arXiv subjects

Vladimir I. Fal'ko

Publications and source records attributed to Vladimir I. Fal'ko.

At least 73 records · Page 4Linked to original sources

arXiv2017

Cooling of chiral heat transport in the quantum Hall effect graphene

In the quantum Hall effect (QHE) regime, heat is carried by electrons in the edge states of Landau levels. Here, we study cooling of hot electrons propagating along the edge of graphene at the filling factor $ν=\pm2$, mediated by acoustic phonons. We determine the temperature profile extended from a hot spot, where the Hall current is injected into graphene from a metallic contact, taking into account specifics of boundary conditions for lattice displacements in graphene in a van der Waals heterostructure with an insulating substrate. Our calculations, performed using generic boundary conditions for Dirac electrons, show that emission of phonons can explain a short cooling length observed in graphene-based QHE devices by Nahm, Hwang and Lee [PRL 110, 226801 (2013)].

cond-mat.mes-hall↗

arXiv2017

Oscillating magnetoresistance in graphene p-n junctions at intermediate magnetic fields

We report on the observation of magnetoresistance oscillations in graphene p-n junctions. The oscillations have been observed for six samples, consisting of single-layer and bilayer graphene, and persist up to temperatures of 30 K, where standard Shubnikov-de Haas oscillations are no longer discernible. The oscillatory magnetoresistance can be reproduced by tight-binding simulations. We attribute this phenomenon to the modulated densities of states in the n- and p- regions.

cond-mat.mes-hall↗

arXiv2017

Dark trions and biexcitons in WS2 and WSe2 made bright by e-e scattering

The direct band gap character and large spin-orbit splitting of the valence band edges (at the K and K' valleys) in monolayer transition metal dichalcogenides have put these two-dimensional materials under the spot-light of intense experimental and theoretical studies. In particular, for Tungsten dichalcogenides it has been found that the sign of spin splitting of conduction band edges makes ground state excitons radiatively inactive (dark) due to spin and momentum mismatch between the constituent electron and hole. One might similarly assume that the ground states of charged excitons and biexcitons in these monolayers are also dark. Here, we show that the intervalley K$\leftrightarrows$K' electron-electron scattering mixes bright and dark states of these complexes, and estimate the radiative lifetimes in the ground states of these "semi-dark" trions and biexcitons to be ~ 10ps, and analyse how these complexes appear in the temperature-dependent photoluminescence spectra of WS2 and WSe2 monolayers.

cond-mat.mes-hall↗

arXiv2016

Auger recombination of dark excitons in ${\rm WS_2}$ and ${\rm WSe_2}$ monolayers

We propose a novel phonon assisted Auger process unique to the electronic band structure of monolayer transition metal dichalcogenides (TMDCs), which dominates the radiative recombination of ground state excitons in Tungsten based TMDCs. Using experimental and DFT computed values for the exciton energies, spin-orbit splittings, optical matrix element, and the Auger matrix elements, we find that the Auger process begins to dominate at carrier densities as low as $10^{9-10}~{\rm cm^{-2}}$, thus providing a plausible explanation for the low quantum efficiencies reported for these materials.

cond-mat.mes-hall↗

arXiv2016

Magnetic ratchet effect in bilayer graphene

We consider the orbital effect of an in-plane magnetic field on electrons in bilayer graphene, deriving linear-in-field contributions to the low-energy Hamiltonian arising from the presence of either skew interlayer coupling or interlayer potential asymmetry, the latter being tunable by an external metallic gate. To illustrate the relevance of such terms, we consider the ratchet effect in which a dc current results from the application of an alternating electric field in the presence of an in-plane magnetic field and inversion-symmetry breaking. By comparison with recent experimental observations in monolayer graphene [C. Drexler et al., Nature Nanotech. 8, 104 (2013)], we estimate that the effect in bilayer graphene can be two orders of magnitude greater than that in monolayer, illustrating that the bilayer is an ideal material for the realization of optoelectronic effects that rely on inversion-symmetry breaking.

cond-mat.mes-hall↗

arXiv2016

Edge effects in the magnetic interference pattern of a ballistic SNS junction

We investigate the Josephson critical current $I_c(Φ)$ of a wide superconductor-normal metal-superconductor (SNS) junction as a function of the magnetic flux $Φ$ threading it. Electronic trajectories reflected from the side edges alter the function $I_c(Φ)$ as compared to the conventional Fraunhofer-type dependence. At weak magnetic fields, $B\lesssim Φ_0/d^2$, the edge effect lifts zeros in $I_c(Φ)$ and gradually shifts the minima of that function toward half-integer multiples of the flux quantum. At $B>Φ_0/d^2$, the edge effect leads to an accelerated decay of the critical current $I_c(Φ)$ with increasing $Φ$. At larger fields, eventually, the system is expected to cross into a regime of "classical" mesoscopic fluctuations that is specific for wide ballistic SNS junctions with rough edges.

cond-mat.supr-con↗

arXiv2016

Ballistic miniband conduction in a graphene superlattice

Rational design of artificial lattices yields effects unavailable in simple solids, and vertical superlattices of multilayer semiconductors are already used in optical sensors and emitters. Manufacturing lateral superlattices remains a much bigger challenge, with new opportunities offered by the use of moire patterns in van der Waals heterostructures of graphene and hexagonal crystals such as boron nitride (h-BN). Experiments to date have elucidated the novel electronic structure of highly aligned graphene/h-BN heterostructures, where miniband edges and saddle points in the electronic dispersion can be reached by electrostatic gating. Here we investigate the dynamics of electrons in moire minibands by transverse electron focusing, a measurement of ballistic transport between adjacent local contacts in a magnetic field. At low temperatures, we observe caustics of skipping orbits extending over hundreds of superlattice periods, reversals of the cyclotron revolution for successive minibands, and breakdown of cyclotron motion near van Hove singularities. At high temperatures, we study the suppression of electron focusing by inelastic scattering.

cond-mat.mes-hall↗

arXiv2016

Valley and spin pump by scattering at non-magnetic disorders

In solid, the crystalline structure can endow electron an internal degree of freedom known as valley, which characterizes the degenerate energy minima in momentum space. The recent success in optical pumping of valley polarization in 2D transition metal dichalcogenides (TMDs) has greatly promoted the concept of valley-based informatics and electronics. However, between the demonstrated valley polarization of transient electron-hole pair excitations and practical valleytronic operations, there exist obvious gaps to fill, among which is the valley pump of long-lived charge carriers. Here we discover that the quested valley pump of electrons or holes can be realized simply by scattering at the ubiquitous nonmagnetic disorders, not relying on any specific material property. The mechanism is rooted in the nature of valley as a momentum space index: the intervalley backscattering in general has valley contrasted rate due to the distinct momentum transfers, causing a net transfer of population from one valley to another. As examples, we numerically demonstrate the sizable valley pump effects driven by charge current in nanoribbons of monolayer TMDs, where the spin-orbit scattering by non-magnetic disorders also realizes spin pump for the spin-valley locked holes. Our finding points to an unexpected new opportunity towards valley-spintronics, turning disorders from a deleterious factor to a resource of valley and spin polarization.

cond-mat.mes-hall↗

arXiv2016

Influence of spin dynamics of defects on weak localization in paramagnetic 2D metals

Spin-flip scattering of charge carriers in metals with magnetic defects leads to the low-temperature saturation of the decoherence time, $τ_φ$, of electrons at the value comparable to their spin relaxation time, $τ_s$. In two-dimensional (2D) conductors such a saturation can be lifted by an in-plane magnetic field, $B_\parallel$, which polarizes spins of scatterers without affecting orbital motion of free carriers. Here, we show that in 2D conductors with substantially different values of the g-factors of electrons ($g_e$) and magnetic defects ($g_i$), the decoherence time $τ_φ(B_\parallel)$ (reflected by the curvature of magnetoconductance) displays an anomaly: it first gets shorter, decaying on the scale $B_\parallel\sim \hbar/|g_i-g_e|μ_B τ_s$, before becoming longer at higher values of $B_\parallel$.

cond-mat.mes-hall↗

arXiv2015

Hierarchy of gaps and magnetic minibands in graphene in the presence of the Abrikosov vortex lattice

We determine the structure of band and gaps in graphene encapsulated in hexagonal boron nitride and subjected to magnetic field of Abrikosov lattice of vortices in the underlying superconducting film. The spectrum features one non-dispersive magnetic miniband at zero energy, separated by the largest gaps in the miniband spectrum from a pair of minibands resembling slightly broadened first Landau levels in graphene, suggesting the persistence of $ν= \pm 2$ quantum Hall effect states. Also, we identify occasional merging point of magnetic minibands which feature Dirac-type dispersion at the consecutive miniband edges.

cond-mat.mes-hall↗

arXiv2014

Spin-valley relaxation and quantum transport regimes in two-dimensional transition metal dichalcogenides

Quantum transport and spintronics regimes are studied in p- and n-doped atomic layers of hexagonal transition metal dichalcogenides (TMDCs), subject to the interplay between the valley structure and spin-orbit coupling. We find how spin relaxation of carriers depends on their areal density and show that it vanishes for holes near the band edge, leading to the density-independent spin diffusion length, and we develop a theory of weak localisation/antilocalisation, describing the crossovers between the orthogonal, double-unitary and symplectic regimes of quantum transport in TMDCs.

cond-mat.mes-hall↗

arXiv2014

Anomalous sequence of quantum Hall liquids revealing tunable Lifshitz transition in bilayer graphene

Fermi surface topology plays an important role in determining the electronic properties of metals. In bulk metals, the Fermi energy is not easily tunable at the energy scale needed for reaching conditions for the Lifshitz transition - a singular point in the band structure where the connectivity of the Fermi surface changes. Bilayer graphene is a unique system where both Fermi energy and the low-energy electron dispersion can be tuned using the interplay between trigonal warping and a band gap opened by a transverse electric field. Here, we drive the Lifshitz transition to experimentally controllable carrier densities by applying large transverse electric fields through a h-BN-encapsulated bilayer graphene structure, and detect it by measuring the degeneracies of Landau levels. These degeneracies are revealed by filling factor -3 and -6 quantum Hall effect states of holes at low magnetic fields reflecting the existence of three maxima on the top of the valence band dispersion. At high magnetic fields all integer quantum Hall states are observed, indicating that deeper in the valence band the constant energy contours are singly-connected. The fact that we observe ferromagnetic quantum Hall states at odd-integer filling factors testifies to the high quality of our sample, and this enables us to identify several phase transitions between correlated quantum Hall states at intermediate magnetic fields, in agreement with the calculated evolution of the Landau level spectrum.

cond-mat.mes-hall↗

arXiv2014

Strain-induced modifications of transport in gated graphene nanoribbons

We investigate the effects of homogeneous and inhomogeneous deformations and edge disorder on the conductance of gated graphene nanoribbons. Under increasing homogeneous strain the conductance of such devices initially decreases before it acquires a resonance structure, and finally becomes completely suppressed at larger strain. Edge disorder induces mode mixing in the contact regions, which can restore the conductance to its ballistic value. The valley-antisymmetric pseudo-magnetic field induced by inhomogeneous deformations leads to the formation of additional resonance states, which either originate from the coupling into Fabry-Perot states that extend through the system, or from the formation of states that are localized near the contacts, where the pseudo-magnetic field is largest. In particular, the n=0 pseudo-Landau level manifests itself via two groups of conductance resonances close to the charge neutrality point.

cond-mat.mes-hall↗

arXiv2014

Moire pattern as a magnifying glass for strain and dislocations in van der Waals heterostructures

We consider the role of deformations in graphene heterostructures with hexagonal crystals (including strain, wrinkles and dislocations) on the geometrical properties of moire patterns characteristic for a pair of two incommensurate misaligned isostructural crystals. By, employing a phenomenological theory to describe generic moire perturbation in van der Waals heterostructures of graphene and hexagonal crystals we investigate the electronic properties of such heterostructure.

cond-mat.mes-hall↗

arXiv2014

Charge-density-wave states in double-layer graphene structures in a high magnetic field

We study the phases of correlated charge-density waves that form at a high magnetic field in two parallel graphene flakes separated by a thin insulator. The predicted phases include the square and hexagonal charge-density-wave bubbles, and a quasi-one-dimensional stripe phase. We find that the transition temperature for such phases is within the experimentally accessible range and that formation of interlayer-correlated states produces a negative compressibility contribution to the differential capacitance of this system.

cond-mat.mes-hall↗

arXiv2014

Observation of even denominator fractional quantum Hall effect in suspended bilayer graphene

We investigate low-temperature magneto-transport in recently developed, high-quality multi-terminal suspended bilayer graphene devices, enabling the independent measurement of the longitudinal and transverse resistance. We observe clear signatures of the fractional quantum Hall effect, with different states that are either fully developed, and exhibit a clear plateau in the transverse resistance with a concomitant dip in longitudinal resistance, or incipient, and exhibit only a longitudinal resistance minimum. All observed states scale as a function of filling factor nu, as expected. An unprecedented even-denominator fractional state is observed at nu = -1/2 on the hole side, exhibiting a clear plateau in Rxy quantized at the expected value of 2h/e^2 with a precision of ~0.5%. Many of our observations, together with a recent electronic compressibility measurement performed in graphene bilayers on hexagonal boron-nitride (hBN) substrates, are consistent with a recent theory that accounts for the effect of the degeneracy between the N=0 and N=1 Landau levels in the fractional quantum Hall effect, and predicts the occurrence of a Moore-Read type nu = -1/2 state. Owing to the experimental flexibility of bilayer graphene --which has a gate-dependent band structure, can be easily accessed by scanning probes, and can be contacted with materials such as superconductors--, our findings offer new possibilities to explore the microscopic nature of even-denominator fractional quantum Hall effect.

cond-mat.mes-hall↗

arXiv2013

Infrared absorption by graphene-hBN heterostructures

We propose a theory of optical absorption in monolayer graphene-hexagonal boron nitride (hBN) heterostructures. In highly oriented heterostructures, the hBN underlay produces a long-range moiré superlattice potential for the graphene electrons which modifies the selection rules for absorption of incoming photons in the infrared to visible frequency range. The details of the absorption spectrum modification depend on the relative strength of the various symmetry-allowed couplings between the graphene electrons and the hBN, and the resulting nature of the reconstructed band structure.

cond-mat.mes-hall↗

arXiv2013

Monolayer MoS2: trigonal warping, "Γ-valley", and spin-orbit coupling effects

We use a combined ab-initio calculations and k.p theory based approach to derive a low-energy effective Hamiltonian for monolayer MoS2 at the K point of the Brillouin zone. It captures the features which are present in first-principles calculations but not explained by the theory of Xiao et al. [Phys Rev Lett 108, 196802 (2012)], namely the trigonal warping of the valence and conduction bands, the electron-hole symmetry breaking, and the spin-splitting of the conduction band. We also consider other points in the Brillouin zone which might be important for transport properties. Our findings lead to a more quantitative understanding of the properties of this material in the ballistic limit.

cond-mat.mes-hall↗