arXiv subjects

D. Baeriswyl

Publications and source records attributed to D. Baeriswyl.

At least 19 recordsLinked to original sources

arXiv2023

Infrared ellipsometry study of the charge dynamics in K3p-terphenyl

We report an infrared ellipsometry study of the charge carrier dynamics in polycrystalline Kxp-terphenyl samples with nominal $x=3$, for which signatures of high-temperature superconductivity were previously reported. The infrared spectra are dominated by two Lorentzian bands with maxima around 4 000 cm$^{-1}$ and 12 000 cm$^{-1}$ which, from a comparison with calculations based on a Hückel model are assigned to intra-molecular excitations of $π$ electrons of the anionic p-terphenyl molecules. The inter-molecular electronic excitations are much weaker and give rise to a Drude peak and a similarly weak Lorentzian band around 220 cm$^{-1}$. A dc resistivity of about 0.3 $Ω$ cm at 300 K is deduced from the IR data, comparable to values measured by electrical resistivity on a twin sample. The analysis of the temperature dependence of the low-frequency response reveals a gradual decrease of the plasma frequency and the scattering rate of the Drude peak below 300 K that gets anomalously enhanced below 90 K. The corresponding missing spectral weight of the Drude peak appears blue-shifted towards the Lorentz-band at 220 cm$^{-1}$. This characteristic blue-shift signifies an enhanced localization of the charge carriers at low temperatures and contrasts the behavior expected for a bulk superconducting state for which the missing spectral weight would be redshifted to a delta-function at zero frequency that accounts for the loss-free response of the superconducting condensate. Our data might still be compatible with a filamentary superconducting state with a volume fraction well below the percolation limit for which the spatial confinement of the condensate can result in a plasmonic resonance at finite frequency.

cond-mat.mtrl-sci↗

arXiv2022

Frustrated mixed-spin ladders: Evidence for a bond order wave phase between rung-singlet and Haldane phases

In frustrated spin ladders the interplay of frustration and correlations leads to the familiar Haldane (H) and rung-singlet (RS) phases. The nature of the transition between these two phases is still under debate. In this paper we tackle this issue using tools of quantum information theory. We consider frustrated mixed-spin-(1, 1/2) ladders with antiferromagnetic leg, rung and diagonal couplings, and calculate various quantities, such as the entanglement entropy (EE), the Schmidt gap, and the level degeneracy of the entanglement spectrum (ES). We use two numerical techniques, the infinite time-evolving block decimation (iTEBD) and the density matrix renormalization group (DMRG). We demonstrate that there exists an intermediate phase in which the ES levels do not exhibit the characteristic degeneracies of the H and RS phases. To understand the underlying physics in this phase, we investigate short-range spin correlations along legs, rungs and diagonals and show that in this intermediate phase long-wavelength modulations occur, akin to bond order waves.

cond-mat.str-el↗

arXiv2020

Topological and non-topological features of generalized Su-Schrieffer-Heeger models

The (one-dimensional) Su-Schrieffer-Heeger Hamiltonian, augmented by spin-orbit coupling and longer-range hopping, is studied at half filling for an even number of sites. The ground-state phase diagram depends sensitively on the symmetry of the model. Charge-conjugation (particle-hole) symmetry is conserved if hopping is only allowed between the two sublattices of even and odd sites. In this case (of BDI symmetry) we find a variety of topologically non-trivial phases, characterized by different numbers of edge states (or, equivalently, different quantized Zak phases). The transitions between these phases are clearly signalled by the entanglement entropy. Charge-conjugation symmetry is broken if hopping within the sublattices is admitted (driving the system into the AI symmetry class). We study specifically next-nearest-neighbor hopping with amplitudes $t_a$ and $t_b$ for the $A$ and $B$ sublattices, respectively. For $t_a=t_b$ parity is conserved, and also the quantized Zak phases remain unchanged in the gapped regions of the phase diagram. However, metallic patches appear due to the overlap between conduction and valence bands in some regions of parameter space. The case of alternating next-nearest neighbor hopping, $t_a=-t_b$, is also remarkable, as it breaks both charge-conjugation $C$ and parity $P$ but conserves the product $CP$. Both the Zak phase and the entanglement spectrum still provide relevant information, in particular about the broken parity. Thus the Zak phase for small values of $t_a$ measures the disparity between bond strengths on $A$ and $B$ sublattices, in close analogy to the proportionality between the Zak phase and the polarization in the case of the related Aubry-André model.

cond-mat.mes-hall↗

arXiv2013

Optical probe of ferroelectric order in bulk and thin film perovskite titanates

We have measured the temperature dependence of the direct band gap, $E_g$, in SrTi$^{16}$O$_3$ and BaTiO$_3$ and related materials with quantum-paraelectric and ferroelectric properties using optical spectroscopy. We show that $E_g$ exhibits an anomalous temperature dependence with pronounced changes in the vicinity of the ferroelectric transition that can be accounted for in terms of the Fröhlich electron-phonon interaction with an optical phonon mode, the so-called soft mode. We demonstrate that these characteristic changes of $E_g$ can be readily detected even in very thin films of SrTi$^{16}$O$_3$ with a strain-induced ferroelectric order. Optical spectroscopy thus can be used as a relatively simple but sensitive probe of ferroelectric order in very thin films of these titanates and probably also in subsequent multilayers and devices.

cond-mat.str-el↗

arXiv2012

Quantum XX chain with interface

The quantum XX chain - or rather ring - is studied as a toy model of an interface. Two transverse field patterns are used to define the interface, on the one hand a staggered field, on the other hand a step-like configuration, from -h to +h. The interface leads to Friedel oscillations and proximity effects, in particular close to the quantum phase transition of the bulk, which is a metal-insulator transition in the fermionic language. The most prominent interface effects appear for odd-numbered rings, for which - in contrast to even chains - the ground state is doubly degenerate. In the regime where the bulk energy spectrum is gapped a level appears close to midgap, with a wave function localized in the region of the interface. The two members of the ground state doublet have two different particle number parities and spin components S_z=+-1/2. They also have different energy levels and thus the degeneracy does not originate from different occupancies of a rigid band structure, but rather from a global symmetry. The unitary transformation linking the two degenerate ground states resembles a Majorana operator. Coherent superpositions of the two states may be suitable candidates for well protected qubits.

cond-mat.mes-hall↗

arXiv2009

Variational ground states of the two-dimensional Hubbard model

Recent refinements of analytical and numerical methods have improved our understanding of the ground-state phase diagram of the two-dimensional (2D) Hubbard model. Here we focus on variational approaches, but comparisons with both Quantum Cluster and Gaussian Monte Carlo methods are also made. Our own ansatz leads to an antiferromagnetic ground state at half filling with a slightly reduced staggered order parameter (as compared to simple mean-field theory). Away from half filling, we find d-wave superconductivity, but confined to densities where the Fermi surface passes through the antiferromagnetic zone boundary (if hopping between both nearest-neighbour and next-nearest-neighbour sites is considered). Our results agree surprisingly well with recent numerical studies using the Quantum Cluster method. An interesting trend is found by comparing gap parameters (antiferromagnetic or superconducting) obtained with different variational wave functions. They vary by an order of magnitude and thus cannot be taken as a characteristic energy scale. In contrast, the order parameter is much less sensitive to the degree of sophistication of the variational schemes, at least at and near half filling.

cond-mat.str-el↗

arXiv2009

Critical properties of the band-insulator-to-Mott-insulator transition in the strong-coupling limit of the ionic Hubbard model

We investigate the neutral-to-ionic insulator-insulator transition in one-dimensional materials by treating a strong-coupling effective model based on the ionic Hubbard model using the density-matrix renormalization group and finite-size scaling. The effective model, formulated in a spin-one representation, contains a single parameter. We carry out an extensive finite-size scaling analysis of the relevant gaps and susceptibilities to characterize the two zero-temperature transitions. We find that the transition from the ionic band-insulating phase to an intermediate spontaneously dimerized phase is Ising, and the transition from the dimerized phase to the Mott-insulating phase is Kosterlitz-Thouless, in agreement with the field-theory-based predictions.

cond-mat.str-el↗

arXiv2008

Superconductivity in the 2D Hubbard model: Electron doping is different

A variational Monte Carlo calculation is used for studying the ground state of the two-dimensional Hubbard model, including hopping between both nearest and next-nearest neighbor sites. Superconductivity with d-wave symmetry is found to be restricted to densities where the Fermi surface crosses the magnetic zone boundary. The condensate energy is much larger for hole doping than for electron doping. Superconductivity is kinetic energy driven for hole doping, but potential energy driven for electron doping. Our findings agree surprisingly well with experimental data for layered cuprates, both for electron- and hole-doped materials.

cond-mat.str-el↗

arXiv2007

Superconductivity and antiferromagnetism in the two-dimensional Hubbard model: a variational study

A variational ground state of the repulsive Hubbard model on a square lattice is investigated numerically for an intermediate coupling strength (U = 8t) and for moderate sizes (from 6 x 6 to 10 x 10). Our ansatz is clearly superior to other widely used variational wave functions. The results for order parameters and correlation functions provide new insight for the antiferromagnetic state at half filling as well as strong evidence for a superconducting phase away from half filling.

cond-mat.str-el↗

arXiv2006

Superconductivity in the two-dimensional Hubbard model?

A refined variational wave function for the two-dimensional repulsive Hubbard model is studied numerically, with the aim of approaching the difficult crossover regime of intermediate values of U. The issue of a superconducting ground state with d-wave symmetry is investigated for an average electron density n=0.8125 and for U=8t. Due to finite-size effects a clear-cut answer to this fundamental question has not yet been reached.

cond-mat.str-el↗

arXiv2006

Cohesion, Conductance, and Charging Effects in a Metallic Nanocontact

The conducting and thermodynamic properties of ballistic metallic nanocontacts with smooth shapes are investigated. All properties are related to the electronic scattering matrix, which is evaluated in the WKB approximation for independent electrons and in the self-consistent Hartree approximation for interacting electrons. Mesoscopic oscillations of order 1nN in the cohesive force and of order e in the contact charge are predicted when a metallic nanocontact is pulled apart, which are synchronized with quantized jumps in the conductance.

cond-mat.mes-hall↗

arXiv2006

Nature of the metal-insulator transition in the half--filled t-t' Hubbard chain

We study the quantum phase transition from an insulator to a metal realized at t'=t'_c > 0.5t in the ground state of the half-filled Hubbard chain with both nearest-neighbor (t) and next-nearest-neighbor (t') hopping. The study is carried out using the bosonization approach and density matrix renormalization group calculations. An effective low-energy Hamiltonian that describes the insulator-metal transition is derived. We find that the gross features of the phase diagram are well-described by the standard theory of commensurate-incommensurate transitions in a wide range of parameters. We also obtain an analytical expression for the insulator-metal transition line t'_c(U,t). We argue that close to the insulator-metal transition line, a crossover to a regime corresponding to an infinite-order transition takes place.We present results of density-matrix-renormalization-group calculations of spin and charge distribution in various sectors of the phase diagram. The numerical results support the picture derived from the effective theory and give evidence for the complete separation of the transitions involving spin and charge degrees of freedom.

cond-mat.str-el↗

arXiv2005

Fate of the Wigner crystal on the square lattice

The phase diagram of a system of electrons hopping on a square lattice and interacting through long-range Coulomb forces is studied as a function of density and interaction strength. The presence of a lattice strongly enhances the stability of the Wigner crystal phase as compared to the case of the two-dimensional electron gas.

cond-mat.str-el↗

arXiv2005

Competing effects of interactions and spin-orbit coupling in a quantum wire

We study the interplay of electron-electron interactions and Rashba spin-orbit coupling in one-dimensional ballistic wires. Using the renormalization group approach we construct the phase diagram in terms of Rashba coupling, Tomonaga-Luttinger stiffness and backward scattering strength. We identify the parameter regimes with a dynamically generated spin gap and show where the Luttinger liquid prevails. We also discuss the consequences for the operation of the Datta-Das transistor.

cond-mat.mes-hall↗

arXiv2004

Phase Diagram of the Heisenberg Spin Ladder with Ring Exchange

We investigate the phase diagram of a generalized spin-1/2 quantum antiferromagnet on a ladder with rung, leg, diagonal, and ring-exchange interactions. We consider the exactly soluble models associated with the problem, obtain the exact ground states which exist for certain parameter regimes, and apply a variety of perturbative techniques in the regime of strong ring-exchange coupling. By combining these approaches with considerations related to the discrete Z_4 symmetry of the model, we present the complete phase diagram.

cond-mat.str-el↗

arXiv2003

Exactly soluble isotropic spin-1/2 ladder models

The undeformed limit of the dilute two-color braid-monoid algebra gives a natural basis for the description of spin-1/2 ladder models, and allows different Baxterization Ansatze. Based on this observation we find an entire class of exactly soluble generalized isotropic spin-1/2 lader models.

cond-mat.str-el↗

arXiv2003

Weakly interacting electrons and the renormalization group

We present a general method to study weak-coupling instabilities of a large class of interacting electron models in a controlled and unbiased way. Quite generally, the electron gas is unstable towards a superconducting state even in the absence of phonons, since high-energy spin fluctuations create an effective attraction between the quasi-particles. As an example, we show the occurrence of d-wave pairing in the repulsive Hubbard model in two dimensions. In one dimension or if the Fermi surface is nested, there are several competing instabilities. The required renormalization group formalism for this case is presented to lowest (one-loop) order on a most elementary level, connecting the idea of the ``parquet summation'' to the more modern concept of Wilson's effective action. The validity and restrictions of the one-loop approximation are discussed in detail. As a result, three different renormalization group approaches known in the literature are shown to be equivalent within the regime of applicability. We also briefly discuss the open problem of a two-dimensional Fermi system at Van Hove filling without nesting.

cond-mat.str-el↗

arXiv2003

Incipient quantum melting of the one-dimensional Wigner lattice

A one--dimensional tight--binding model of electrons with long--range Coulomb interactions is studied in the limit where double site occupancy is forbidden and the Coulomb coupling strength $V$ is large with respect to the hopping amplitude $t$. The quantum problem of a kink--antikink pair generated in the Wigner lattice (the classical ground state for $t=0$) is solved for fillings $n=1/s$, where $s$ is an integer larger than 1. The pair energy becomes negative for a relatively high value of $V$, $V_c/t\approx s^3$. This signals the initial stage of the quantum melting of the Wigner lattice.

cond-mat.str-el↗