arXiv subjects

Ribhu K. Kaul

Publications and source records attributed to Ribhu K. Kaul.

52 records · Page 3Linked to original sources

arXiv2011

Exotic Gapless Mott Insulators of Bosons on Multi-Leg Ladders

We present evidence for an exotic gapless insulating phase of hard-core bosons on multi-leg ladders with a density commensurate with the number of legs. In particular, we study in detail a model of bosons moving with direct hopping and frustrating ring exchange on a 3-leg ladder at $ν=1/3$ filling. For sufficiently large ring exchange, the system is insulating along the ladder but has two gapless modes and power law transverse density correlations at incommensurate wave vectors. We propose a determinantal wave function for this phase and find excellent comparison between variational Monte Carlo and density matrix renormalization group calculations on the model Hamiltonian, thus providing strong evidence for the existence of this exotic phase. Finally, we discuss extensions of our results to other $N$-leg systems and to $N$-layer two-dimensional structures.

cond-mat.str-el↗

arXiv2009

Interplay of Quantum Criticality and Geometric Frustration in Columbite

Motivated by CoNb2O6 (belonging to the columbite family of minerals), we theoretically study the physics of quantum ferromagnetic Ising chains coupled anti-ferromagnetically on a triangular lattice in the plane perpendicular to the chain direction. We combine exact solutions of the chain physics with perturbative approximations for the transverse couplings. When the triangular lattice has an isosceles distortion (which occurs in the real material), the T=0 phase diagram is rich with five different states of matter: ferrimagnetic, Néel, anti-ferromagnetic, paramagnetic and incommensurate phases, separated by quantum phase transitions. Implications of our results to experiments on CoNb2O6 are discussed.

cond-mat.str-el↗

arXiv2008

Imaging bond order near non-magnetic impurities in square lattice antiferromagnets

We study the textures of generalized "charge densities" (scalar objects invariant under time reversal), in the vicinity of non-magnetic impurities in square-lattice quantum anti-ferromagnets, by order parameter field theories. Our central finding is the structure of the "vortex" in the generalized density wave order parameter centered at the non-magnetic impurity. Using exact numerical data from quantum Monte Carlo simulations on an antiferromagnetic spin model, we are able to verify the results of our field theoretic study. We extend our phenomenological approach to the period-4 bond-centered density wave found in the underdoped cuprates.

cond-mat.str-el↗

arXiv2008

Destruction of Neel order in the cuprates by electron-doping

Motivated by the evidence in PCCO and NCCO of a magnetic quantum critical point at which Neel order is destroyed, we study the evolution with doping of the T=0 quantum phases of the electron doped cuprates. At low doping, there is a metallic Neel state with small electron Fermi pockets, and this yields a fully gapped d_{x^2-y^2} superconductor with co-existing Neel order at low temperatures. We analyze the routes by which the spin-rotation symmetry can be restored in these metallic and superconducting states. In the metal, the loss of Neel order leads to a topologically ordered `doublon metal' across a deconfined critical point with global O(4) symmetry. In the superconductor, in addition to the conventional spin density wave transition, we find a variety of unconventional possibilities, including transitions to a nematic superconductor and to valence bond supersolids. Measurements of the spin correlation length and of the anomalous dimension of the Neel order by neutron scattering or NMR should discriminate these unconventional transitions from spin density wave theory.

cond-mat.str-el↗

arXiv2008

Large-N estimates of universal amplitudes of the CP^{N-1} theory and comparison with the JQ model

We present computations of certain finite-size scaling functions and universal amplitude ratios in the large-N limit of the CP^{N-1} field theory. We pay particular attention to the uniform susceptibility, the spin stiffness and the specific heat. Field theoretic arguments have shown that the long-wavelength description of the phase transition between the Neel and valence bond solid states in square lattice S=1/2 anti-ferromagnets is expected to be the non-compact CP^1 field theory. We provide a detailed comparison between our field theoretic calculations and quantum Monte Carlo data close to the Neel -VBS transition on a S=1/2 square-lattice model with competing four-spin interactions (the JQ model).

cond-mat.str-el↗

arXiv2008

Scaling in the Fan of an Unconventional Quantum Critical Point

We present results of extensive finite-temperature Quantum Monte Carlo simulations on a SU(2) symmetric S=1/2 quantum antiferromagnet with a four-spin interaction [Sandvik, Phys. Rev. Lett. 98, 227202 (2007)]. Our simulations, which are free of the sign-problem and carried out on lattices containing in excess of 1.6 X 10^4 spins, indicate that the four-spin interaction destroys the Néel order at an unconventional z=1 quantum critical point, producing a valence-bond solid paramagnet. Our results are consistent with the `deconfined quantum criticality' scenario.

cond-mat.str-el↗

arXiv2008

Quantum criticality of U(1) gauge theories with fermionic and bosonic matter in two spatial dimensions

We consider relativistic U(1) gauge theories in 2+1 dimensions, with N_b species of complex bosons and N_f species of Dirac fermions at finite temperature. The quantum phase transition between the Higgs and Coulomb phases is described by a conformal field theory (CFT). At large N_b and N_f, but for arbitrary values of the ratio N_b/N_f, we present computations of various critical exponents and universal amplitudes for these CFTs. We make contact with the different spin-liquids, charge-liquids and deconfined critical points of quantum magnets that these field theories describe. We compute physical observables that may be measured in experiments or numerical simulations of insulating and doped quantum magnets.

cond-mat.str-el↗

arXiv2007

Algebraic charge liquids

High temperature superconductivity emerges in the cuprate compounds upon changing the electron density of an insulator in which the electron spins are antiferromagnetically ordered. A key characteristic of the superconductor is that electrons can be extracted from them at zero energy only if their momenta take one of four specific values (the `nodal points'). A central enigma has been the evolution of the zero energy electrons in the metallic state between the antiferromagnet and the superconductor, and recent experiments yield apparently contradictory results. The oscillation of the resistance in this metal as a function of magnetic field indicate that the zero energy electrons carry momenta which lie on elliptical `Fermi pockets', while ejection of electrons by high intensity light indicates that the zero energy electrons have momenta only along arc-like regions. We present a theory of new states of matter, which we call `algebraic charge liquids', which arise naturally between the antiferromagnet and the superconductor, and reconcile these observations. Our theory also explains a puzzling dependence of the density of superconducting electrons on the total electron density, and makes a number of unique predictions for future experiments.

cond-mat.str-el↗

arXiv2007

Hole dynamics in an antiferromagnet across a deconfined quantum critical point

We study the effects of a small density of holes, delta, on a square lattice antiferromagnet undergoing a continuous transition from a Neel state to a valence bond solid at a deconfined quantum critical point. We argue that at non-zero delta, it is likely that the critical point broadens into a non-Fermi liquid `holon metal' phase with fractionalized excitations. The holon metal phase is flanked on both sides by Fermi liquid states with Fermi surfaces enclosing the usual Luttinger area. However the electronic quasiparticles carry distinct quantum numbers in the two Fermi liquid phases, and consequently the limit of the ratio A_F/delta, as delta tends to zero (where A_F is the area of a hole pocket) has a factor of 2 discontinuity across the quantum critical point of the insulator. We demonstrate that the electronic spectrum at this transition is described by the `boundary' critical theory of an impurity coupled to a 2+1 dimensional conformal field theory. We compute the finite temperature quantum-critical electronic spectra and show that they resemble "Fermi arc" spectra seen in recent photoemission experiments on the pseudogap phase of the cuprates.

cond-mat.str-el↗

arXiv2007

Fermi surface change across a deconfined quantum critical point

The quantum phase transitions of metals have been extensively studied in the rare-earth "heavy electron" materials, the cuprates, and related compounds. The Fermi surface of the metal often has different shapes in the states well away from the critical point. It has been proposed that these differences can persist up to the critical point, setting up a discontinuous Fermi surface change across a continuous quantum transition. We study square lattice antiferromagnets undergoing a continuous transition from a Neel state to a valence bond solid, and examine the fate of a small density of holes in the two phases. Fermi surfaces of charge e, spin 1/2 quasiparticles appear in both phases, enclosing the usual Luttinger area. However, additional quantum numbers cause the area enclosed by each Fermi surface pocket to jump by a factor of 2 across the transition in the limit of small density. We demonstrate that the electronic spectrum across this transition is described by a critical theory of a localized impurity coupled to a 2+1 dimensional conformal field theory. This critical theory also controls the more complex Fermi surface crossover at fixed density, which likely involves intermediate phases with exotic fractionalized excitations. We suggest that such theories control the electronic spectrum in the pseudogap phase of the cuprates.

cond-mat.str-el↗

arXiv2006

Spin excitations in fluctuating stripe phases of doped cuprate superconductors

Using a phenomenological lattice model of coupled spin and charge modes, we determine the spin susceptibility in the presence of fluctuating stripe charge order. We assume the charge fluctuations to be slow compared to those of the spins, and combine Monte Carlo simulations for the charge order parameter with exact diagonalization of the spin sector. Our calculations unify the spin dynamics of both static and fluctuating stripe phases and support the notion of a universal spin excitation spectrum in doped cuprate superconductors.

cond-mat.supr-con↗

arXiv2006

Strongly Inhomogeneous Phases and Non-Fermi Liquid Behavior in Randomly Depleted Kondo Lattices

We investigate the low-temperature behavior of Kondo lattices upon random depletion of the local $f$-moments, by using strong-coupling arguments and solving SU($N$) saddle-point equations on large lattices. For a large range of intermediate doping levels, between the coherent Fermi liquid of the dense lattice and the single-impurity Fermi liquid of the dilute limit, we find strongly inhomogeneous states that exhibit distinct non-Fermi liquid characteristics. In particular, the interplay of dopant disorder and strong interactions leads to rare weakly screened moments which dominate the bulk susceptibility. Our results are relevant to compounds like Ce_{x}La_{1-x}CoIn_5 and Ce_{x}La_{1-x}Pb_3

cond-mat.str-el↗

arXiv2006

Spectroscopy of the Kondo Problem in a Box

Motivated by experiments on double quantum dots, we study the problem of a single magnetic impurity confined in a finite metallic host. We prove an exact theorem for the ground state spin, and use analytic and numerical arguments to map out the spin structure of the excitation spectrum of the many-body Kondo-correlated state, throughout the weak to strong coupling crossover. These excitations can be probed in a simple tunneling-spectroscopy transport experiment; for that situation we solve rate equations for the conductance.

cond-mat.mes-hall↗

arXiv2005

On the Sign Problem in the Hirsch-Fye Algorithm for Impurity Problems

We show that there is no fermion sign problem in the Hirsch and Fye algorithm for the single-impurity Anderson model. Beyond the particle-hole symmetric case for which a simple proof exists, this has been known only empirically. Here we prove the nonexistence of a sign problem for the general case by showing that each spin trace for a given Ising configuration is separately positive. We further use this insight to analyze under what conditions orbitally degenerate Anderson models or the two-impurity Anderson model develop a sign.

cond-mat.str-el↗

arXiv2005

Mesoscopic Kondo Problem

We study the effect of mesoscopic fluctuations on a magnetic impurity coupled to a spatially confined electron gas with a temperature in the mesoscopic range (i.e. between the mean level spacing $Δ$ and the Thouless energy $E_{\rm Th}$). Comparing ``poor-man's scaling'' with exact Quantum Monte Carlo, we find that for temperatures larger than the Kondo temperature, many aspects of the fluctuations can be captured by the perturbative technique. Using this technique in conjunction with semi-classical approximations, we are able to calculate the mesoscopic fluctuations for a wide variety of systems. For temperatures smaller than the Kondo temperature, we find large fluctuations and deviations from the universal behavior.

cond-mat.mes-hall↗

arXiv2005

Cluster Algorithms for Quantum Impurity Models and Mesoscopic Kondo Physics

Nanoscale physics and dynamical mean field theory have both generated increased interest in complex quantum impurity problems and so have focused attention on the need for flexible quantum impurity solvers. Here we demonstrate that the mapping of single quantum impurity problems onto spin-chains can be exploited to yield a powerful and extremely flexible impurity solver. We implement this cluster algorithm explicitly for the Anderson and Kondo Hamiltonians, and illustrate its use in the ``mesoscopic Kondo problem''. To study universal Kondo physics, a large ratio between the effective bandwidth $D_\mathrm{eff}$ and the temperature $T$ is required; our cluster algorithm treats the mesoscopic fluctuations exactly while being able to approach the large $D_\mathrm{eff}/T$ limit with ease. We emphasize that the flexibility of our method allows it to tackle a wide variety of quantum impurity problems; thus, it may also be relevant to the dynamical mean field theory of lattice problems.

cond-mat.mes-hall↗