arXiv subjects

Allan H. MacDonald

Publications and source records attributed to Allan H. MacDonald.

At least 109 records · Page 6Linked to original sources

arXiv2020

Current Driven Magnetization Reversal in Orbital Chern Insulators

Graphene multilayers with flat moiré minibands can exhibit the quantized anomalous Hall effect due to the combined influence of spontaneous valley polarization and topologically non-trival valley-projected bands. The sign of the Hall effect in these Chern insulators can be reversed either by applying an external magnetic field, or by driving a transport current through the system. We propose a current-driven mechanism whereby reversal occurs along lines in the (current $I$, magnetic-field $B$) control parameter space with slope $dI/dB = (e/h)\, M A_{M} \, (1-γ^2)/γ$, where $M$ is the magnetization, $A_M$ is the moiré unit cell area, and $γ<1$ is the ratio of the chemical potential difference between valleys along a domain wall to the electrical bias $eV$.

cond-mat.mes-hall↗

arXiv2020

Higgs modes in two-dimensional spatially-indirect exciton condensates

Higgs modes in condensed matter physics have drawn attention because of analogies to the Higgs bosons of particle physics. Here we use a microscopic time-dependent mean-field theory to study the collective mode spectra of two-dimensional spatially indirect exciton (electron-hole pair) condensates, focusing on the Higgs modes, i.e., those that have a large weight in electron-hole pair amplitude response functions. We find that in the low exciton density (BEC) limit, the dominant Higgs modes of spatially indirect exciton condensates correspond to adding electron-hole pairs that are orthogonal to the condensed pair state. We comment on the previously studied Higgs-like collective excitations of superconductors in light of this finding.

cond-mat.mes-hall↗

arXiv2020

Graphene Bilayers with a Twist

Near a magic twist angle, bilayer graphene transforms from a weakly correlated Fermi liquid to a strongly correlated two-dimensional electron system with properties that are extraordinarily sensitive to carrier density and to controllable environmental factors such as the proximity of nearby gates and twist-angle variation. Among other phenomena magic-angle twisted bilayer graphene hosts superconductivity, interaction induced insulating states, magnetism, electronic nematicity, linear-in-T low-temperature resistivity, and quantized anomalous Hall states. We highlight some key research results in this field, point to important questions that remain open, and comment on the place of magic angle twisted bilayer graphene in the strongly correlated quantum matter world.

cond-mat.mes-hall↗

arXiv2020

Scattering of magnons at graphene quantum-Hall-magnet junctions

Motivated by recent non-local transport studies of quantum-Hall-magnet (QHM) states formed in monolayer graphene's $N=0$ Landau level, we study the scattering of QHM magnons by gate-controlled junctions between states with different integer filling factors $ν$. For the $ν=1|-1|1$ geometry we find magnons are weakly scattered by electric potential variation in the junction region, and that the scattering is chiral when the junction lacks a mirror symmetry. For the $ν=1|0|1$ geometry, %in which the scattering region contains a $ν=0$ canted antiferromagnet, we find that kinematic constraints completely block magnon transmission if the incident angle exceeds a critical value. Our results explain the suppressed non-local-voltage signals observed in the $ν=1|0|1$ case. We use our theory to propose that valley-waves generated at $ν=-1|1$ junctions and magnons can be used in combination to probe the spin/valley flavor structure of QHM states at integer and fractional filling factors.

cond-mat.mes-hall↗

arXiv2020

Linear magnetoresistance induced by intra-scattering semiclassics of Bloch electrons

The weak field magnetoresistance has seen a revived interest due to the distinct role played by the momentum-space Berry curvature of Bloch electrons. While most previous studies in this regard focus on the inter-scattering motion of semiclassical Bloch electrons in electromagnetic fields, the intra-scattering effects of the semiclassical dynamics augmented by the Berry curvature, magnetic moment and shift vector on the magnetoresistance have been largely overlooked. Here we uncover that these intra-scattering effects, which are neglected in the field-independent relaxation time approximation to the Boltzmann collision integral, can be as important as the inter-scattering ones. Concrete calculations on the two dimensional gapped Dirac model show that the sign of the negative linear magnetoresistance given by the Berry curvature alone is reversed when one considers the magnetic moment and shift vector.

cond-mat.mtrl-sci↗

arXiv2020

Unveiling Defect-Mediated Carrier Dynamics in Monolayer Semiconductors by Spatiotemporal Microwave Imaging

The optoelectronic properties of atomically thin transition-metal dichalcogenides are strongly correlated with the presence of defects in the materials, which are not necessarily detrimental for certain applications. For instance, defects can lead to an enhanced photoconduction, a complicated process involving charge generation and recombination in the time domain and carrier transport in the spatial domain. Here, we report the simultaneous spatial and temporal photoconductivity imaging in two types of WS2 monolayers by laser-illuminated microwave impedance microscopy. The diffusion length and carrier lifetime were directly extracted from the spatial profile and temporal relaxation of microwave signals respectively. Time-resolved experiments indicate that the critical process for photo-excited carriers is the escape of holes from trap states, which prolongs the apparent lifetime of mobile electrons in the conduction band. As a result, counterintuitively, the photoconductivity is stronger in CVD samples than exfoliated monolayers with a lower defect density. Our work reveals the intrinsic time and length scales of electrical response to photo-excitation in van der Waals materials, which is essential for their applications in novel optoelectronic devices.

cond-mat.mes-hall↗

arXiv2020

Crescent states in charge-imbalanced polariton condensates

We study two-dimensional charge-imbalanced electron-hole systems embedded in an optical microcavity. We find that strong coupling to photons favors states with pairing at zero or small center of mass momentum, leading to a condensed state with spontaneously broken time-reversal and rotational symmetry, and unpaired carriers that occupy an anisotropic crescent-shaped sliver of momentum space. The crescent state is favoured at moderate charge imbalance, while a Fulde--Ferrel--Larkin--Ovchinnikov-like state --- with pairing at large center of mass momentum --- occurs instead at strong imbalance. The crescent state stability results from long-range Coulomb interactions in combination with extremely long-range photon-mediated interactions.

cond-mat.quant-gas↗

arXiv2020

Tunneling and Fluctuating Electron-Hole Cooper Pairs in Double Bilayer Graphene

A strong low-temperature enhancement of the tunneling conductance between graphene bilayers has been reported recently, and interpreted as a signature of equilibrium electron-hole pairing, first predicted in bilayers more than forty years ago but previously unobserved. Here we provide a detailed theory of conductance enhanced by fluctuating electron-hole Cooper pairs, which are a precursor to equilibrium pairing, that accounts for specific details of the multi-band double graphene bilayer system which supports several different pairing channels. Above the equilibrium condensation temperature, pairs have finite temporal coherence and do not support dissipationless tunneling. Instead, they strongly boost the tunneling conductivity via a fluctuational internal Josephson effect. Our theory makes predictions for the dependence of the zero bias peak in the differential tunneling conductance on temperature and electron-hole density imbalance that capture important aspects of the experimental observations. In our interpretation of the observations, cleaner samples with longer disorder scattering times would condense at temperatures $T_c$ up to $\sim 50 {\rm K}$, compared to the record $T_c \sim 1.5 $K achieved to date in the experiment.

cond-mat.mes-hall↗

arXiv2020

Vortex Lattice Structure and Topological Superconductivity in the Quantum Hall Regime

Chiral topological superconductors are expected to appear as intermediate states when a quantum anomalous Hall system is proximity coupled to an s-wave superconductor and the magnetization direction is reversed. In this paper we address the edge state properties of ordinary quantum Hall systems proximity coupled to s-wave superconductors, accounting explicitly for Landau quantization. We find that the appearance of topological superconducting phases with an odd number of Majorana edge modes is dependent on the structure of the system's vortex lattice. More precisely, vortex lattices containing odd number of superconducting flux quanta per unit cell, always support an even number of chiral edge channels and are therefore adiabatically connected to normal quantum Hall insulators. We discuss strategies to engineer chiral topological superconductivity in proximity-coupled quantum Hall systems by manipulating vortex lattice structure.

cond-mat.mes-hall↗

arXiv2019

On the nature of the correlated insulator states in twisted bilayer graphene

We use self-consistent Hartree-Fock calculations performed in the full $π$-band Hilbert space to assess the nature of the recently discovered correlated insulator states in magic-angle twisted bilayer graphene (TBG). We find that gaps between the flat conduction and valence bands open at neutrality over a wide range of twist angles, sometimes without breaking the system's valley projected ${\cal C}_{2}{\cal T}$ symmetry. Broken spin/valley flavor symmetries then enable gapped states to form not only at neutrality, but also at total moiré band filling $n = \pm p/4$ with integer $p = 1, 2, 3$, when the twist angle is close to the magic value at which the flat bands are most narrow. Because the magic-angle flat band quasiparticles are isolated from remote band quasiparticles only for effective dielectric constants larger than $ \sim 20$, the gapped states do not necessarily break \CT symmetry and as a consequence the insulating states at $n = \pm 1/4$ and $n = \pm 3/4$ need not exhibit a quantized anomalous Hall effect.

cond-mat.str-el↗

arXiv2019

Correlated Insulating States in Twisted Double Bilayer Graphene

We present a combined experimental and theoretical study of twisted double bilayer graphene with twist angles between 1° and 1.35°. Consistent with moiré band structure calculations, we observe insulators at integer moiré band fillings one and three, but not two. An applied transverse electric field separates the first moiré conduction band from neighbouring bands, and favors the appearance of correlated insulators at 1/4, 1/2, and 3/4 band filling. Insulating states at 1/4 and 3/4 band filling emerge only in a parallel magnetic field (B_{||}), whereas the resistance at half band filling is weakly dependent on B_{||}. These findings suggest that correlated insulators are favored when a moiré flat band is spectrally isolated, with spin polarization at 1/4 and 3/4 band filling and valley polarization at 1/2 band filling.

cond-mat.mes-hall↗

arXiv2019

Resonant photovoltaic effect in doped magnetic semiconductors

The rectified non-linear response of a clean undoped semiconductor to an AC electric field includes a well known intrinsic contribution -- the shift current. We show that when Kramers degeneracy is broken, a distinct second order rectified response appears that is due to Bloch state anomalous velocities in a system with an oscillating Fermi surface. This effect, which we refer to as the resonant photovoltaic effect (RPE), produces a resonant galvanic current peak at the interband absorption threshold in doped semiconductors or semimetals with approximate particle-hole symmetry. We evaluate the RPE for a model of the surface states of a magnetized topological insulator.

cond-mat.mes-hall↗

arXiv2019

Electronic structure of carbon nanotubes on graphene substrates

Allotropes of carbon, including one-dimensional carbon nanotubes and two-dimensional graphene sheets, continue to draw attention as promising platforms for probing the physics of electrons in lower dimensions. Recent research has shown that the electronic properties of graphene multilayers are exquisitely sensitive to the relative orientation between sheets, and in the bilayer case exhibit strong electronic correlations when close to a magic twist angle. Here, we investigate the electronic properties of a carbon nanotube deposited on a graphene sheet by deriving a low-energy theory that accounts both for rotations and rigid displacements of the nanotube with respect to the underlying graphene layer. We show that this heterostructure is described by a translationally invariant, a periodic or a quasi-periodic Hamiltonian, depending on the orientation and the chirality of the nanotube. Furthermore, we find that, even for a vanishing twist angle, rigid displacements of a nanotube with respect to a graphene substrate can alter its electronic structure qualitatively. Our results identify a promising new direction for strong correlation physics in low dimensions.

cond-mat.mes-hall↗

arXiv2019

Tunneling density of states, correlation energy, and spin polarization in the fractional quantum Hall regime

We derive exact sum-rules that relate the tunneling density of states (TDOS) of spinful electrons in the fractional quantum Hall (FQH) regime to the spin-dependent many body ground state correlation energy. Because the tunneling process is spin-conserving, the 2D (two-dimensional) to 2D tunneling current $I$ at a given bias voltage $V$ in a spin-polarized system is a sum of majority and minority spin contributions. The sum rules can be used to define spin-dependent gaps that we associate with peaks in 2D to 2D tunneling $I-V$ curves. We discuss different scenarios of tunneling between layers at different fillings in relation to our sum rules. We comment on what recent tunneling experiments say about spin-dependent correlations in light of our sum rules, and propose additional measurements that could provide more specific experimental estimates of spin-dependent correlation energies.

cond-mat.str-el↗

arXiv2019

Electronic Structure of Single-Twist Trilayer Graphene

Small-twist-angle bilayer graphene supports strongly correlated insulating states and superconductivity. Twisted few-layer graphene systems are likely to open up new directions for strong correlation physics in moiré superlattices. We derive and study moiré band models that describe the electronic structure of graphene trilayers in which one of the three layers is twisted by a small angle relative to perfect AAA, ABA, or ABC stacking arrangements. We find that the electronic structure depends very strongly on the starting stackings arrangement and on which layer is twisted. We identify ABA stacking with a middle-layer twist as a promising system for itinerant electron magnetism or even more robust superconductivity, because it exhibits both large and small velocity bands at energies near the Fermi level.

cond-mat.mtrl-sci↗

arXiv2019

Collective modes of Dirac and Weyl semimetals in strong magnetic fields

Dirac and Weyl semimetals provide a new example of three-dimensional electron gases which are sensitive to strong magnetic fields. In this paper we address their collective excitations in the extreme quantum limit in which the Hamiltonian can be projected to the $N=0$ Landau level. We show that the energy of the plasmon collective mode is not shifted by interactions, even though the quasiparticle bands are strongly renormalized, and that both optical and acoustic magnetoplasmons emerge from the particle-hole continuum when the chemical potential is not at the band-crossing point. The rare presence of a sharp acoustic magnetoplasmon excitation can be understood as a consequence of the unusual particle-hole excitation phase space of these pseudo-1D systems, and could prove valuable as a sample characterization tool. We comment on the relationship between the collective mode dispersion and the non-Fermi-liquid character of these interacting electron systems.

cond-mat.str-el↗

arXiv2019

Terahertz Faraday and Kerr rotation spectroscopy of Bi$_{1-x}$Sb$_x$ films in high magnetic fields up to 30 Tesla

We report results of terahertz Faraday and Kerr rotation spectroscopy measurements on thin films of $\text{Bi}_{1-x}\text{Sb}_{x}$, an alloy system that exhibits a semimetal-to-topological-insulator transition as the Sb composition $x$ increases. By using a single-shot time-domain terahertz spectroscopy setup combined with a table-top pulsed mini-coil magnet, we conducted measurements in magnetic fields up to 30~T, observing distinctly different behaviors between semimetallic ($x < 0.07$) and topological insulator ($x > 0.07$) samples. Faraday and Kerr rotation spectra for the semimetallic films showed a pronounced dip that blue-shifted with the magnetic field, whereas spectra for the topological insulator films were positive and featureless, increasing in amplitude with increasing magnetic field and eventually saturating at high fields ($>$20~T). Ellipticity spectra for the semimetallic films showed resonances, whereas the topological insulator films showed no detectable ellipticity. To explain these observations, we developed a theoretical model based on realistic band parameters and the Kubo formula for calculating the optical conductivity of Landau-quantized charge carriers. Our calculations quantitatively reproduced all experimental features, establishing that the Faraday and Kerr signals in the semimetallic films predominantly arise from bulk hole cyclotron resonances while the signals in the topological insulator films represent combined effects of surface carriers originating from multiple electron and hole pockets. These results demonstrate that the use of high magnetic fields in terahertz magnetopolarimetry, combined with detailed electronic structure and conductivity calculations, allows us to unambiguously identify and quantitatively determine unique contributions from different species of carriers of topological and nontopological nature in Bi$_{1-x}$Sb$_x$.

cond-mat.mes-hall↗

arXiv2019

Spin Hall effects without spin currents in magnetic insulators

The spin Hall effect (SHE) is normally discussed in terms of a spin current, which is ill-defined in strongly spin-orbit-coupled systems because of spin non-conservation. In this work we propose an alternative view of SHE phenomena by relating them to a spin analog of charge polarization induced by an electric field. The spin density polarization is most conveniently defined in insulators, which can have a SHE if they break time-reversal symmetry, i.e. if they are magnetic. The reciprocal of this SHE is a counterpart of the inverse SHE (ISHE), and is manifested in magnetic insulators as a charge polarization induced by a Zeeman field gradient. We use a modified Kane-Mele model to illustrate the magnetic spin Hall effect, and to discuss its bulk-boundary relationship.

cond-mat.mes-hall↗