arXiv subjects

A. Ralko

Publications and source records attributed to A. Ralko.

At least 19 recordsLinked to original sources

arXiv2023

Transient localization from the interaction with quantum bosons

We carefully revisit the electron-boson scattering problem, going beyond popular semi-classical treatments. By providing numerically exact results valid at finite temperatures, we demonstrate the existence of a regime of electron-boson scattering where quantum localization processes become relevant despite the absence of extrinsic disorder. Localization in the Anderson sense is caused by the emergent randomness resulting from a large thermal boson population, being effective at transient times before diffusion can set in. Compelling evidence of this transient localization phenomenon is provided by the observation of a distinctive displaced Drude peak (DDP) in the optical absorption and the ensuing suppression of conductivity. Our findings identify a general route for anomalous metallic behavior that can broadly apply in interacting quantum matter.

cond-mat.str-el↗

arXiv2022

A quantum theory of the nearly frozen charge glass

We study long-range interacting electrons on the triangular lattice using mixed quantum/classical simulations going beyond the usual classical descriptions of the lattice Coulomb fluid. Our results in the strong interaction limit indicate that the emergence and proliferation of quantum defects governs the low-temperature dynamics of this strongly frustrated system, in a way that crucially depends on the degree of anisotropy of the electronic structure. The present theoretical findings explain the phenomenology observed in the $θ$-ET$_2$X charge ordering materials as they fall out of equilibrium. The approach devised here can be easily generalized to address other systems where charge frustration is lifted by quantum fluctuations.

cond-mat.str-el↗

arXiv2018

Spin molecular-orbit coupling and magnetic properties of the decorated honeycomb layers of Mo3S7(dmit)3 crystals

We explore the magnetic properties of isolated a-b planes of trinuclear organometallic crystals, Mo3S7(dmit)3, in which an interplay of strong electronic correlations and spin molecular-orbital coupling (SMOC) occurs. The magnetic properties can be described by a XXZ+120, S=1 Heisenberg model on a honeycomb lattice with single-spin anisotropy, D, which depends strongly on SMOC. Based on ab initio estimates of SMOC in Mo3S7(dmit)3 crystals, we predict that the honeycomb layers of Mo3S7(dmit)3 are Neel ordered. However, in materials with a greater degree of magnetic frustration, Neel order can give way to the large-D phase.

cond-mat.str-el↗

arXiv2018

Role of quantum fluctuations on spin liquids and ordered phases in the Heisenberg model on the honeycomb lattice

Motivated by the rich physics of honeycomb magnetic materials, we obtain the phase diagram and analyze magnetic properties of the spin-1/2 and spin-1 J1-J2-J3 Heisenberg model on the honeycomb lattice. Based on the SU(2) and SU(3) symmetry representations of the Schwinger boson approach, which treats disordered spin liquids and magnetically ordered phases on an equal footing, we obtain the complete phase diagrams in the (J2,J3)plane. This is achieved using a fully unrestricted approach which does not assume any pre-defined Ansatze. For S=1/2, we find a quantum spin liquid (QSL) stabilized between the Néel, spiral and collinear antiferromagnetic phases in agreement with previous theoretical work. However, by increasing S from 1/2 to 1, the QSL is quickly destroyed due to the weakening of quantum fluctuations indicating that the model already behaves as a quasi-classical system. The dynamical structure factors and temperature dependence of the magnetic susceptibility are obtained in order to characterize all phases in the phase diagrams. Moreover, motivated by the relevance of the single-ion anisotropy, D, to various S=1 honeycomb compounds, we have analyzed the destruction of magnetic order based on a SU(3) representation of the Schwinger bosons. Our analysis provides a unified understanding of the magnetic properties of honeycomb materials realizing the J1-J2-J3 Heisenberg model from the strong quantum spin regime at S=1/2 to the S=1 case. Neutron scattering and magnetic susceptibility experiments can be used to test the destruction of the QSL phase when replacing S=1/2 by S=1 localized moments in certain honeycomb compounds.

cond-mat.str-el↗

arXiv2015

Multiorbital kinetic effects on charge ordering of frustrated electrons on the triangular lattice

The role of the multiorbital effects on the emergence of frustrated electronic orders on the triangular lattice at half filling is investigated through an extended spinless fermion Hubbard model. By using two complementary approaches, unrestricted Hartree-Fock and exact diagonalizations, we unravel a very rich phase diagram controlled by the strength of both local and off-site Coulomb interactions and by the interorbital hopping anisotropy ratio $t'/t$. Three robust unconventional electronic phases, a pinball liquid, an inverse pinball liquid, and a large-unit-cell $\sqrt{12} \times \sqrt{12}$ droplet phase, are found to be generic in the triangular geometry, being controlled by the band structure parameters. The latter are also stabilized in the isotropic limit of our microscopic model, which recovers the standard SU(2) spinful extended single-band Hubbard model.

cond-mat.str-el↗

arXiv2014

Competing supersolids of Bose-Bose mixtures in a triangular lattice

We study the ground state properties of a frustrated two-species mixture of hard-core bosons on a triangular lattice, as a function of tunable amplitudes for tunnelling and interactions. By combining three different methods, a self-consistent cluster mean-field, exact diagonalizations and effective theories, we unravel a very rich and complex phase diagram. More specifically, we discuss the existence of three original mixture supersolids: (i) a commensurate with frozen densities and supersolidity in spin degrees of freedom, in a regime of strong interspecies interactions; and (ii) when this interaction is weaker, two mutually competing incommensurate supersolids. Finally, we show how these phases can be stabilized by a quantum fluctuation enhancement of peculiar insulating parent states.

cond-mat.str-el↗

arXiv2012

Statistics of holes and nature of superfluid phases in Quantum dimer models

Quantum Dimer Models (QDM) arise as low energy effective models for frustrated magnets. Some of these models have proven successful in generating a scenario for exotic spin liquid phases with deconfined spinons. Doping, i.e. the introduction of mobile holes, has been considered within the QDM framework and partially studied. A fundamental issue is the possible existence of a superconducting phase in such systems and its properties. For this purpose, the question of the statistics of the mobile holes (or "holons") shall be addressed first. Such issues are studied in details in this paper for generic doped QDM defined on the most common two-dimensional lattices (square, triangular, honeycomb, kagome,...) and involving general resonant loops. We prove a general "statistical transmutation" symmetry of such doped QDM by using composite operators of dimers and holes. This exact transformation enables to define duality equivalence classes (or families) of doped QDM, and provides the analytic framework to analyze dynamical statistical transmutations. We discuss various possible superconducting phases of the system. In particular, the possibility of an exotic superconducting phase originating from the condensation of (bosonic) charge-e holons is examined. A numerical evidence of such a superconducting phase is presented in the case of the triangular lattice, by introducing a novel gauge-invariant holon Green's function. We also make the connection with a Bose-Hubbard model on the kagome lattice which gives rise, as an effective model in the limit of strong interactions, to a doped QDM on the triangular lattice.

cond-mat.str-el↗

arXiv2012

Statistical transmutation in doped quantum dimer models

We prove a "statistical transmutation" symmetry of doped quantum dimer models on the square, triangular and kagome lattices: the energy spectrum is invariant under a simultaneous change of statistics (i.e. bosonic into fermionic or vice-versa) of the holes and of the signs of all the dimer resonance loops. This exact transformation enables to define duality equivalence between doped quantum dimer Hamiltonians, and provides the analytic framework to analyze dynamical statistical transmutations. We investigate numerically the doping of the triangular quantum dimer model, with special focus on the topological Z2 dimer liquid. Doping leads to four (instead of two for the square lattice) inequivalent families of Hamiltonians. Competition between phase separation, superfluidity, supersolidity and fermionic phases is investigated in the four families.

cond-mat.str-el↗

arXiv2011

Geometrical frustration effects on charge-driven quantum phase transitions

The interplay of Coulomb repulsion and geometrical frustration on charge-driven quantum phase transitions is explored. The ground state phase diagram of an extended Hubbard model on an anisotropic triangular lattice relevant to quarter-filled layered organic materials contains homogeneous metal, 'pinball' and three-fold charge ordered metallic phases. The stability of the 'pinball' phase occurring for strong Coulomb repulsions is found to be strongly influenced by geometrical frustration. A comparison with a spinless model reproduces the transition from the homogeneous metallic phase to a pinball liquid, which indicates that the spin correlations should play a much smaller role than the charge correlations in the metallic phase close to the charge ordering transition. Spin degeneracy is, however, essential to describe the dependence of the system on geometrical frustration. Based on finite temperature Lanczos diagonalization we find that the effective Fermi temperature scale, T*, of the homogeneous metal vanishes at the quantum phase transition to the ordered metallic phase driven by the Coulomb repulsion. Above this temperature scale 'bad' metallic behavior is found which is robust against geometrical frustration in general. Quantum critical phenomena are not found whenever nesting of the Fermi surface is strong, possibly indicating a first order transition instead. 'Reentrant' behavior in the phase diagram is encountered whenever the 2kF-CDW instability competes with the Coulomb driven three-fold charge order transition. The relevance of our results to the family of quarter-filled materials: theta-(BEDT-TTF)2X is discussed.

cond-mat.str-el↗

arXiv2011

Resonating color state and emergent chromodynamics in the kagome antiferromagnet

We argue that the spin-wave breakdown in the Heisenberg kagome antiferromagnet signals an instability of the ground state and leads, through an emergent local constraint, to a quantum dynamics described by a gauge theory similar to that of chromodynamics. For integer spins, we show that the quantum fluctuations of the gauge modes select the sqrt(3)xsqrt(3) Neel state with an on-site moment renormalized by color resonances. We find non-magnetic low-energy excitations that may be responsible for a deconfinement "transition" at experimentally accessible temperatures which we estimate.

cond-mat.str-el↗

arXiv2010

Impurity-doped Kagome Antiferromagnet: A Quantum Dimer Model Approach

The doping of quantum Heisenberg antiferromagnets on the kagome lattice by non-magnetic impurities is investigated within the framework of a generalized quantum dimer model (QDM) describing a) the valence bond crystal (VBC), b) the dimer liquid and c) the critical region on equal footing. Following the approach by Ralko et al. [Phys. Rev. Lett. 101, 117204 (2008)] for the square and triangular lattices, we introduce the (minimal) extension of the QDM on the Kagome lattice to account for spontaneous creation of mobile S=1/2 spinons at finite magnetic field. Modulations of the dimer density (at zero or finite magnetic field) and of the local field-induced magnetization in the vicinity of impurities are computed using Lanczos Exact Diagonalization techniques on small clusters (48 and 75 sites). The VBC is clearly revealed from its pinning by impurities, while, in the dimer liquid, crystallization around impurities involves only two neighboring dimers. We also find that a next-nearest-neighbor ferromagnetic coupling favors VBC order. Unexpectedly, a small size spinon-impurity bound state appears in some region of the the dimer liquid phase. In contrast, in the VBC phase the spinon delocalizes within a large region around the impurity, revealing the weakness of the VBC confining potential. Lastly, we observe that an impurity concentration as small as 4% enhances dimerization substantially. These results are confronted to the Valence Bond Glass scenario [R.R.P. Singh, Phys. Rev. Lett. 104, 177203 (2010)] and implications to the interpretation of the Nuclear Magnetic Resonance spectra of the Herbertsmithite compound are outlined.

cond-mat.str-el↗

arXiv2010

Quantum Melting of Valence Bond Crystal Insulators and Novel Supersolid Phase at Commensurate Density

Bosonic and fermionic Hubbard models on the checkerboard lattice are studied numerically for infinite on-site repulsion. At particle density n=1/4 and strong nearest-neighbor repulsion, insulating Valence Bond Crystals (VBC) of resonating particle pairs are stabilized. Their melting into superfluid/metallic phases under increasing hopping is investigated at T=0K. More specifically, we identify a novel and unconventional commensurate VBC supersolid region, precursor to the melting of the bosonic crystal. Hardcore bosons (spins) are compared to fermions (electrons), as well as positive to negative (frustrating) hoppings.

cond-mat.str-el↗

arXiv2009

A Generalized Quantum Dimer Model Applied to the Frustrated Heisenberg Model on the Square Lattice: Emergence of a Mixed Columnar-Plaquette Phase

Aiming to describe frustrated quantum magnets with non-magnetic singlet ground states, we have extended the Rokhsar-Kivelson (RK) loop-expansion to derive a generalized Quantum Dimer Model containing only connected terms up to arbitrary order. For the square lattice frustrated Heisenberg antiferromagnet (J1-J2-J3 model), an expansion up to 8th order shows that the leading correction to the original RK model comes from dimer moves on length-6 loops. This model free of the original sign problem is treated by advanced numerical techniques. The results suggest that a rotationally anisotropic plaquette phase is the ground state of the Heisenberg model in the parameter region of maximum frustration.

cond-mat.str-el↗

arXiv2008

Magnetic field-induced transition in a quantum magnet described by the Quantum Dimer Model

The effect of a magnetic field on a gapped quantum magnet is described within the framework of the Quantum Dimer Model. A minimal model describing the proliferation of itinerant spinons above a critical field is proposed and investigated by Lanczos exact diagonalizations and quantum Monte Carlo simulations. For both square and triangular lattices, it is shown that spinons are fully polarized and Bose-condense. This offers a novel scenario of a Quantum Critical Point in the dimer-liquid phase (triangular lattice) characterized by the continuous appearance of a spinon superfluid density, contrasting with the usual triplet condensation picture. The possible role of other spinon kinetic terms neglected in the model are discussed.

cond-mat.str-el↗

arXiv2007

Generic mixed columnar-plaquette phases in Rokhsar-Kivelson models

We revisit the phase diagram of Rokhsar-Kivelson models, which are used in fields such as superconductivity, frustrated magnetism, cold bosons, and the physics of Josephson junction arrays. From an extended height effective theory, we show that one of two simple generic phase diagrams contains a mixed phase that interpolates continuously between columnar and plaquette states. For the square lattice quantum dimer model we present evidence from exact diagonalization and Green's function Monte Carlo techniques that this scenario is realised, by combining an analysis of the excitation gaps of different symmetry sectors with information on plaquette structure factors. This presents a natural framework for resolving the disagreement between previous studies.

cond-mat.str-el↗

arXiv2007

Phase separation and flux quantization in the doped quantum dimer model on the square and triangular lattices

The doped two-dimensional quantum dimer model is investigated by numerical techniques on the square and triangular lattices, with significantly different results. On the square lattice, at small enough doping, there is always a phase separation between an insulating valence-bond solid and a uniform superfluid phase, whereas on the triangular lattice, doping leads directly to a uniform superfluid in a large portion of the RVB phase. Under an applied Aharonov-Bohm flux, the superfluid exhibits quantization in terms of half-flux quanta, consistent with Q=2e elementary charge quanta in transport properties.

cond-mat.str-el↗

arXiv2006

The emergence of Resonating Valence Bond physics in spin-orbital models

We discuss how orbital degeneracy, which is usually removed by a cooperative Jahn-Teller distortion, could under appropriate circumstances lead rather to a Resonating Valence Bond spin-orbital liquid. The key points are: i) The tendency to form spin-orbital dimers, a tendency already identified in several cases; ii) The mapping onto Quantum Dimer Models, which have been shown to possess Resonating Valence Bond phases on the triangular lattice. How this program can be implemented is explained in some details starting from a microscopic model of LiNiO$_2$.

cond-mat.str-el↗

arXiv2002

Behaviour of three charged particles on a plane under perpendicular magnetic field

We consider the problem of three identical charged particles on a plane under a perpendicular magnetic field and interacting through Coulomb repulsion. This problem is treated within Taut's framework, in the limit of vanishing center of mass vector $\vec{R} \to \vec{0}$, which corresponds to the strong magnetic field limit, occuring for example in the Fractional Quantum Hall Effect. Using the solutions of the biconfluent Heun equation, we compute the eigenstates and show that there is two sets of solutions. The first one corresponds to a system of three independent anyons which have their angular momenta fixed by the value of the magnetic field and specified by a dimensionless parameter $C \simeq \frac{l_B}{l_0}$, the ratio of $l_B$, the magnetic length, over $l_0$, the Bohr radius. This anyonic character, consistent with quantum mechanics of identical particles in two dimensions, is induced by competing physical forces. The second one corresponds to the case of the Landau problem when $C \to 0$. Finally we compare these states with the quantum Hall states and find that the Laughlin wave functions are special cases of our solutions under certains conditions.

cond-mat.mes-hall↗