretched-)exponentially small for weak interactions. We also consider the cases with a harmonic trap, reaching similar (though not identical) conclusions. Finally, we comment on the implications of our results for spectroscopic experiments and for (de)coherence phenomena.

cond-mat.stat-mech↗

arXiv2004

Quantum Phase Transitions and Correlated Electrons

This article is aimed at a pedagogical introduction to the physics of quantum phase transitions that is unique to metallic systems. It has been recognized for some time that quantum criticality can result in a breakdown of Landau's Fermi liquid theory. Its converse, however, has not been appreciated until very recently: non-Fermi liquid behavior can in turn lead to new classes of quantum phase transition. A concrete example is provided by ``local quantum critical points''. I summarize the theoretical reasoning and experimental evidence for local quantum criticality, in the context of heavy fermion metals. The underlying physics is likely to be relevant to other correlated electron systems including doped Mott insulators.

cond-mat.str-el↗

arXiv2004

Quantum Critical Metals: beyond the Order Parameter Fluctuations

The standard description of quantum critical points takes into account only fluctuations of the order parameter, and treats quantum fluctuations as extra dimensions of classical fluctuations. This picture can break down in a qualitative fashion in quantum critical metals: non-Fermi liquid electronic excitations are formed precisely at the quantum critical point and appear as a part of the quantum-critical spectrum. In the case of heavy fermion metals, it has been proposed that the non-Fermi liquid behavior is characterized by the destruction of the Kondo effect. The latter invalidates Hertz's Gaussian theory of paramagnons and leads to an interacting theory that is ``locally quantum critical''. We summarize the theoretical and experimental developments on the subject. We also discuss their broader implications, and make contact with recent work on quantum critical magnets.

cond-mat.str-el↗

arXiv2003

Effects of a Collective Spin Resonance Mode on the STM Spectra of D-Wave Superconductors

A high-energy spin resonance mode is known to exist in many high-temperature superconductors. Motivated by recent scanning tunneling microscopy (STM) experiments in superconducting Bi$_2$Sr$_2$CaCu$_2$O$_{8+δ}$, we study the effects of this resonance mode on the local density of states (LDOS). The coupling between the electrons in a d-wave superconductor and the resonance mode produces high-energy peaks in the LDOS, which displays a two-unit-cell periodic modulation around a nonmagnetic impurity. This suggests a new means to not only detect the dynamical spin collective mode but also study its coupling to electronic excitations.

cond-mat.supr-con↗

arXiv2003

Transitions from small to large Fermi momenta in a one-dimensional Kondo lattice model

We study a one-dimensional system that consists of an electron gas coupled to a spin-1/2 chain by Kondo interaction away from half-filling. We show that zero-temperature transitions between phases with "small" and "large" Fermi momenta can be continuous. Such a continuous but Fermi-momentum-changing transition arises in the presence of spin anisotropy, from a Luttinger liquid with a small Fermi momentum to a Kondo-dimer phase with a large Fermi momentum. We have also added a frustrating next-nearest-neighbor interaction in the spin chain to show the possibility of a similar Fermi-momentum-changing transition, between the Kondo phase and a spin-Peierls phase, in the spin isotropic case. This transition, however, appears to involve a region in which the two phases coexist.

cond-mat.str-el↗

arXiv2003

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.

cond-mat.str-el↗

arXiv2003

Local fluctuations in quantum critical metals

We show that spatially local, yet low-energy, fluctuations can play an essential role in the physics of strongly correlated electron systems tuned to a quantum critical point. A detailed microscopic analysis of the Kondo lattice model is carried out within an extended dynamical mean-field approach. The correlation functions for the lattice model are calculated through a self-consistent Bose-Fermi Kondo problem, in which a local moment is coupled both to a fermionic bath and to a bosonic bath (a fluctuating magnetic field). A renormalization-group treatment of this impurity problem--perturbative in $ε=1-γ$, where $γ$ is an exponent characterizing the spectrum of the bosonic bath--shows that competition between the two couplings can drive the local-moment fluctuations critical. As a result, two distinct types of quantum critical point emerge in the Kondo lattice, one being of the usual spin-density-wave type, the other ``locally critical.'' Near the locally critical point, the dynamical spin susceptibility exhibits $ω/T$ scaling with a fractional exponent. While the spin-density-wave critical point is Gaussian, the locally critical point is an interacting fixed point at which long-wavelength and spatially local critical modes coexist. A Ginzburg-Landau description for the locally critical point is discussed. It is argued that these results are robust, that local criticality provides a natural description of the quantum critical behavior seen in a number of heavy-fermion metals, and that this picture may also be relevant to other strongly correlated metals.

cond-mat.str-el↗

arXiv2003

Continuous quantum phase transition in a Kondo lattice model

We study the magnetic quantum phase transition in an anisotropic Kondo lattice model. The dynamical competition between the RKKY and Kondo interactions is treated using an extended dynamic mean field theory (EDMFT) appropriate for both the antiferromagnetic and paramagnetic phases. A quantum Monte Carlo approach is used, which is able to reach very low temperatures, of the order of 1% of the bare Kondo scale. We find that the finite-temperature magnetic transition, which occurs for sufficiently large RKKY interactions, is first order. The extrapolated zero-temperature magnetic transition, on the other hand, is continuous and locally critical.

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↗

arXiv2003

Locally critical point in an anisotropic Kondo lattice

We report the first numerical identification of a locally quantum critical point, at which the criticality of the local Kondo physics is embedded in that associated with a magnetic ordering. We are able to numerically access the quantum critical behavior by focusing on a Kondo-lattice model with Ising anisotropy. We also establish that the critical exponent for the q-dependent dynamical spin susceptibility is fractional and compares well with the experimental value for heavy fermions.

cond-mat.str-el↗

arXiv2003

Hall Effect in Nested Antiferromagnets Near the Quantum Critical Point

We investigate the behavior of the Hall coefficient in the case of antiferromagnetism driven by Fermi surface nesting, and find that the Hall coefficient should abruptly increase with the onset of magnetism, as recently observed in vanadium doped chromium. This effect is due to the sudden removal of flat portions of the Fermi surface upon magnetic ordering. Within this picture, the Hall coefficient should scale as the square of the residual resistivity divided by the impurity concentration, which is consistent with available data.

cond-mat.str-el↗

arXiv2003

Spectral functions in a magnetic field as a probe of spin-charge separation in a Luttinger liquid

We show that the single-particle spectral functions in a magnetic field can be used to probe spin-charge separation of a Luttinger liquid. Away from the Fermi momentum, the magnetic field splits both the spinon peak and holon peak; here the spin-charge separation nature is reflected in the different magnitude of the two splittings. At the Fermi momentum, the magnetic field splits the zero-field peak into {\it four} peaks. The feasibility of experimentally studying this effect is discussed.

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↗

arXiv2002

Critical local-moment fluctuations, anomalous exponents, and omega/T scaling in the Kondo problem with a pseudogap

Experiments in heavy-fermion metals and related theoretical work suggest that critical local-moment fluctuations can play an important role near a zero-temperature phase transition. We study such fluctuations at the quantum critical point of a Kondo impurity model in which the density of band states vanishes as |E|^r at the Fermi energy (E = 0). The local spin response is described by a set of critical exponents that vary continuously with r. For 0 < r < 1, the dynamical susceptibility exhibits omega/T scaling with a fractional exponent, implying that the critical point is interacting.

cond-mat.str-el↗

arXiv2001

Locally critical quantum phase transitions in strongly correlated metals

When a metal undergoes a continuous quantum phase transition, non-Fermi liquid behaviour arises near the critical point. It is standard to assume that all low-energy degrees of freedom induced by quantum criticality are spatially extended, corresponding to long-wavelength fluctuations of the order parameter. However, this picture has been contradicted by recent experiments on a prototype system: heavy fermion metals at a zero-temperature magnetic transition. In particular, neutron scattering from CeCu$_{6-x}$Au$_x$ has revealed anomalous dynamics at atomic length scales, leading to much debate as to the fate of the local moments in the quantum-critical regime. Here we report our theoretical finding of a locally critical quantum phase transition in a model of heavy fermions. The dynamics at the critical point are in agreement with experiment. We also argue that local criticality is a phenomenon of general relevance to strongly correlated metals, including doped Mott insulators.

cond-mat.str-el↗

arXiv2000

Heat transport and spin-charge separation in the normal state of high temperature superconductors

Hill et al. have recently measured both the thermal and charge conductivities in the normal state of a high temperature superconductor. Based on the vanishing of the Wiedemann-Franz ratio in the extrapolated zero temperature limit, they conclude that the charge carriers in this material are not fermionic. Here I make a simple observation that the prefactor in the temperature dependence of the measured thermal conductivity is unusually large, corresponding to an extremely small energy scale $T_0 \approx 0.15$ K. I argue that $T_0$ should be interpreted as a collective scale. Based on model-independent considerations, I also argue that the experiment leads to two possibilities: 1) The charge-carrying excitations are non-fermionic. And much of the heat current is in fact carried by distinctive charge-neutral excitations; 2) The charge-carrying excitations are fermionic, but a subtle ordering transition occurs at $T_0$.

cond-mat.supr-con↗

arXiv2000

Probing spin-charge separation using spin transport

Pedagogical discussions are given on what constitutes a signature of spin-charge separation. A proposal is outlined to probe spin-charge separation in the normal state of the high $T_c$ cuprates using spin transport. Specifically, the proposal is to compare the temperature dependences of the spin resistivity and electrical resistivity: Spin-charge separation will be manifested in the different temperature dependences of these two resistivities. We also estimate the spin diffusion length and spin relaxation time scales, and we argue that it should be experimentally feasible to measure the spin transport properties in the cuprates using the spin-injection technique. The on-going spin-injection experiments in the cuprates and related theoretical issues are also discussed.

cond-mat.supr-con↗