arXiv subjects

Yafis Barlas

Publications and source records attributed to Yafis Barlas.

At least 37 records · Page 2Linked to original sources

arXiv2018

Topological Classification Table Implemented with Classical Passive Meta-Materials

Topological condensed matter systems from class A and class AII of the classification table have received classical electromagnetic and mechanical analogs and protected wave-guiding with such systems has been demonstrated experimentally. Here we introduce a map which generates classical analogs for all entries of the classification table, using only passive elements. Physical mechanical models are provided for all strong topological phases in dimension 2, as well as for three classes in dimension 3. This includes topological super-conducting phases, which have never been attempted with classical systems.

cond-mat.mes-hall↗

arXiv2018

Counter-propagating Fractional Hall states in mirror-symmetric Dirac semi-metals

The Landau bands of mirror symmetric 2D Dirac semi-metals (for example odd-layers of ABA-graphene) can be identified by their parity with respect to mirror symmetry. This symmetry facilitates a new class of counter-propagating Hall states at opposite but equal electron and hole filling factors $|ν_{\pm}|=1/m$ ({\it m} odd). Here, we propose a Laughlin-like correlated liquid wavefunction, at the charge neutrality point, that exhibits fractionally charged quasi-particle/hole pair excitation of opposite parity. Using a bosonized one-dimensional edge state theory, we show that the longitudinal conductance of this state, $σ_{xx} = 2e^2/(m h)$, is robust to short-ranged inter-mode interactions.

cond-mat.mes-hall↗

arXiv2018

Gate controlled Majorana Zero Modes on 2D heterostructures

Half-integer conductance, the signature of Majorana edge modes, has been recently observed in a quantum anomalous Hall insulator/superconductor heterostructure. Here, we analyze a scheme for gate-tunable control of degenerate ground states of Majorana zero modes (MZM) in thin film topological superconductors. Gating the top surface of a thin film magnetic topological insulator controls the topological phase in the region underneath the gate. The voltage of the transition depends on the gate width, and narrower gates require larger voltages. Relatively long gates are required, on the order of a micron, to prevent hybridization of the end modes and to allow the creation of MZMs at low gate voltages. Applying a voltage to T{shaped and I{shaped gates localizes the Majoranas at their ends. This scheme may provide a facile method for implementing quantum gates for topological quantum computing.

cond-mat.mes-hall↗

arXiv2018

Spin superfluid Josephson oscillator

The magnetic analogue of the Josephson effect can be exploited to develop a new class of nano-spin oscillators that we denote as spin superfluid Josephson oscillators. Such a device, consisting of two exchange coupled easy-plane metallic ferromagnets separated by a thin normal metal spacer, is proposed and analyzed. A spin chemical potential difference drives a $2π$ precession of the in-plane magnetization of each ferromagnet. The $2 π$ precession angle gives maximum values of the giant magnetoresistance, resulting in large output power compared to conventional spin Hall oscillators. An applied ac current results in a time-averaged magnetoresistance with Shapiro-like steps. The multistate mode-locking behavior exhibited by the spin Shapiro steps may be explored for applications in neuromorphic computing. As an experimental characterization method, electrical measurements of spin superfluid Josephson junctions can provide additional signatures of spin superfluidity.

cond-mat.mes-hall↗

arXiv2018

Theory and Experimental Investigation of the Quantum Valley Hall Effect

The quantum valley Hall effect (QVHE) has been observed in a variety of experimental setups, both quantum and classical. While extremely promising for applications, one should be reminded that QVHE is not an exact topological phenomenon and that, so far, it has been fully understood only qualitatively in certain extreme limits. Here we present a technique to relate QVHE systems with exact quantum spin-Hall insulators that accept real-space representations, without taking any extreme limit. Since the bulk-boundary correspondence is well understood for the latter, we are able to formulate precise quantitative statements about the QVHE regime and its robustness against disorder. We further investigate the effect using a novel experimental platform based on magnetically coupled spinners. Visual renderings, quantitative data and various tests of the domain-wall modes are supplied, hence giving an unprecedented insight into the effect.

cond-mat.mes-hall↗

arXiv2018

Twist Angle-Dependent Bands and Valley Inversion in 2D Materials/hBN Heterostructures

The use of relative twist angle between adjacent atomic layers in a van der Waals heterostructure, has emerged as a new degree of freedom to tune electronic and optoelectronic properties of devices based on 2D materials. Using ABA-stacked trilayer (TLG) graphene as the model system, we show that, contrary to conventional wisdom, the band structures of 2D materials are systematically tunable depending on their relative alignment angle between hexagonal BN (hBN), even at very large twist angles. Moreover, addition or removal of the hBN substrate results in an inversion of the K and K' valley in TLG's lowest Landau level (LL). Our work illustrates the critical role played by substrates in van der Waals heterostructures and opens the door towards band structure modification and valley control via substrate and twist angle engineering.

cond-mat.mes-hall↗

arXiv2018

Tunable Lifshitz Transitions and Multiband Transport in Tetralayer Graphene

As the Fermi level and band structure of two-dimensional materials are readily tunable, they constitute an ideal platform for exploring Lifshitz transition, a change in the topology of a material's Fermi surface. Using tetralayer graphene that host two intersecting massive Dirac bands, we demonstrate multiple Lifshitz transitions and multiband transport, which manifest as non-monotonic dependence of conductivity on charge density n and out-of-plane electric fieldD, anomalous quantum Hall sequences and Landau level crossings that evolve with n, D and B.

cond-mat.mes-hall↗

arXiv2018

Long-Distance Spin Transport Through a Graphene Quantum Hall Antiferromagnet

Antiferromagnetic insulators (AFMI) are robust against stray fields, and their intrinsic dynamics could enable ultrafast magneto-optics and ultrascaled magnetic information processing. Low dissipation, long distance spin transport and electrical manipulation of antiferromagnetic order are much sought-after goals of spintronics research. Here, we report the first experimental evidence of robust long-distance spin transport through an AFMI, in our case the gate-controlled, canted antiferromagnetic (CAF) state that appears at the charge neutrality point of graphene in the presence of an external magnetic field. Utilizing gate-controlled quantum Hall (QH) edge states as spin-dependent injectors and detectors, we observe large, non-local electrical signals across a 5 micron-long, insulating channel only when it is biased into the nu=0 CAF state. Among possible transport mechanisms, spin superfluidity in an antiferromagnetic state gives the most consistent interpretation of the non-local signal's dependence on magnetic field, temperature and filling factors. This work also demonstrates that graphene in the QH regime is a powerful model system for fundamental studies of antiferromagnetic, and in the case of a large in-plane field, ferromagnetic spintronics.

cond-mat.mes-hall↗

arXiv2017

Exciton condensate in bilayer transition metal dichalcogenides: strong coupling regime

Exciton condensation in an electron-hole bilayer system of monolayer transition metal dichalcogenides is analyzed at three different levels of theory to account for screening and quasiparticle renormalization. The large effective masses of the transition metal dichalcogenides place them in a strong coupling regime. In this regime, mean field (MF) theory with either an unscreened or screened interlayer interaction predicts a room temperature condensate. Interlayer and intralayer interactions renormalize the quasiparticle dispersion, and this effect is included in a GW approximation. The renormalization reverses the trends predicted from the unscreened or screened MF theories. In the strong coupling regime, intralayer interactions have a large impact on the magnitude of the order parameter and its functional dependencies on effective mass and carrier density.

cond-mat.mes-hall↗

arXiv2016

Interlayer Transport through a Graphene / Rotated-Boron-Nitride / Graphene Heterostructure

Interlayer electron transport through a graphene / hexagonal boron-nitride (h-BN) / graphene heterostructure is strongly affected by the misorientation angle $θ$ of the h-BN with respect to the graphene layers with different physical mechanisms governing the transport in different regimes of angle, Fermi level, and bias. The different mechanisms and their resulting signatures in resistance and current are analyzed using two different models, a tight-binding, non-equilibrium Green function model and an effective continuum model, and the qualitative features resulting from the two different models compare well. In the large-angle regime ($θ> 4^\circ$), the change in the effective h-BN bandgap seen by an electron at the $K$ point of the graphene causes the resistance to monotonically increase with angle by several orders of magnitude reaching a maximum at $θ= 30^\circ$. It does not affect the peak-to-valley current ratios in devices that exhibit negative differential resistance. In the small-angle regime ($θ< 4^\circ$), Umklapp processes open up new conductance channels that manifest themselves as non-monotonic features in a plot of resistance versus Fermi level that can serve as experimental signatures of this effect. For small angles and high bias, the Umklapp processes give rise to two new current peaks on either side of the direct tunneling peak.

cond-mat.mtrl-sci↗

arXiv2016

Tunable symmetries of integer and fractional quantum Hall phases in heterostructures with multiple Dirac bands

The co-presence of multiple Dirac bands in few-layer graphene leads to a rich phase diagram in the quantum Hall regime. Using transport measurements, we map the phase diagram of BN-encapsulated ABA-stacked trilayer graphene as a function charge density n, magnetic field B and interlayer displacement field D, and observe transitions among states with different spin, valley, orbital and parity polarizations. Such rich pattern arises from crossings between Landau levels from different subbands, which reflect the evolving symmetries that are tunable in situ. At D=0, we observe fractional quantum Hall (FQH) states at filling factors 2/3 and -11/3. Unlike those in bilayer graphene, these FQH states are destabilized by a small interlayer potential that hybridizes the different Dirac bands.

cond-mat.mes-hall↗

arXiv2016

Spin Josephson effects in Exchange coupled Anti-ferromagnets

The energy of exchange coupled antiferromagnetic insulators (AFMIs) is a periodic function of the relative in-plane orientation of the Néel vector fields. We show that this leads to oscillations in the relative magnetization of exchange coupled AFMIs separated by a thin metallic barrier. These oscillations pump a spin current ($I_{S}$) through the metallic spacer that is proportional to the rate of change of the relative in-plane orientation of the Néel vector fields. By considering spin-transfer torque induced by a spin chemical potential ($V_{S}$) at one of the interfaces, we predict non-Ohmic $I_{S}$-$V_{S}$ characteristics of AFMI exchange coupled hetero-structures, which leads to a non-local voltage across a spin-orbit coupled metallic spacer.

cond-mat.mes-hall↗

arXiv2015

Proximity induced quantum anomalous Hall effect in graphene/EuO hetero-structures

In a heterostructure of graphene and the ferromagnetic insulator EuO, the Eu atoms induce proximity exchange and inter-valley interactions in the graphene layer. Constrained by the lattice symmetries, and guided by ab initio calculations, a model Hamiltonian is constructed that describes the low-energy bands. Band parameters such as proximity induced exchange splitting, spin orbit coupling, and inter-valley interaction are determined. Calculations of the Chern number identify the conditions under which the hetero-structures exhibit topologically non-trivial bands that give rise to the quantum anomalous Hall effect with a Hall conductivity of $σ_{xy} = 2 e^2/h$.

cond-mat.mtrl-sci↗

arXiv2015

Natural Regulation of Energy Flow in a Green Quantum Photocell

Manipulating the flow of energy in nanoscale and molecular photonic devices is of both fundamental interest and central importance for applications in light harvesting optoelectronics. Under erratic solar irradiance conditions, unregulated power fluctuations in a light harvesting photocell lead to inefficient energy storage in conventional solar cells and potentially fatal oxidative damage in photosynthesis. Here, we show that regulation against these fluctuations arises naturally within a two-channel quantum heat engine photocell, thus enabling the efficient conversion of varying incident solar spectrum at Earth's surface. Remarkably, absorption in the green portion of the spectrum is avoided, as it provides no inherent regulatory benefit. Our findings illuminate a quantum structural origin of regulation, provide a novel optoelectronic design strategy, and may elucidate the link between photoprotection in photosynthesis and the predominance of green plants on Earth.

physics.optics↗

arXiv2014

Gate Tunable Quantum Oscillations in Air-Stable and High Mobility Few-Layer Phosphorene Heterostructures

As the only non-carbon elemental layered allotrope, few-layer black phosphorus or phosphorene has emerged as a novel two-dimensional (2D) semiconductor with both high bulk mobility and a band gap. Here we report fabrication and transport measurements of phosphorene-hexagonal BN (hBN) heterostructures with one-dimensional (1D) edge contacts. These transistors are stable in ambient conditions for >300 hours, and display ambipolar behavior, a gate-dependent metal-insulator transition, and mobility up to 4000 $cm^2$/Vs. At low temperatures, we observe gate-tunable Shubnikov de Haas (SdH) magneto-oscillations and Zeeman splitting in magnetic field with an estimated g-factor ~2. The cyclotron mass of few-layer phosphorene holes is determined to increase from 0.25 to 0.31 $m_e$ as the Fermi level moves towards the valence band edge. Our results underscore the potential of few-layer phosphorene (FLP) as both a platform for novel 2D physics and an electronic material for semiconductor applications.

cond-mat.mes-hall↗

arXiv2014

Proximity-induced ferromagnetism in graphene revealed by anomalous Hall effect

We demonstrate the anomalous Hall effect (AHE) in single-layer graphene exchange-coupled to an atomically flat yttrium iron garnet (YIG) ferromagnetic thin film. The anomalous Hall conductance has magnitude of ~0.09(2e2/h) at low temperatures and is measurable up to ~ 300 K. Our observations indicate not only proximity-induced ferromagnetism in graphene/YIG with large exchange interaction, but also enhanced spin-orbit coupling which is believed to be inherently weak in ideal graphene. The proximity-induced ferromagnetic order in graphene can lead to novel transport phenomena such as the quantized AHE which are potentially useful for spintronics.

cond-mat.mes-hall↗

arXiv2014

Quasiparticle renormalization in ABC graphene trilayers

We investigate the effect of electron-electron interactions in ABC stacked graphene trilayers. In the gapless regime, we show that the self-energy corrections lead to the renormalization of the dynamical exponent $z=3+α_{1}/N$, with $α_{1}\approx0.52$ and $N$ is the number of fermionic species. Although the quasiparticle residue is suppressed near the neutrality point, the lifetime has a sublinear scaling with the energy and the quasiparticles are well defined even at zero energy. We calculate the renormalization of different physical observables, which can be directly measured in experiments.

cond-mat.mes-hall↗

arXiv2014

Adiabatic cooling of Majorana zero-modes in topological superconductors

We show that the presence of Majorana zero-modes in a one-dimensional topological superconductor can be detected by adiabatic cooling. This cooling effect results from an increase of the topological entropy associated with the ground state degeneracy due to such modes. Here, we consider an experimentally feasible topological superconductor: a strong spin-orbit coupled semiconducting quantum wire interfaced with an s-wave superconductor and subjected to a magnetic field. Numerical simulations of realistic experimental geometries indicate that the creation of Majorana zero-modes, which can be achieved by tuning of electronic gates or the external magnetic field, results in a measurable cooling effect. We also argue that this cooling effect results in an increase in the zero bias peak conductance.

cond-mat.mes-hall↗