arXiv subjects

Huanzhi Hu

Publications and source records attributed to Huanzhi Hu.

6 recordsLinked to original sources

arXiv2026

Microscopic origin of hard-plane antiferromagnetism in the Kondo lattice Ce2Rh3Ge5

Hard plane antiferromagnetic order where ordered moments lie perpendicular to the single-ion crystal electric field easy axis is rare in Ce-based Kondo lattices and is a subject of active interest. Here we show that Ce$_2$Rh$_3$Ge$_5$ realizes a hard-plane antiferromagnetic state in which partial delocalization of the local moment gives rise to an RKKY exchange that overturns the single-ion easy-axis preference. Neutron diffraction reveals moments in the $ab$ plane, while inelastic neutron scattering and susceptibility establish a magnetic easy axis along $c$ in the paramagnetic regime, highlighting a clear inversion between single-ion and ordered-state anisotropies. In this work, we establish a unified microscopic framework to consistently account for partial $4f$-moment delocalization, enhanced in-plane RKKY exchange, and the resulting hard-plane antiferromagnetic order. Ce$_2$Rh$_3$Ge$_5$ thus provides a benchmark system in which single-ion anisotropy, Kondo screening, and RKKY exchange compete on comparable energy scales, revealing a cooperative route to hard-axis ordering in strongly hybridized Kondo lattices.

cond-mat.str-el↗

arXiv2025

Quantum Criticality of Type-I and Critically Tilted Dirac Semimetals

We investigate the universality of an Ising symmetry breaking phase transition of tilted two-dimensional Dirac fermions, in the type-I phase as well as at the Lifshitz transition between a type-I and a type-II semimetal, where the Fermi surface changes from point-like to one with electron and hole pockets that touch at the overtilted Dirac cones. We compute the Landau damping of long-wavelength order parameter fluctuations by tilted Dirac fermions and use the resulting IR propagator as input for a renormalisation-group analysis of the resulting Gross-Neveu-Yukawa field theory. We first demonstrate that the criticality of tilted type-I fermions is controlled by a line of fixed points along which the poles of the renormalised Green function correspond to an untilted Dirac spectrum with varying anisotropy of Fermi velocities. At the phase transition the Lorentz invariance is restored, resulting in the same critical exponents as for conventional Dirac systems. The multicritical point is given by the endpoint of the fixed-point line. It can be approached along any path in parameter space that avoids the fixed point line of the critical type-I semimetal. We show that the critical exponents at the Lifshitz point are different and that Lorentz invariance is broken.

cond-mat.str-el↗

arXiv2024

Nature of Topological Phase Transition of Kitaev Quantum Spin Liquids

We investigate the nature of the topological quantum phase transition between the gapless and gapped Kitaev quantum spin liquid phases away from the exactly solvable point. The transition is driven by anisotropy of the Kitaev couplings. At the critical point the two Dirac points of the gapless Majorana modes merge, resulting in the formation of a semi-Dirac point with quadratic and linear band touching directions. We derive an effective Gross-Neveu-Yukawa type field theory that describes the topological phase transition in the presence of additional magnetic interactions. We obtain the infrared scaling form of the propagator of the dynamical Ising order parameter field and perform a renormalization-group analysis. The universality of the transition is found to be different to that of symmetry-breaking phase transitions of semi-Dirac electrons. However, as in the electronic case, the Majorana fermions acquire an anomalous dimension, indicative of the breakdown of the fractionalized quasiparticle description.

cond-mat.str-el↗

arXiv2023

Majorana Fermion Mean-Field Theories of Kitaev Quantum Spin Liquids

We determine the phase diagrams of anisotropic Kitaev-Heisenberg models on the honeycomb lattice using parton mean-field theories based on different Majorana fermion representations of the $S=1/2$ spin operators. Firstly, we use a two-dimensional Jordan-Wigner transformation (JWT) involving a semi-infinite snake string operator. In order to ensure that the fermionized Hamiltonian remains local we consider the limit of extreme Ising exchange anisotropy in the Heisenberg sector. Secondly, we use the conventional Kitaev representation in terms of four Majorana fermions subject to local constraints, which we enforce through Lagrange multipliers. For both representations we self-consistently decouple the interaction terms in the bond and magnetization channels and determine the phase diagrams as a function of the anisotropy of the Kitaev couplings and the relative strength of the Ising exchange. While both mean-field theories produce identical phase boundaries for the topological phase transition between the gapless and gapped Kitaev quantum spin liquids, the JWT fails to correctly describe the the magnetic instability and finite-temperature behavior. Our results show that the magnetic phase transition is first order at low temperatures but becomes continuous above a certain temperature. At this energy scale we also observe a finite temperature crossover on the quantum spin-liquid side, from a fractionalized paramagnet at low temperatures, in which gapped flux excitations are frozen out, to a conventional paramagnet at high temperatures.

cond-mat.str-el↗

arXiv2023

Stability of the Néel quantum critical point in the presence of Dirac fermions

We investigate the stability of the Néel quantum critical point of two-dimensional quantum antiferromagnets, described by a non-linear $σ$ model (NL$σ$M), in the presence of a Kondo coupling to $N_f$ flavours of two-component Dirac fermion fields. The long-wavelength order parameter fluctuations are subject to Landau damping by electronic particle-hole fluctuations. Using momentum-shell RG, we demonstrate that the Landau damping is weakly irrelevant at the Néel quantum critical point, despite the fact that the corresponding self-energy correction dominates over the quadratic gradient terms in the IR limit. In the ordered phase, the Landau damping increases under the RG, indicative of damped spin-wave excitations. Although the Kondo coupling is weakly relevant, sufficiently strong Landau damping renders the Néel quantum critical point quasi-stable for $N_f\ge 4$ and thermodynamically stable for $N_f<4$. In the latter case, we identify a new multi-critical point which describes the transition between the Néel critical and Kondo run-away regimes. The symmetry breaking at this fixed point results in the opening of a gap in the Dirac fermion spectrum. Approaching the multi-critical point from the disordered phase, the fermionic quasiparticle residue vanishes, giving rise to non-Fermi-liquid behavior.

cond-mat.str-el↗

arXiv2022

Magnetic hard-direction ordering in anisotropic Kondo systems

We present a generic mechanism that explains why many Kondo materials show magnetic ordering along directions that are not favoured by the crystal-field anisotropy. Using a renormalization-group (RG) analysis of single impurity Kondo models with single-ion anisotropy, we demonstrate that strong fluctuations above the Kondo temperature drive a moment re-orientation over a wide range of parameters, e.g. for different spin values $S$ and number of Kondo channels $N$. In tetragonal systems this can happen for both easy-plane or easy axis anisotropy. The characteristic crossing of magnetic susceptibilities is not an artefact of the weak-coupling RG treatment but can be reproduced in brute-force perturbation theory. Employing numerical renormalization group (NRG), we show that for an under-screened moment ($S=1$, $N=1$) with easy-plane anisotropy, a crossing of magnetic susceptibilities can also occur in the strong-coupling regime (below the Kondo temperature). This suggests that collective magnetic ordering of such under-screened moments would develop along the magnetic hard axis.

cond-mat.str-el↗