arXiv subjects

W. A. Atkinson

Publications and source records attributed to W. A. Atkinson.

At least 19 recordsLinked to original sources

arXiv2025

Spin-orbit coupling and the Edelstein effect at conducting ferroelectric domain walls

Head-to-head ferroelectric domain walls are intrinsically charged, and are typically compensated by a mix of oppositely charged defects and free electrons. The free electrons form a two-dimensional electron gas (2DEG) along the domain wall. In many cases, inversion symmetry is broken at the wall, which implies that the 2DEG is subject to nontrivial spin-orbit coupling. Here, we use symmetry arguments to construct a generic six-band tight-binding electronic Hamiltonian for a $90^\circ$ head-to-head ferroelectric domain wall. The model, which includes spin-orbit physics and has a multi-orbital $t_{2g}$ band structure that is common to transition-metal perovskites, is applied to BaTiO$_3$. We find that the 2DEG develops an Ising spin texture, with spins aligned perpendicular to the domain wall. We contrast this with the Rashba spin texture that should emerge at weakly conducting $90^\circ$ head-to-tail domain walls. We then show that the head-to-head domain walls should have a measurable Edelstein effect (that is, a current-induced magnetization), even in the dilute limit and at room temperature, and describe a simple experiment to measure it.

cond-mat.mes-hall↗

arXiv2025

Dynamics of Conducting Ferroelectric Domain Walls

We report on the dynamics of a conducting domain wall under applied dc and ac voltages. These dynamics are modeled for a thin film that hosts an ideal charged domain wall via a combination of time-dependent Ginzburg-Landau equations for the polarization, the Schr\"odinger equation for the electron gas, and Poisson's equation for the electrostatic potential. The electron dynamics are treated within a Born-Oppenheimer approximation. We find that the electron gas introduces an additional degree of freedom, beyond polarization relaxation, that modifies the dynamical response of the domain wall. While marginally relevant for the dc response, the electron dynamics have a pronounced effect on the film's ac dielectric function. The dielectric function has an intrinsic contribution, due to the bulk susceptibility of the film, and an extrinsic contribution due to the domain-wall displacement. The elecron gas affects the dielectric function by changing both the amplitude and phase of the displacement.

cond-mat.mtrl-sci↗

arXiv2025

Inhomogeneity, Fluctuations, and Gap Filling in Overdoped Cuprates

Several recent experiments have challenged the premise that cuprate high-temperature superconductors approach conventional Landau-BCS behavior in the high-doping limit. We argue, based on an analysis of their superconducting spectra, that anomalous properties seen in the most-studied overdoped cuprates require a pairing interaction that is strongly inhomogeneous on nm length scales. This is consistent with recent proposals that the "strange-metal" phase above $T_c$ in the same doping range arises from a spatially random interaction. We show, via mean-field Bogoliubov-de Gennes (BdG) calculations and time-dependent Ginzburg-Landau (TDGL) simulations, that key features of the observed tunneling spectra are reproduced when both inhomogeneity and thermal phase fluctuations are accounted for. In accord with experiments, BdG calculations find that low-$T$ spectra are highly inhomogeneous and exhibit a low-energy spectral shoulder and broad coherence peaks. However, the spectral gap in this approach becomes homogeneous at high $T$, in contrast to experiments. This is resolved when thermal fluctuations are included; in this case, global phase coherence is lost at the superconducting $T_c$ via a broadened BKT transition, while robust phase-coherent superconducting islands persist well above $T_c$. The local spectrum remains inhomogeneous at $T_c$, and the gap is found to fill instead of close with increasing temperature.

cond-mat.supr-con↗

arXiv2024

Charge order near the antiferromagnetic quantum critical point in the trilayer high $T_c$ cuprate HgBa$_2$Ca$_2$Cu$_3$O$_{8+ \delta}$

We study the transport properties of underdoped trilayer cuprate HgBa$_2$Ca$_2$Cu$_3$O$_{8+ \delta}$ with doping level $p$ = 0.1 - 0.12 in magnetic field up to 88 T. We report for the first time in a cuprate superconductor a dramatic change of the quantum oscillation spectrum versus temperature, which is accompanied by a sign change of the Hall effect below $T \approx$ 10 K. Based on numerical simulations, we infer a Fermi surface reconstruction in the inner plane from an antiferromagnetic state (hole pockets) to a biaxial charge density wave state (electron pockets). We show that both orders compete and share the same hotspots of the Fermi surface and we discuss our result in the context of spin-fermion models.

cond-mat.supr-con↗

arXiv2023

Influence of a Realistic Multiorbital Band Structure on Conducting Domain Walls in Perovskite Ferroelectrics

Domain wall morphologies in ferroelectrics are believed to be largely shaped by electrostatic forces. Here, we show that for conducting domain walls, the morphology also depends on the details of the charge-carrier band structure. For concreteness, we focus on transition-metal perovskites like BaTiO$_3$ and SrTiO$_3$. These have a triplet of $t_{2g}$ orbitals attached to the Ti atoms that form the conduction bands when electron doped. We solve a set of coupled equations -- Landau-Ginzburg-Devonshire (LGD) equations for the polarization, tight-binding Schrödinger equations for the electron bands, and Gauss' law for the electric potential -- to obtain polarization and electron density profiles as a function of electron density. We find that at low electron densities, the electron gas is pinned to the surfaces of the ferroelectric by a Kittel-like domain structure. As the electron density increases, the domain wall evolves smoothly through a zigzag head-to-head structure, eventually becoming a flat head-to-head domain wall at high density. We find that the Kittel-like morphology is protected by orbital asymmetry at low electron densities, while at large electron densities the high density of states of the multiorbital band structure provides effective screening of depolarizing fields and flattens the domain wall relative to single-orbital models. Finally, we show that in the zigzag phase, the electron gas develops tails that extend away from the domain wall, in contrast to naïve expectations.

cond-mat.mes-hall↗

arXiv2023

Intrinsic Hallmarks of Phonon-Induced Charge Order in Cuprates

Charge-density wave (CDW) modulations in underdoped high-temperature cuprate superconductors remain a central puzzle in condensed matter physics. However, despite a substantial experimental verification of this ubiquitous phase in a large class of high $T_{\mathrm{c}}$ cuprates, a complete theoretical explanation of this phase is still missing. Here, we build upon our recent proposal that the CDW in underdoped cuprates (Y- and Bi- based compounds) emerges from a unique cooperation of the B$_{1g}$ bond-buckling phonon with strong electronic correlations. We assume a static mean-field lattice distortion with B$_{1g}$ symmetry, regardless of its origin, with a commensurate wave vector $\mathbf{q}^*=(2π/3,0)/(0,2π/3)$. We show that such a phonon-induced CDW (both uni- and bi-axial) reconstructs the Fermi surface, leading to electron and hole pockets, with relevant quantum oscillation frequencies in close consistency with the experiments. Furthermore, a systematic analysis of the symmetry of the intra-unit-cell charge modulations on the copper-oxygen planes is provided. We find that the atomic charge modulation on the CuO$_2$ unit cell is predominantly of $s$-wave character -- in support of the recent experimental observation.

cond-mat.str-el↗

arXiv2022

Simulating Superconducting Properties of Overdoped Cuprates: the Role of Inhomogeneity

Theoretical studies of disordered $d$-wave superconductors have focused, with a few exceptions, on optimally doped models with strong scatterers. Addressing recent controversies about the nature of the overdoped cuprates, however, requires studies of the weaker scattering associated with dopant atoms. Here we study simple models of such systems in the self-consistent Bogoliubov-de Gennes (BdG) framework, and compare to disorder-averaged results using the self-consistent-T-matrix-approximation (SCTMA). Despite surprisingly linear in energy behavior of the low-energy density of states even for quite disordered systems, the superfluid density in such cases retains a quadratic low-temperature variation of the penetration depth, unlike other BdG results reported recently. We trace the discrepancy to smaller effective system size employed in that work. Overall, the SCTMA performs remarkably well, with the exception of highly disordered systems with strongly suppressed superfluid density. We explore this interesting region where gap inhomogeneity dominates measured superconducting properties, and compare with overdoped cuprates.

cond-mat.supr-con↗

arXiv2022

Evolution of domain structure with electron doping in ferroelectric thin films

To minimize their electrostatic energy, insulating ferroelectric films tend to break up into nanoscale ``Kittel'' domains of opposite polarization that are separated by uncharged 180$^\circ$ domain walls. Here, I report on self-consistent solutions of coupled Landau-Ginzburg-Devonshire and Schrödinger equations for an electron-doped ferroelectric thin film. The model is based on LaAlO$_3$/SrTiO$_3$ interfaces in which the SrTiO$_3$ substrate is made ferroelectric by cation substitution or strain. I find that electron doping destabilizes the Kittel domains. As the two-dimensional electron density $n_\mathrm{2D}$ increases, there is a smooth crossover to a zigzag domain wall configuration. The domain wall is positively charged, but is compensated by the electron gas, which attaches itself to the domain wall and screens depolarizing fields. The domain wall approaches a flat head-to-head configuration in the limit of perfect screening. The polarization profile may be manipulated by an external bias voltage and the electron gas may be switched between surfaces of the ferroelectric film.

cond-mat.mes-hall↗

arXiv2022

Mechanism for Switchability in Electron-Doped Ferroelectric Interfaces

With the recent experimental verification that ferroelectric lattice distortions survive in the metallic phase of some materials, there is a desire to create devices that are both switchable and take advantage of the novel functionalities afforded by polar interfaces. In this work, we explore a simple model for such an interface and demonstrate a mechanism by which a metallic ferroelectric substrate may be switched by a bias voltage. This finding is in contrast to the reasonable expectation that hysteresis is prevented by screening of external fields in ferroelectric metals. Instead, the electron gas binds to polarization gradients to form a compensated state. Uncompensated electrons, which may screen external fields, are generated either when the electron density exceeds the ferroelectric polarization or when the bias voltage exceeds a "spillover" threshold. We propose that switchable thin films may be optimized by choosing an electron density that is slightly less than the lattice polarization. In addition to the high-polarization states, we find that thin metallic films also have a low-polarization state with average polarization near zero. Unlike in insulating films, where the polarization is small everywhere in this state, the low-polarization state in the metallic films consists of two head-to-head domains of opposite polarization. This domain formation is enabled by the screening of depolarizing fields by the electron gas.

cond-mat.mes-hall↗

arXiv2021

Correlations Among STM Observables in Disordered Unconventional Superconductors

New developments in scanning tunneling spectroscopy now allow for the spatially resolved measurement of the Josephson critical current $I_c$ between a tip and a superconducting sample, a nearly direct measurement of the true superconducting order parameter. However, it is unclear how these $I_c$ measurements are correlated with previous estimates of the spectral gap taken from differential conductance measurements. In particular, recent such experiments on an iron-based superconductor found almost no correlation between $I_c$ and the spectral gap obtained from differential conductance $g=dI/dV$ spectra, reporting instead a more significant correlation between $I_c$ and the the coherence-peak height. Here we point out that the correlation--or the lack thereof--between these various quantities can be naturally explained by the effect of disorder on unconventional superconductivity. Using large scale numerical simulations of a BCS $d$-wave pair Hamiltonian with many-impurity potentials, we observe that "substitutional" disorder models with weak pointlike impurities lead to a situation in which the true superconducting order parameter and $I_c$ are both uncorrelated with the spectral gap from $dI/dV$ measurements and highly correlated with the coherence-peak heights. The underlying mechanism appears to be the disorder-induced transfer of spectral weight away from the coherence peaks. On the other hand, smooth impurity potentials with a length scale larger than the lattice constant lead to a large positive correlation between the true superconducting order parameter and the spectral gap, in addition to a large correlation between the order parameter and the coherence-peak height. We discuss the applicability of our results to recent Josephson scanning tunneling spectroscopy experiments on iron-based and cuprate high-temperature superconductors.

cond-mat.supr-con↗

arXiv2020

Intrinsic Mechanism for Magneto-Thermal Conductivity Oscillations in Spin-Orbit-Coupled Nodal Superconductors

We describe a mechanism by which the longitudinal thermal conductivity $κ_{xx}$, measured in an in-plane magnetic field, oscillates as a function of field angle in layered nodal superconductors. These oscillations occur when the spin-orbit splitting at the nodes is larger than the nodal scattering rate, and are complementary to vortex-induced oscillations identified previously. In sufficiently anisotropic materials, the spin-orbit mechanism may be dominant. As a particular application, we focus on the cuprate high-temperature superconductor YBa$_2$Cu$_3$O$_{6+x}$. This material belongs to the class of Rashba bilayers, in which individual CuO$_2$ layers lack inversion symmetry although the crystal itself is globally centrosymmetric. We show that spin-orbit coupling endows $κ_{xx}/T$ with a characteristic dependence on magnetic field angle that should be easily detected experimentally, and argue that for underdoped samples the spin-orbit contribution is larger than the vortex contribution. A key advantage of the magneto-thermal conductivity is that it is a bulk probe of spin-orbit physics, and therefore not sensitive to inversion breaking at surfaces.

cond-mat.supr-con↗

arXiv2020

Emergent Charge Order from Correlated Electron-Phonon Physics in Cuprates

Charge-density wave order is now understood to be a widespread feature of underdoped cuprate high-temperature superconductors, although its origins remain unclear. While experiments suggest that the charge-ordering wavevector is determined by Fermi-surface nesting, the relevant sections of the Fermi surface are featureless and provide no clue as to the underlying mechanism. Here, focusing on underdoped YBa$_2$Cu$_3$O$_{6+x}$, we propose a scenario that traces the charge-density wave formation to the incipient softening of a bond-buckling phonon. The momentum dependence of its coupling to the electrons in the copper-oxygen planes favourably selects the incommensurate and axial ordering wavevector found in experiments. But, it requires strong electronic correlations via their cuprate-specific renormalization of the weight and the dispersion of quasiparticles to enable a unique enhancement of the charge susceptibility near the B$_{1g}$-phonon selected wavevector. The frequency of the B$_{1g}$ phonon softens by a few percent, and a lattice instability with concomitant finite-range charge-density wave correlations will form locally, if nucleated by defects or dopant disorder. These results offer the perspective that the complex phase diagram of underdoped cuprates cannot be understood in the context of strong electronic correlations alone.

cond-mat.str-el↗

arXiv2020

Modified transverse Ising model for the dielectric properties of SrTiO$_3$ films and interfaces

The transverse Ising model (TIM), with pseudospins representing the lattice polarization, is often used as a simple description of ferroelectric materials. However, we demonstrate that the TIM, as it is usually formulated, provides an incorrect description of SrTiO$_{3}$ films and interfaces because of its inadequate treatment of spatial inhomogeneity. We correct this deficiency by adding a pseudospin anisotropy to the model. We demonstrate the physical need for this term by comparison of the TIM to a typical Landau-Ginzburg-Devonshire model. We then demonstrate the physical consequences of the modification for two model systems: a ferroelectric thin film, and a metallic LaAlO$_{3}$/SrTiO$_{3}$ interface. We show that, in both cases, the modified TIM has a substantially different polarization profile than the conventional TIM. In particular, at low temperatures the formation of quantized states at LaAlO$_{3}$/SrTiO$_{3}$ interfaces only occurs in the modified TIM.

cond-mat.mes-hall↗

arXiv2019

Microscopic model for the hidden Rashba effect in YBa$_2$Cu$_3$O$_{6+x}$

Each unit cell in YBa$_2$Cu$_3$O$_{6+x}$ contains a pair of two-dimensional CuO$_2$ layers. While the crystal structure is globally inversion symmetric, the individual layers are not. This leads, necessarily, to a nonvanishing Rashba spin-orbit coupling (SOC) in the CuO$_2$ layers, with opposite signs of the coupling constant in each layer. These so-called Rashba bilayers generate hidden spin textures, with a vansishing net spin at each $k$-point in the Brillouin zone, but nonvanishing spin textures in each layer separately. Here, we trace the microscopic origin of the Rashba splitting through the orbital structure of the CuO$_2$ conduction bands, obtain a generic three-orbital model Hamiltonian, and show that the magnitude of the spin-splitting predicted by density functional theory is $\sim 10$~meV.

cond-mat.supr-con↗

arXiv2018

Possible Flexoelectric Origin of the Lifshitz Transition in LaAlO$_3$/SrTiO$_3$ Interfaces

Multiple experiments have observed a sharp transition in the band structure of LaAlO$_3$/SrTiO$_3$ (001) interfaces as a function of applied gate voltage. This Lifshitz transition, between a single occupied band at low electron density and multiple occupied bands at high density, is remarkable for its abruptness. In this work, we propose a mechanism by which such a transition might happen. We show via numerical modeling that the simultaneous coupling of the dielectric polarization to the interfacial strain ("electrostrictive coupling") and strain gradient ("flexoelectric coupling") generates a thin polarized layer whose direction reverses at a critical density. The Lifshitz transition occurs concomitantly with the polarization reversal and is first-order at $T=0$. A secondary Lifshitz transition, in which electrons spread out into semiclassical tails, occurs at a higher density.

cond-mat.mes-hall↗

arXiv2018

Structure of the Charge-Density Wave in Cuprate Superconductors: Lessons from NMR

Using a mix of numerical and analytic methods, we show that recent NMR $^{17}$O measurements provide detailed information about the structure of the charge-density wave (CDW) phase in underdoped YBa$_2$Cu$_3$O$_{6+x}$. We perform Bogoliubov-de Gennes (BdG) calculations of both the local density of states and the orbitally resolved charge density, which are closely related to the magnetic and electric quadrupole contributions to the NMR spectrum, using a microscopic model that was shown previously to agree closely with x-ray experiments. The BdG results reproduce qualitative features of the experimental spectrum extremely well. These results are interpreted in terms of a generic "hotspot" model that allows one to trace the origins of the NMR lineshapes. We find that four quantities---the orbital character of the Fermi surface at the hotspots, the Fermi surface curvature at the hotspots, the CDW correlation length, and the magnitude of the subdominant CDW component---are key in determining the lineshapes.

cond-mat.str-el↗

arXiv2017

Temperature-Dependent Band Structure of SrTiO$_3$ Interfaces

We build a theoretical model for the electronic properties of the two-dimensional (2D) electron gas that forms at the interface between insulating SrTiO$_3$ and a number of polar cap layers, including LaTiO$_3$, LaAlO$_3$, and GdTiO$_3$. The model treats conduction electrons within a tight-binding approximation, and the dielectric polarization via a Landau-Devonshire free energy that incorporates strontium titanate's strongly nonlinear, nonlocal, and temperature-dependent dielectric response. The self-consistent band structure comprises a mix of quantum 2D states that are tightly bound to the interface, and quasi-three-dimensional (3D) states that extend hundreds of unit cells into the SrTiO$_3$ substrate. We find that there is a substantial shift of electrons away from the interface into the 3D tails as temperature is lowered from 300 K to 10 K. This shift is least important at high electron densities ($\sim 10^{14}$ cm$^{-2}$), but becomes substantial at low densities; for example, the total electron density within 4~nm of the interface changes by a factor of two for 2D electron densities $\sim 10^{13}$ cm$^{-2}$. We speculate that the quasi-3D tails form the low-density high-mobility component of the interfacial electron gas that is widely inferred from magnetoresistance measurements.

cond-mat.str-el↗

arXiv2016

Influence of the ferroelectric quantum critical point on SrTiO$_3$ interfaces

We study a model SrTiO$_3$ interface in which conduction $t_{2g}$ electrons couple to the ferroelectric (FE) phonon mode. We treat the FE mode within a self-consistent phonon theory that captures its quantum critical behavior, and show that proximity to the quantum critical point leads to universal tails in the electron density of the form $n(z) \sim (λ+z)^{-2}$, where $λ\sim T^{2-d/\mathfrak{z}}$, with $d=3$ the dimensionality and $\mathfrak{z}=1$ the dynamical critical exponent. Implications for the metal-insulator transition at low electron density are discussed.

cond-mat.str-el↗