arXiv subjects

Yashar Komijani

Publications and source records attributed to Yashar Komijani.

At least 19 recordsLinked to original sources

arXiv2026

Coherent manipulation of Kondo Majoranas in two-channel Kondo setups

We study coherent manipulation of Majorana zero modes emerging in overscreened two-channel Kondo systems. Using compactified lattice models, we show that these interacting Kondo Majoranas support non-local qubits and admit teleportation, fusion, and braiding operations. In particular, we identify a distinction between non-topological and genuinely topological Y-junction geometries, the latter realizing a non-Abelian geometric holonomy. Our results establish a proof-of-principle route toward coherent control of non-Abelian anyons beyond conventional free-fermion platforms.

cond-mat.str-el↗

arXiv2025

Critical fluctuations and conserved dynamics in a strange ferromagnetic metal

The origin of the strange metallic behavior observed in a wide range of quantum materials is an open challenge to condensed matter physics. Historically, strange metals were uniquely associated with antiferromagnetic quantum critical points (QCPs), but a new generation of materials reveals their association with uniform order parameters, such as ferromagnetism, valley or nematic order, suggesting a deeper common denominator. At a QCP, order parameter fluctuations are characterized by the dynamical critical exponent $z$, which quantifies the space-time scaling asymmetry. Here, we report the observation of a divergence in the Grüneisen ratio at the QCP of the strange-metal ferromagnet CeRh$_6$Ge$_4$ with a dynamical critical exponent $z=3$, signaling that the underlying quantum singularity involves a conserved degree of freedom. Yet the magnetization of this easy-plane ferromagnet is not conserved. We argue that the $z=3$ strange criticality requires a description beyond the Landau paradigm, proposing a link with the gauge modes of the small-to-large Fermi surface transition and the associated gauge charge of the delocalizing heavy electrons.

cond-mat.str-el↗

arXiv2025

Higher-Dimensional Chirally Stabilized Fixed Points and Their Deformations

Non-Fermi liquids in $d>2$ remain poorly understood, particularly when relevant perturbations destabilize them. In one spatial dimension, chirally stabilized fixed points provide a rare class of analytically tractable non-Fermi-liquid critical points, but their higher-dimensional analogues have been elusive. Here, we develop a Wilsonian operator-product-expansion renormalization group scheme that captures power-divergent terms and use it to construct finite-$N$ higher-dimensional analogues of chirally stabilized fixed points in arbitrary dimension $d\le4$. This exposes a conformal window at finite $N$. We further show that symmetry-breaking masses, far from being trivial, can collapse this window and drive the system to strong coupling, triggering dynamical mass generation.

cond-mat.str-el↗

arXiv2024

Topological ground state degeneracy of the two-channel Kondo lattice

There are indications from the large-N analysis that multi-channel Kondo lattices have topological order. We use the coupled-wire construction to study the channel paramagnetic regime of a two-channel Kondo lattice model of spin-1/2 SU(2) spins. Using abelian bosonization we show that in presence of particle-hole symmetry, each wire is described by a [SO(5)$\times$Ising]/Z$_2\times$ SU(2) symmetric theory. When the wires are coupled together and the time-reversal symmetry is broken, the system exhibits topological order with fractional edge states and anyonic excitations. By an explicit construction of the Heisenberg algebra acting on the ground state manifold, we demonstrate that in presence of particle-hole symmetry, the ground state on a torus is eight-fold degenerate. This is also discussed using a heuristic approach which is applicable to other topologically ordered states.

cond-mat.str-el↗

arXiv2024

Dynamic-RKKY induced time-reversal symmetry breaking and chiral spin liquids

We study the Ruderman-Kittel-Kasuya-Yosida (RKKY) interaction in various Kondo lattice systems. We argue that the weak Kondo-coupling expansion contains certain physics which is lost in the usual static approximation to the spin susceptibility. Most notably, while the former is sensitive to the time-reversal symmetry breaking, the latter is blind to it. Using exact diagonalization on small systems, we show that this enables inducing spin chirality by an external magnetic field. To study larger systems, we use a large-N approximation to capture the effect of dynamic-RKKY interaction on U(1) spin liquids. On a honeycomb Kondo lattice with Haldane fluxes for electrons, we show that the non-trivial topology and chiral edge states are induced on the spinons. Our results suggest that dynamic RKKY in combination with external magnetic field or in proximity to topological electronic materials, can be used as a tunable Dzyaloshinskii-Moriya even in centrosymmetric materials.

cond-mat.str-el↗

arXiv2024

Dynamic mass generation and topological order in overscreened Kondo lattices

Multichannel Kondo lattice models are examples of strongly correlated electronic systems that exhibit non-Fermi-liquid behavior due to the presence of a continuous channel symmetry. Mean-field analyses have predicted that these systems undergo channel symmetry breaking at low temperature. We use the dynamical large-$N$ technique to study temporal and spatial fluctuations of the multichannel Kondo model on a honeycomb lattice and find that this prediction is not generally true. Rather, we find a 2+1D conformally invariant fixed point, governed by critical exponents that are found numerically. When we break time-reversal symmetry by adding a Haldane mass to the conduction electrons, three phases, separated by continuous transitions, are discernible: one characterized by dynamic mass generation and spontaneous breaking of the channel symmetry, one where topological defects restore channel symmetry but preserve the gap, and one with a Kondo-coupled chiral spin liquid. We argue that the last phase is a fractional Chern insulator with anyonic excitations.

cond-mat.str-el↗

arXiv2024

Towards Entanglement Entropy of Random Large-N Theories

A large class of strongly correlated quantum systems can be described in certain large-N limits by quadratic in field actions along with self-consistency equations that determine the two-point functions. We use the replica approach and the notion of shifted Matsubara frequency to compute von Neumann and Rényi entanglement entropies for generic bi-partitioning of such systems. We argue that the von Neumann entropy can be computed from equilibrium spectral functions w/o partitioning, while the Rényi entropy requires re-calculating the spectrum in the interacting case. We demonstrate the flexibility of the method by applying it to examples of a two-site problem in presence of decoherence, and coupled Sachdev-Ye-Kitaev models.

cond-mat.str-el↗

arXiv2024

A Mean-Field Study of Quantum Oscillations in Two-Dimensional Kondo Insulators

Magnetic oscillations in strongly correlated insulating systems have garnered interest due to oscillations seemingly originating from the bulk, despite an anticipated gapped spectrum. We use the large-$N$ mean-field theory to study the behavior of normal and topological Kondo insulators under a magnetic field. In both cases spinons acquire a charge and hybridize with electrons, producing magnetic oscillations that resemble two-band noninteracting systems. We show that in such band insulators magnetic oscillations are exponentially suppressed at weak magnetic fields. A self-consistent mean-field calculation for the Kondo insulators reveals that the temperature dependence of the oscillations departs from the noninteracting case due to the temperature and magnetic-field dependence of the hybridization, even though mean-field parameters remain homogeneous at low fields. Larger magnetic fields result in the Kondo breakdown, where the magnetic oscillation is solely due to the decoupled conduction electrons. These findings offer new insights into the magnetic properties of Kondo insulators, with implications for interpreting experimental results in heavy fermion materials like SmB$_6$.

cond-mat.str-el↗

arXiv2023

Spin fractionalization in a Kondo-lattice superconductor heterostructure

Kondo lattices are one of the classic models of strongly correlated systems where despite a long history, a full understanding of the excitation spectra is still not available. Here we propose that recent progress in engineering heterostructures can be leveraged to gain insight into and even tune this spectra. We use a strong Kondo coupling expansion to study spin-1 excitations of a Kondo lattice in both one and two-dimension to see whether or not paramagnons in a Kondo insulator fractionalize into spin-1/2 excitations. We show that while paramagnons are stable in the strong Kondo coupling limit, presence of sufficient proximity-induced superconducting pairing can favor fractionalization. Our results can be checked using a neutron scattering study of Kondo lattice heterostructures and paves the way toward engineering strongly correlated electronic systems.

cond-mat.str-el↗

arXiv2023

Triplet pairing, orbital selectivity and correlations in Iron-based superconductors

We use a slave-boson approach to study the band renormalization and pair susceptibility in the normal state of Iron-based superconductors in presence of strong Coulomb repulsion and Hund's interaction. Our results show orbital selectivity toward localization of $xy$ orbitals and its interplay with superconductivity. We also compare the recently proposed triplet resonating valence bond theory of superconductivity in Iron-based superconductors with the more conventional $s_\pm$ pairing and show that both favor a superconductivity when the $xy$ orbital is delocalized.

cond-mat.supr-con↗

arXiv2023

Matrix Product Study of Spin Fractionalization in the 1D Kondo Insulator

The Kondo lattice is one of the classic examples of strongly correlated electronic systems. We conduct a controlled study of the Kondo lattice in one dimension, highlighting the role of excitations created by the composite fermion operator. Using time-dependent matrix-product-state methods we compute various correlation functions and contrast them with both large-N mean-field theory and the strong-coupling expansion. We show that the composite fermion operator creates long-lived, charge-e and spin-1/2 excitations, which cover the low-lying single-particle excitation spectrum of the system. Furthermore, spin excitations can be thought to be composed of such fractionalized quasi-particles with a residual interaction which tend to disappear at weak Kondo coupling.

cond-mat.str-el↗

arXiv2022

Interplay of charge and spin fluctuations in a Hund's coupled impurity

In Hund's metals, the local ferromagnetic interaction between orbitals leads to an emergence of complex electronic states with large and slowly fluctuating magnetic moments. Introducing the Hund's coupled mixed valence quantum impurity, we gain analytic insight into recent numerical renormalization group studies. We show that valence fluctuations drastically impede the development of a large fluctuating moment over a wide range of temperatures and energy, characterized by quenched orbital degrees of freedom and a singular logarithmic behavior of the spin susceptibility $χ_{\rm sp}''(ω) \propto [ω\ln(ω/T_K^{\rm eff})^2]^{-1}$, closely resembling power-law scaling $χ_{\rm sp}''(ω) \sim ω^{-γ}$. Such singular spin fluctuations are suspected to play an important role in future models of Hund's driven Cooper pairing.

cond-mat.str-el↗

arXiv2022

Emergent Spinon Dispersion and Symmetry Breaking in Two-Channel Kondo Lattices

Two-channel Kondo lattice serves as a model for a growing family of heavy-fermion compounds. We employ the dynamical large-N technique and go beyond the independent bath approximation to study this model both numerically and analytically using renormalization group ideas. We show that the Kondo effect induces dynamic magnetic correlations that lead to an emergent spinon dispersion. Furthermore, we develop a quantitative framework that interpolates between infinite dimension where the channel-symmetry broken results of mean-field theory are confirmed, and one-dimension where the channel symmetry is restored and a critical fractionalized mode is found.

cond-mat.str-el↗

arXiv2022

Observation of a Critical Charge Mode in a Strange Metal

Quantum electronic matter has long been understood in terms of two limiting behaviors of electrons: one of delocalized metallic states, and the other of localized magnetic states. Understanding the strange metallic behavior which develops at the brink of localization demands new probes of the underlying electronic charge dynamics. Using a state-of-the-art technique, synchrotron-radiation-based Mossbauer spectroscopy, we have studied the longitudinal charge fluctuations of the strange metal phase of beta-YbAlB4 as a function of temperature and pressure. We find that the usual single absorption peak in the Fermi-liquid regime splits into two peaks upon entering the critical regime. This spectrum is naturally interpreted as a single nuclear transition, modulated by nearby electronic valence fluctuations whose long time-scales are further enhanced, due to the formation of charged polarons. Our results represent a direct observation of critical charge fluctuations as a new signature of strange metals.

cond-mat.str-el↗

arXiv2021

Triplet resonating valence bond theory and transition metal chalcogenides

We develop a quantum spin liquid theory for quantum magnets with easy-plane ferromagnetic exchange. These strongly entangled quantum states are obtained by dimer coverings of 2D lattices with triplet $S = 1, m_z = 0$ bonds, forming a triplet resonating valence bond (tRVB) state. We discuss the conditions and the procedure to transfer well-known results from conventional singlet resonating valence bond theory to tRVB. Additionally, we present mean field theories of Abrikosov fermions on 2D triangular and square lattices, which can be controlled in an appropriate large $N$ limit. We also incorporate the effect of charge doping which stabilizes $p+ip$-wave superconductivity. Beyond the pure theoretical interest, our study may help to resolve contradictory statements on certain transition metal chalcogenides, including 1T-TaS$_2$, as a potential tRVB spin-liquid.

cond-mat.str-el↗

arXiv2021

Frustrated Kondo impurity triangle: A simple model of deconfinement

The concepts of deconfinement and topological order are of great current interest for quantum information science and for our understanding of quantum materials. Here, we introduce a simple model of three antiferromagnetically coupled Kondo impurities, a Kondo triangle, which can be used to further extend the application of these concepts to electronic systems. We show that, by tuning the magnetic frustration, the Kondo triangle undergoes a quantum phase transition between two phases of unbroken symmetry, signaling a phase transition beyond the Landau paradigm. We demonstrate that the frustrated spin liquid phase is described by a three-channel Kondo (3CK) fixed point and thus displays an irrational ground state degeneracy. Using an Abrikosov pseudofermion representation, this quantum state is categorized by an emergent U(1) gauge field and its projective symmetry group. The gauge theory is deconfining in the sense that a miniature Wilson loop orders and topological defects (instantons in the gauge field) are expelled. This phase persists in the presence of moderate Kondo screening until proliferation of topological defects leads to a quantum phase transition to an unfrustrated Fermi liquid phase. Based on this evidence, we propose that the 3CK phase displays topological order in a similar sense as gapless spin liquids.

cond-mat.str-el↗

arXiv2021

Emergent moments in a Hund's impurity

Motivated by the relevance of Hund's coupling in the context of multiorbital superconductors, we revisit the problem of a multiorbital Kondo impurity with Hund's interaction. Using dynamical large-N techniques, we propose an efficient approach that retains the essential physics at play, while providing a pathway to scalable quantum impurity solvers. We are able to follow the ground state, dynamic, and thermodynamic properties of this system over many decades of temperature. Our approach captures the emergence of large moments, and follows the stretched evolution of the physics down to their exponentially suppressed Kondo temperature. We focus our analysis on the intermediate finite temperature phase which presents an alternate paramagnetic state due to the emergent moment, and discuss the relevance of this regime to Hund's metals.

cond-mat.str-el↗

arXiv2020

Kondo-based Qubits for Topological Quantum Computation

We propose to use residual parafermions of the overscreened Kondo effect for topological quantum computation. A superconducting proximity gap of $Δ<T_K$ can be utilized to isolate the parafermion from the continuum of excitations and stabilize the non-trivial fixed point. We use weak-coupling renormalization group, dynamical large-N technique and bosonization to show that the residual entropy of multichannel Kondo impurities survives in a superconductor. We find that while (in agreement with recent numerical studies) the non-trivial fixed point is unstable against intra-channel pairing, it is robust in presence of a finite inter-channel pairing. Based on this observation, we suggest a superconducting charge Kondo setup for isolating and detecting the Majorana fermion in the two-channel Kondo system.

cond-mat.str-el↗