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Joseph J. Betouras

Publications and source records attributed to Joseph J. Betouras.

At least 37 records · Page 2Linked to original sources
arXiv2019

Multicritical Fermi surface topological transitions

A wide variety of complex phases in quantum materials are driven by electron-electron interactions, which are enhanced through density of states peaks. A well known example occurs at van Hove singularities where the Fermi surface undergoes a topological transition. Here we show that higher order singularities, where multiple disconnected leaves of Fermi surface touch all at once, naturally occur at points of high symmetry in the Brillouin zone. Such multicritical singularities can lead to stronger divergences in the density of states than canonical van Hove singularities, and critically boost the formation of complex quantum phases via interactions. As a concrete example of the power of these Fermi surface topological transitions, we demonstrate how they can be used in the analysis of experimental data on Sr$_3$Ru$_2$O$_7$. Understanding the related mechanisms opens up new avenues in material design of complex quantum phases.

cond-mat.str-el↗
arXiv2019

First order Fermi surface topological (Lifshitz) transition in an interacting two-dimensional Rashba Fermi liquid

We study the Fermi surface topological transition of the pocket-opening type in a two dimensional Fermi liquid with spin-orbit coupling of Rashba type. We find that the interactions, far from instabilities, drive the transition first order at zero temperature surprisingly in a more pronounced way than in the case of interacting Fermi liquid without spin-orbit coupling. We first gain insight from second order perturbation theory in the self-energy. We then extend the results to stronger interaction, using the self-consistent fluctuation approximation. We discuss existing experimental work on the system BiTeI and suggest further experiments in the light of these results .

cond-mat.str-el↗
arXiv2018

Nematic phase in a two-dimensional Hubbard model at weak coupling and finite temperature

We apply the self-consistent renormalized perturbation theory to the Hubbard model on the square lattice, at finite temperatures in order to study the evolution of the Fermi-surface (FS) as a function of temperature and doping. Previously, a nematic phase for the same model has been reported to appear at weak coupling near a Lifshitz transition from closed to open FS at zero temperature where the self-consistent renormalized perturbation theory was shown to be sensitive to small deformations of the FS. We find that the competition with the superconducting order leads to a maximal nematic order appearing at non-zero temperature. We explicitly observe the two competing phases near the onset of nematic instability and, by comparing the grand canonical potentials, we find that the transitions are first-order. We explain the origin of the interaction-driven spontaneous symmetry breaking to a nematic phase in a system with several symmetry-related Van Hove points and discuss the required conditions.

cond-mat.str-el↗
arXiv2018

Novel phases in twisted bilayer graphene at magic angles as a result of van Hove singularities and interactions

The discovery of different phases as a result of correlations, especially in low-dimensional materials, has been always an exciting and fundamental subject of research. Recent experiments on twisted bilayer graphene have revealed reentrant unconventional superconductivity as a function of doping as well as a Mott-like insulating phase when the two layers are twisted with respect to each other at certain "magic" angles for doping corresponding to two particles per moire unit cell. In this work we propose a microscopic model that takes into account interactions and the van Hove singularities in the density of states of the twisted bilayer graphene at doping corresponding to one particle ($ν$ =1) per moiré unit cell and study how superconductivity emerges. We identify the possible symmetry of the order parameter as $s^{\pm}$, while if the inter-valley coupling is negligible the symmetry is $s^{++}$. In addition, we find and characterise the insulating region of the system, as a region with a uniform charge instability where there is coexistence of the metallic and insulating phases.

cond-mat.str-el↗
arXiv2018

Effects of Lifshitz transitions in ferromagnetic superconductors: the case of URhGe

In ferromagnetic superconductors, like URhGe, superconductivity co-exists with magnetism near zero field, but then re-appears again in a finite field range, where the system also displays mass enhancement in the normal state. We present theoretical understanding of this non-monotonic behavior. We explore the multi-band nature of URhGe and associate re-entrant superconductivity and mass enhancement with the finite field Lifshitz transition in one of the bands. We found good agreement between our theory and a number of experimental results for URhGe, such as weakly first order reentrant transition, the dependence of superconducting $T_c$ on a magnetic field, and the field dependence of the effective mass, the specific heat and the resistivity in the normal state. Our theory can be applied to other ferromagnetic multi-band superconductors.

cond-mat.str-el↗
arXiv2018

Pechukas-Yukawa formalism for Landau-Zener transitions in the presence of external noise

Quantum systems are prone to decoherence due to both intrinsic interactions as well as random fluctuations from the environment. Using the Pechukas-Yukawa formalism, we investigate the influence of noise on the dynamics of an adiabatically evolving Hamiltonian which can describe a quantum computer. Under this description, the level dynamics of a parametrically perturbed quantum Hamiltonian are mapped to the dynamics of 1D classical gas. We show that our framework coincides with the results of the classical Landau-Zener transitions upon linearisation. Furthermore, we determine the effects of external noise on the level dynamics and its impact on Landau-Zener transitions.

cond-mat.stat-mech↗
arXiv2017

BBGKY chain and kinetic equations for the level dynamics in an externally perturbed quantum system

Theoretical description and simulation of large quantum coherent systems out of equilibrium remains a daunting task. Here we are developing a new approach to it based on the Pechukas-Yukawa formalism, which is especially convenient in case of an adiabatically slow external perturbation. In this formalism the dynamics of energy levels in an externally perturbed quantum system as a function of the perturbation parameter is mapped on that of a fictitious one-dimensional classical gas of particles with cubic repulsion. Equilibrium statistical mechanics of this Pechukas gas allows to reproduce the random matrix theory of energy levels. In the present work, we develop the nonequilibrium statistical mechanics of the Pechukas gas, starting with the derivation of the Bogoliubov-Born-Green-Kirkwood-Yvon (BBGKY) chain of equations for the appropriate generalized distribution functions. Sets of approximate kinetic equations can be consistently obtained by breaking this chain at a particular point (i.e. approximating all higher-order distribution functions by the products of the lower-order ones). When complemented by the equations for the level occupation numbers and inter-level transition amplitudes, they allow to describe the nonequilibrium evolution of the quantum state of the system, which can describe better a large quantum coherent system than the currently used approaches. In particular, we find that corrections to the factorized approximation of the distribution function scale as 1/N, where N is the number of the "Pechukas gas particles" (i.e. energy levels in the system).

cond-mat.stat-mech↗
arXiv2017

Entanglement scaling and spatial correlations of the transverse field Ising model with perturbations

We study numerically the entanglement entropy and spatial correlations of the one dimensional transverse field Ising model with three different perturbations. First, we focus on the out of equilibrium, steady state with an energy current passing through the system. By employing a variety of matrix-product state based methods, we confirm the phase diagram and compute the entanglement entropy. Second, we consider a small perturbation that takes the system away from integrability and calculate the correlations, the central charge and the entanglement entropy. Third, we consider periodically weakened bonds, exploring the phase diagram and entanglement properties first in the situation when the weak and strong bonds alternate (period two-bonds) and then the general situation of a period of n bonds. In the latter case we find a critical weak bond that scales with the transverse field as $J'_c/J$ = $(h/J)^n$, where $J$ is the strength of the strong bond, $J'$ of the weak bond and $h$ the transverse field. We explicitly show that the energy current is not a conserved quantity in this case.

cond-mat.stat-mech↗
arXiv2015

Magnetic fluctuations and specific heat in NaxCoO2 near a Lifshitz topological transition

We analyze the temperature and doping dependence of the specific heat $C(T)$ in Na$_x$CoO$_2$. This material was conjectured to undergo a Lifshitz -type topological transition at $x =x_c =0.62$, in which a new electron Fermi pocket emerges at the $Γ$ point, in addition to the existing hole pocket with large $k_F$. The data show that near $x =x_c$, the temperature dependence of $C(T)/T$ at low $T$ gets stronger as $x$ approaches $x_c$ from below and then reverses the trend and changes sign at $x \geq x_c$. We argue that this behavior can be quantitatively explained within the spin-fluctuation theory. We show that magnetic fluctuations are enhanced near $x_c$ at momenta around $k_F$ and their dynamics changes between $x \leq x_c$ and $x >x_c$, when the new pocket forms. We demonstrate that this explains the temperature dependence of $C(T)/T$. We show that at larger $x$ ($x > 0.65$) the system enters a magnetic quantum critical regime where $C(T)/T$ roughly scales as $\log T$. This behavior extends to progressively lower $T$ as $x$ increases towards a magnetic instability at $x \approx 0.75$.

cond-mat.str-el↗
arXiv2014

Dirac fermion time-Floquet crystal: manipulating Dirac points

We demonstrate how to control the spectra and current flow of Dirac electrons in both a graphene sheet and a topological insulator by applying either two linearly polarized laser fields with frequencies $ω$ and $2ω$ or a monochromatic (one-frequency) laser field together with a spatially periodic static potential(graphene/TI superlattice). Using the Floquet theory and the resonance approximation, we show that a Dirac point in the electron spectrum can be split into several Dirac points whose relative location in momentum space can be efficiently manipulated by changing the characteristics of the laser fields. In addition, the laser-field controlled Dirac fermion band structure -- Dirac fermion time-Floquet crystal -- allows the manipulation of the electron currents in graphene and topological insulators. Furthermore, the generation of dc currents of desirable intensity in a chosen direction occurs when applying the bi-harmonic laser field which can provide a straightforward experimental test of the predicted phenomena.

cond-mat.mes-hall↗
arXiv2014

Non-Landau damping of magnetic excitations in systems with localized and itinerant electrons

We discuss the form of the damping of magnetic excitations in a metal near a ferromagnetic instability. The paramagnon theory predicts that the damping term should have the form $Ω/Γ(q)$ with $Γ(q) \propto q$ (the Landau damping). However, the experiments on uranium metallic compounds UGe$_2$ and UCoGe showed that $Γ(q)$ tends to a constant value at vanishing $q$. A non-zero $Γ(0)$ is impossible in systems with one type of carriers (either localized or itinerant) because it would violate the spin conservation. It has been conjectured recently that a non-zero $Γ(q)$ in UGe$_2$ and UCoGe may be due to the presence of both localized and itinerant electrons in these materials, with ferromagnetism involving predominantly localized spins. We present microscopic analysis of the damping of near-critical localized excitations due to interaction with itinerant carriers. We show explicitly how the presence of two types of electrons breaks the cancellation between the contributions to $Γ(0)$ from self-energy and vertex correction insertions into the spin polarization bubble and discuss the special role of the Aslamazov-Larkin processes. We show that $Γ(0)$ increases with $T$ both in the paramagnetic and ferromagnetic regions, but in-between it has a peak at $T_c$. We compare our theory with the available experimental data.

cond-mat.str-el↗
arXiv2014

Distinct magnetic phase transition at the surface of an antiferromagnet

In the majority of magnetic systems the surface is required to order at the same temperature as the bulk. In the present study, we report a distinct and unexpected surface magnetic phase transition, uniquely at a lower temperature than the Néel temperature. Employing grazing incidence X-ray resonant magnetic scattering we have observed the near surface behavior of uranium dioxide. UO$_2$ is a non-collinear, triple-q, antiferromagnet with the U ions on an face-centered-cubic lattice. Theoretical investigations establish that at the surface the energy increase, due to the lost bonds, reduces when the spins near the surface rotate, gradually losing their normal to the surface component. At the surface the lowest-energy spin configuration has a double-q (planar) structure. With increasing temperature, thermal fluctuations saturate the in-plane crystal field anisotropy at the surface, leading to soft excitations that have ferromagnetic $XY$ character and are decoupled from the bulk. The structure factor of a finite two-dimensional $XY$ model, fits the experimental data well for several orders of magnitude of the scattered intensity. Our results support a distinct magnetic transition at the surface in the Kosterlitz-Thouless universality class.

cond-mat.str-el↗
arXiv2013

Effect of paramagnetic fluctuations on a Fermi surface topological transition in two dimensions

We study the Fermi surface topological transition of the pocket-opening type in a two dimensional Fermi liquid. We find that the paramagnetic fluctuations in an interacting Fermi liquid typically drive the transition first order at zero temperature. We first gain insight from a calculation using second order perturbation theory in the self-energy. This is valid for weak interaction and far from instabilities. We then extend the results to stronger interaction, using the self-consistent fluctuation approximation. Experimental signatures are given in the light of these results.

cond-mat.str-el↗
arXiv2011

Fermionic quantum dimer and fully-packed loop models on the square lattice

We consider fermionic fully-packed loop and quantum dimer models which serve as effective low-energy models for strongly correlated fermions on a checkerboard lattice at half and quarter filling, respectively. We identify a large number of fluctuationless states specific to each case, due to the fermionic statistics. We discuss the symmetries and conserved quantities of the system and show that for a class of fluctuating states in the half-filling case, the fermionic sign problem can be gauged away. This claim is supported by numerical evaluation of the low-lying states and can be understood by means of an algebraic construction. The elimination of the sign problem then allows us to analyze excitations at the Rokhsar-Kivelson point of the models using the relation to the height model and its excitations, within the single-mode approximation. We then discuss a mapping to a U(1) lattice gauge theory which relates the considered low-energy model to the compact quantum electrodynamics in 2+1 dimensions. Furthermore, we point out consequences and open questions in the light of these results.

cond-mat.str-el↗
arXiv2010

Lifshitz transitions and crystallization of fully-polarised dipolar Fermions in an anisotropic 2D lattice

We consider a two dimensional model of non-interacting chains of spinless fermions weakly coupled via a small inter-chain hopping and a repulsive inter-chain interaction. The phase diagram of this model has a surprising feature: an abrupt change in the Fermi surface as the interaction is increased. We study in detail this so called meta-nematic transition, and show that the well-known $2 1/2-$order Lifshitz transition is the criticalndpoint of this first order quantum phase transition. Furthermore, in the vicinity of the endpoint, the order parameter has a non-perturbative BCS form. We also study a competing crystallization transition in this model, and derive the full phase diagram. This physics can be demonstrated experimentally in dipolar ultra-cold atomic or molecular gases. In the presence of a harmonic trap, it manifests itself as a sharp jump in the density profile.

cond-mat.str-el↗
arXiv2010

First- order versus unconventional phase transitions in three-dimensional dimer models

We study the phase transition between the Coulomb liquid and the columnar crystal in the 3D classical dimer model, which was found to be continuous in the O(3) universality class. In addition to nearest neighbor interactions which favor parallel dimers, further neighbor interactions are allowed in such a manner that the cubic symmetry of the original system remains intact. We show that the transition in the presence of weak additional, symmetry preserving interactions is first-order. However the universality class of the transition remains continuous when the additional interactions are weakly repulsive. In this way, we verify the existence of a multicritical point near the unperturbed transition and we identify a critical line of unconventional transitions between the Coulomb liquid phase and the $6-$fold columnar phase.

cond-mat.str-el↗
arXiv2009

Deconfinement and quantum liquid crystalline states of dipolar fermions in optical lattices

We describe a simple model of fermions in quasi-one dimension that features interaction induced deconfinement (a phase transition where the effective dimensionality of the system increases as interactions are turned on) and which can be realised using dipolar fermions in an optical lattice. The model provides a relisation of a "soft quantum matter" phase diagram of strongly-correlated fermions, featuring meta-nematic, smectic and crystalline states, in addition to the normal Fermi liquid. In this paper we review the model and discuss in detail the mechanism behind each of these transitions on the basis of bosonization and detailed analysis of the RPA susceptibility.

cond-mat.str-el↗