arXiv subjects

Tomas Lofwander

Publications and source records attributed to Tomas Lofwander.

At least 19 recordsLinked to original sources

arXiv2025

Strengthened correlations near [110] edges of $d$-wave superconductors in the t-J model with the Gutzwiller approximation

We report results of a study of strongly correlated $d$-wave superconducting slabs within a t-J model solved using the statistically consistent Gutzwiller approach. For different dopings, we model slabs cut at 45$^\circ$ relative to the main crystallographic $ab$-axes, i.e., [110] orientation, and conclude that quasiparticle charge is drawn to the edges. Thus, the correlations are locally strengthened, and a region near the edge is closer to the Mott insulating state with reduced hopping amplitudes on the links. Superconductivity is locally weakened near the edges in the correlated state, and the spectral weight of zero-energy Andreev bound states is substantially reduced compared to weak coupling theory. The renormalization of the edge states leaves no room for an extended $s$-wave component to form for hole dopings ranging from strongly underdoped to slightly overdoped.

cond-mat.supr-con↗

arXiv2023

A finite element method for the quasiclassical theory of superconductivity

The Eilenberger-Larkin-Ovchinnikov-Eliashberg quasiclassical theory of superconductivity is a powerful method enabling studies of a wide range of equilibrium and non-equilibrium phenomena in conventional and unconventional superconductors. We introduce here a finite element method, based on a discontinuous Galerkin approach, to self-consistently solve the underlying transport equations for general device geometries, arbitrary mean free path and symmetry of the superconducting order parameter. We present results on i) the influence of scalar impurity scattering on phase crystals in $d$-wave superconducting grains at low temperatures and ii) the current flow and focusing in $d$-wave superconducting weak links, modeling recent experimental realizations of grooved high-temperature superconducting Dayem bridges. The high adaptability of this finite element method for quasiclassical theory paves the way for future investigations of superconducting devices and new physical phenomena in unconventional superconductors.

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arXiv2016

Shot noise in a harmonically driven ballistic graphene transistor

We study time-dependent electron transport and quantum noise in a ballistic graphene field effect transistor driven by an ac gate potential. The non-linear response to the ac signal is computed through Floquet theory for scattering states and Landauer-Büttiker theory for charge current and its fluctuations. Photon-assisted excitation of a quasibound state in the top-gate barrier leads to resonances in transmission that strongly influence the noise properties. For strong doping of graphene under source and drain contacts, when electrons are transmitted through the channel via evanescent waves, the resonance leads to a substantial suppression of noise. The Fano factor is then reduced well below the pseudo-diffusive value, $F<1/3$, also for strong ac drive. The good signal-to-noise ratio (small Fano factor) on resonance suggests that the device is a good candidate for high-frequency (THz) radiation detection. We show analytically that Klein tunneling (total suppression of back-reflection) persists for perpendicular incidence also when the barrier is driven harmonically. Although the transmission is inelastic and distributed among sideband energies, a sum rule leads to total suppression of shot noise.

cond-mat.mes-hall↗

arXiv2016

Nonlinear response of a ballistic graphene transistor with an ac-driven gate: high harmonic generation and THz detection

We present results for time-dependent electron transport in a ballistic graphene field-effect transistor with an ac-driven gate. Nonlinear response to the ac drive is derived utilizing Floquet theory for scattering states in combination with Landauer-Büttiker theory for transport. We identify two regimes that can be useful for applications: (i) low and (ii) high doping of graphene under source and drain contacts, relative to the doping level in the graphene channel, which in an experiment can be varied by a back gate. In both regimes, inelastic scattering induced by the ac drive can excite quasi-bound states in the channel that leads to resonance promotion of higher order sidebands. Already for weak to intermediate ac drive strength, this leads to a substantial change in the direct current between source and drain. For strong ac drive with frequency $Ω$, we compute the higher harmonics of frequencies $nΩ$ ($n$ integer) in the source-drain conductance. In regime (ii), we show that particular harmonics (for instance $n=6$) can be selectively enhanced by tuning the doping level in the channel or by tuning the drive strength. We propose that the device operated in the weak-drive regime can be used to detect THz radiation, while in the strong-drive regime it can be used as a frequency multiplier.

cond-mat.mes-hall↗

arXiv2014

Conductance footprints of impurity scattering in graphene nanoribbons

We report a detailed analytic investigation of the interplay between size quantization and local scattering centers in armchair graphene nanoribbons, as seen in the conductance. The scattering property of a local scattering center is dependent on if it is located on one sublattice (A-site impurity) or both (impurity situated at neighboring carbon atoms, A-B-site impurity). The A-site impurity scatters in a similar way as a localized impurity in a one-dimensional channel made from a two-dimensional electron gas. On the other hand, the A-B-site impurity introduces A- to B-sublattice scattering, which knows about the chirality of Dirac electrons, and heavily influence the conductance. For A-site impurities, interplay between evanescent waves at the impurity and the propagating modes contributing to the conductance, leads to scattering resonances that generate either dips in the conductance or render the impurity completely transparent. The latter occurs at subband bottom energies where the wave vector of the opening mode is zero. The conductance of defect free graphene therefore remains at these energies. This is analagous to the case of a scattering center in a quantum channel made from a two-dimensional electron gas. The conductance dips occur at energies $ΔE$ away from the conductance steps and their location depend directly on the impurity strength. In particular, for repulsive impurities the dips occur for hole-doping, while for attractive impurities for electron doping. For an A-B-site impurity, the A- to B-sublattice scattering interferes with the transmission resonance at the energies of the subband bottoms and the impurity is never transparent and the conductance steps of defect free graphene ribbons are always lost. We derive a generalized Fisher-Lee formula for graphene leads that holds for arbitrary scattering region and arbitrary number of leads.

cond-mat.mes-hall↗

arXiv2014

Destroyed quantum Hall effect in graphene with [0001] tilt grain boundaries

The reason why the half-integer quantum Hall effect (QHE) is suppressed in graphene grown by chemical vapor deposition (CVD) is unclear. We propose that it might be connected to extended defects in the material and present results for the quantum Hall effect in graphene with [0001] tilt grain boundaries connecting opposite sides of Hall bar devices. Such grain boundaries contain 5-7 ring complexes that host defect states that hybridize to form bands with varying degree of metallicity depending on grain boundary defect density. In a magnetic field, edge states on opposite sides of the Hall bar can be connected by the defect states along the grain boundary. This destroys Hall resistance quantization and leads to non-zero longitudinal resistance. Anderson disorder can partly recover quantization, where current instead flows along returning paths along the grain boundary depending on defect density in the grain boundary and on disorder strength. Since grain sizes in graphene made by chemical vapor deposition are usually small, this may help explain why the quantum Hall effect is usually poorly developed in devices made of this material.

cond-mat.mes-hall↗

arXiv2013

Inverse proximity effect and influence of disorder on triplet supercurrents in strongly spin-polarized ferromagnets

We discuss the Josephson effect in strongly spin-polarized ferromagnets where triplet correlations are induced by means of spin-active interface scattering, extending our earlier work [Phys. Rev. Lett. 102, 227005 (2009)] by including impurity scattering in the ferromagnetic bulk and the inverse proximity effect in a fully self-consistent way. Our quasiclassical approach accounts for the differences of Fermi momenta and Fermi velocities between the two spin bands of the ferromagnet, and thereby overcomes an important short-coming of previous work within the framework of Usadel theory. We show that non-magnetic disorder in conjunction with spin-dependent Fermi velocities may induce a reversal of the spin-current as a function of temperature.

cond-mat.supr-con↗

arXiv2013

Quantum Hall effect in graphene with twisted bilayer stripe defects

We analyze the quantum Hall effect in single layer graphene with bilayer stripe defects. Such defects are often encountered at steps in the substrate of graphene grown on silicon carbide. We show that AB or AA stacked bilayer stripes result in large Hall conductivity fluctuations that destroy the quantum Hall plateaux. The fluctuations are a result of the coupling of edge states at opposite edges through currents traversing the stripe. Upon rotation of the second layer with respect to the continuous monolayer (a twisted-bilayer stripe defect), such currents decouple from the extended edge states and develop into long-lived discrete quasi bound states circulating around the perimeter of the stripe. Backscattering of edge modes then occurs only at precise resonant energies, and hence the quantum Hall plateaux are recovered as twist angle grows.

cond-mat.mes-hall↗

arXiv2012

Spectral footprints of impurity scattering in graphene nanoribbons

We report a detailed investigation of the interplay between size quantization and local scattering centers in graphene nanoribbons, as seen in the local density of states. The spectral signatures, obtained after Fourier transformation of the local density of states, include characteristic peaks that can be related to the transverse modes of the nanoribbon. In armchair ribbons, the Fourier transformed density of states of one of the two inequivalent sublattices takes a form similar to that of a quantum channel in a two-dimensional electron gas, modified according to the differences in bandstructure. After addition of the second sublattice contribution, a characteristic modulation of the pattern due to superposition is obtained, similar to what has been obtained in spectra due to single impurity scattering in large-area graphene. We present analytic results for the electron propagator in armchair nanoribbons in the Dirac approximation, including a single scattering center within a T-matrix formulation. For comparison, we have extended the investigation with numerics obtained with an atomistic recursive Green's function approach. The spectral signatures of the atomistic approach include the effects of trigonal warping. The impurity induced oscillations in the local density of states are not decaying at large distance in few-mode nanoribbons.

cond-mat.mes-hall↗

arXiv2012

Theory of superconductor-ferromagnet point contact spectra: the case of strong spin polarization

We study the impact of spin-active scattering on Andreev spectra of point contacts between superconductors(SCs) and strongly spin-polarized ferromagnets(FMs) using recently derived boundary conditions for the Quasiclassical Theory of Superconductivity. We describe the interface region by a microscopic model for the interface scattering matrix. Our model includes both spin-filtering and spin-mixing and is non-perturbative in both transmission and spin polarization. We emphasize the importance of spin-mixing caused by interface scattering, which has been shown to be crucial for the creation of exotic pairing correlations in such structures. We provide estimates for the possible magnitude of this effect in different scenarios and discuss its dependence on various physical parameters. Our main finding is that the shape of the interface potential has a tremendous impact on the magnitude of the spin-mixing effect. Thus, all previous calculations, being based on delta-function or box-shaped interface potentials, underestimate this effect gravely. As a consequence, we find that with realistic interface potentials the spin-mixing effect can easily be large enough to cause spin-polarized sub-gap Andreev bound states in SC/sFM point contacts. In addition, we show that our theory generalizes earlier models based on the Blonder-Tinkham-Klapwijk approach.

cond-mat.supr-con↗

arXiv2010

Is CrO$_2$ Fully Spin-Polarized? - Analysis of Andreev Spectra and Excess Current

We report an extensive theoretical analysis of point-contact Andreev reflection data available in literature on ferromagnetic CrO$_2$. We find that the spectra can be well understood within a model of fully spin-polarized bands in CrO$_2$ together with spin active scattering at the contact. This is in contrast to analyses of the data within extended Blonder-Tinkham-Klapwijk models, which lead to a spin polarization varying between 50~\% and 100~\% depending on the transparency of the interface. We propose to utilize both the temperature dependence of the spectra and the excess current at voltages above the gap to resolve the spin-polarization in CrO$_2$ in a new generation of experiments.

cond-mat.supr-con↗

arXiv2008

Spectrum of Andreev Bound States in a Molecule Embedded Inside a Microwave-Excited Superconducting Junction

Non-dissipative Josephson current through nanoscale superconducting constrictions is carried by spectroscopically sharp energy states, so-called Andreev bound states. Although theoretically predicted almost 40 years ago, no direct spectroscopic evidence of these Andreev bound states exists to date. We propose a novel type of spectroscopy based on embedding a superconducting constriction, formed by a single-level molecule junction, in a microwave QED cavity environment. In the electron-dressed cavity spectrum we find a polariton excitation at twice the Andreev bound state energy, and a superconducting-phase dependent ac Stark shift of the cavity frequency. Dispersive measurement of this frequency shift can be used for Andreev bound state spectroscopy.

cond-mat.supr-con↗

arXiv2007

Impurity Scattering and Mott's Formula in Graphene

We present calculations of the thermal and electric linear response in graphene, including disorder in the self-consistent t-matrix approximation. For strong impurity scattering, near the unitary limit, the formation of a band of impurity states near the Fermi level leads to that Mott's relation holds at low temperature. For higher temperatures, there are strong deviations due to the linear density of states. The low-temperature thermopower is proportional to the inverse of the impurity potential and the inverse of the impurity density. Information about impurity scattering in graphene can be extracted from the thermopower, either measured directly, or extracted via Mott's relation from the electron-density dependence of the electric conductivity.

cond-mat.mes-hall↗

arXiv2006

Phase Diagrams of Ferromagnet-Superconductor Multilayers with Misaligned Exchange Fields

We study the influence of misalignment of the ferromagnetic exchange field on the equilibrium properties of hybrid structures, composed of superconducting (S) and ferromagnetic (F) parts. In particular, we study numerically the superconducting critical temperature Tc in F-S-F trilayers and in F-S-F-S-F Josephson junctions as a function of the misalignment angle theta of the ferromagnetic magnetization. We discuss the corresponding phase diagrams for these hybrid structures. For the Josephson junctions, a transition between the zero-phase and the pi-phase ground state as a function of theta takes place under certain conditions. Within the quasiclassical Green's function technique in the diffusive limit, we introduce a fast and effective method for calculating Tc in such multilayer structures.

cond-mat.supr-con↗

arXiv2005

Large Thermoelectric Effects and Inelastic Scattering in Unconventional Superconductors

The thermoelectric coefficient $η(T)$ in unconventional superconductors is enhanced below $T_c$ by intermediate strength impurity scattering that is intrinsically particle-hole asymmetric. We compute $η(T)$ for a strong-coupling d-wave superconductor and investigate the effects of inelastic scattering originating from electron-boson interactions. We show that $η(T)$ is severely suppressed at temperatures just below $T_c$ by a particle-hole symmetric inelastic scattering rate. At lower temperatures inelastic scattering is frozen out and $η(T)$ recovers and regains its large amplitude. In the limit $T\to 0$, we have $η(T)\sim η_{0} T+{\cal O}[T^3]$, where the slope $η_{0}$ contains information about the Drude plasma frequency, the details of impurity scattering, and the change in effective mass by electron-boson interactions. In this limit $η(T)$ can be used as a probe, complementary to the universal heat and charge conductivities, in investigations of the nature of nodal quasiparticles.

cond-mat.supr-con↗

arXiv2005

Interplay of Magnetic and Superconducting Proximity Effects in FSF Trilayers

We present theoretical results on the interplay of magnetic and superconducting orders in diffusive ferromagnet-superconductor-ferromagnet trilayers. The induced triplet superconducting correlations throughout the trilayer lead to an induced spin magnetization. We include self-consistency of the order parameter in the superconducting layer at arbitrary temperatures, arbitrary interface transparency, and any relative orientation of the exchange fields in the two ferromagnets. We propose to use the torque on the trilayer in an external magnetic field as a probe of the presence of triplet correlations in the superconducting phase.

cond-mat.supr-con↗

arXiv2005

Low-Temperature Thermal Conductivity of Superconductors With Gap Nodes

We report a detailed analytic and numerical study of electronic thermal conductivity in d-wave superconductors. We compare theory of the cross over at low temperatures from T-dependence to T^3-dependence for increasing temperature with recent experiments on YBCO in zero magnetic field for temperatures from 0.04K to 0.4K by Hill et al., Phys. Rev. Lett. 92, 027001 (2004). Transport theory, including impurity scattering and inelastic scattering within strong coupling superconductivity, can consistently fit the temperature dependence of the data in the lower half of the temperature regime. We discuss the conditions under which we expect power-law dependences over wide temperature intervals.

cond-mat.supr-con↗

arXiv2004

Proximity Effect in Normal Metal - High Tc Superconductor Contacts

We study the proximity effect in good contacts between normal metals and high Tc (d-wave) superconductors. We present theoretical results for the spatially dependent order parameter and local density of states, including effects of impurity scattering in the two sides, s-wave pairing interaction in the normal metal side (attractive or repulsive), as well as subdominant s-wave paring in the superconductor side. For the [100] orientation, a real combination d+s of the order parameters is always found. The spectral signatures of the proximity effect in the normal metal includes a suppression of the low-energy density of states and a finite energy peak structure. These features are mainly due to the impurity self-energies, which dominate over the effects of induced pair potentials. For the [110] orientation, for moderate transparencies, induction of a d+is order parameter on the superconductor side, leads to a proximity induced is order parameter also in the normal metal. The spectral signatures of this type of proximity effect are potentially useful for probing time-reversal symmetry breaking at a [110] interface.

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