arXiv subjects

Dmitry Smirnov

Publications and source records attributed to Dmitry Smirnov.

72 records · Page 4Linked to original sources

arXiv2018

Integer and Fractional Quantum Hall effect in Ultra-high Quality Few-layer Black Phosphorus Transistors

As a high mobility two-dimensional semiconductor with strong structural and electronic anisotropy, atomically thin black phosphorus (BP) provides a new playground for investigating the quantum Hall (QH) effect, including outstanding questions such as the functional dependence of Landau level (LL) gaps on magnetic field B, and possible anisotropic fractional QH states. Using encapsulating few-layer BP transistors with mobility up to 55,000 cm2/Vs, we extract LL gaps over an exceptionally wide range of B for QH states at filling factors ν=-1 to -4, which are determined to be linear in B, thus resolving a controversy raised by its anisotropy. Furthermore, a fractional QH state at ν~ -4/3 and an additional feature at -0.56+/- 0.1 are observed, underscoring BP as a tunable 2D platform for exploring electron interactions.

cond-mat.mes-hall↗

arXiv2018

Large tunable intrinsic gap in rhombohedral-stacked tetralayer graphene

In rhombohedral-stacked few-layer graphene, the very flat energy bands near the charge neutrality point are unstable to electronic interactions, giving rise to states with spontaneous broken symmetries. Using transport measurements on suspended rhombohedral-stacked tetralayer graphene, we observe an insulating ground state with a large interaction-induced gap up to 80 meV. This gapped state can be enhanced by a perpendicular magnetic field, and suppressed by an interlayer potential, carrier density, or a critical temperature of ~ 40 K.

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

Metamorphic InAs1-xSbx/InAs1-ySby superlattices with ultra-low bandgap as a Dirac material

It was experimentally demonstrated that short-period metamorphic InAs1-xSbx/InAs1-ySby superlattices with ultra low bandgap have properties of a Dirac material. Cyclotron resonance and interband magneto-absorption peaks in superlattices with ultra-low bandgaps demonstrate a square root dependence on the magnetic field for a range up to 16 T (energy range up to 300meV). This directly indicates the linearity of the electron dispersion. The Fermi velocity can be controlled by varying the overlap between electron and hole states in the superlattice. The dependence of the cyclotron resonance energy on the magnetic field parallel to the superlattice plane demonstrates that the electron dispersion in the growth direction can be characterized by an effective mass of 0.028m0 in a superlattice with a period of 6 nm and 0.045m0 in a superlattice with a period of 7.5 nm. Extreme design flexibility makes the short-period metamorphic InAs1-xSbx/InAs1-ySby superlattice a new prospective platform for studying the effects of charge carrier chirality and topologically nontrivial states in structures with the inverted bandgaps.

cond-mat.mtrl-sci↗

arXiv2017

Surface Transport and Quantum Hall Effect in Ambipolar Black Phosphorus Double Quantum Wells

Quantum wells constitute one of the most important classes of devices in the study of 2D systems. In a double layer QW, the additional "which-layer" degree of freedom gives rise to celebrated phenomena such as Coulomb drag, Hall drag and exciton condensation. Here we demonstrate facile formation of wide QWs in few-layer black phosphorus devices that host double layers of charge carriers. In contrast to tradition QWs, each 2D layer is ambipolar, and can be tuned into n-doped, p-doped or intrinsic regimes. Fully spin-polarized quantum Hall states are observed on each layer, with enhanced Lande g-factor that is attributed to exchange interactions. Our work opens the door for using 2D semiconductors as ambipolar single, double or wide QWs with unusual properties such as high anisotropy.

cond-mat.mes-hall↗

arXiv2016

Magnetic brightening and control of dark excitons in monolayer WSe2

Monolayer transition metal dichalcogenide (TMDC) crystals, as direct-gap materials with unusually strong light-matter interaction, have attracted much recent attention. In contrast to the initial understanding, the minima of the conduction band are predicted to be spin split. Because of this splitting and the spin-polarized character of the valence bands, the lowest-lying excitonic states in WX2 (X=S, Se) are expected to be spin-forbidden and optically dark. To date, however, there has been no direct experimental probe of these dark band-edge excitons, which strongly influence the light emission properties of the material. Here we show how an in-plane magnetic field can brighten the dark excitonic states and allow their properties to be revealed experimentally in monolayer WSe2. In particular, precise energy levels for both the neutral and charged dark excitons were obtained and compared with ab-initio calculations using the GW-BSE approach. Greatly increased emission and valley lifetimes were observed for the brightened dark states as a result of their spin configuration. These studies directly probe the excitonic spin manifold and provide a new route to tune the optical and valley properties of these prototypical two-dimensional semiconductors.

cond-mat.mes-hall↗

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

Energy gaps and layer polarization of integer and fractional quantum Hall states in bilayer graphene

Owing to the spin, valley, and orbital symmetries, the lowest Landau level (LL) in bilayer graphene exhibits multicomponent quantum Hall ferromagnetism. Using transport spectroscopy, we investigate the energy gaps of integer and fractional quantum Hall states in bilayer graphene with controlled layer polarization. The state at filling factor ν=1 has two distinct phases: a layer polarized state that has a larger energy gap and is stabilized by high electric field, and a hitherto unobserved interlayer coherent state with a smaller gap that is stabilized by large magnetic field. In contrast, the ν=2/3 quantum Hall state and a feature at ν=1/2 are only resolved at finite electric field and large magnetic field. These results underscore the importance of controlling layer polarization in understanding the competing symmetries in the unusual QH system of BLG.

cond-mat.mes-hall↗

arXiv2015

Pronounced photovoltaic response from multi-layered transition-metal dichalcogenides PN-junctions

Transition metal dichalcogenides (TMDs) are layered semiconductors with indirect band gaps comparable to Si. These compounds can be grown in large area, while their gap(s) can be tuned by changing their chemical composition or by applying a gate voltage. The experimental evidence collected so far, points towards a strong interaction with light, which contrasts with the small photovoltaic efficiencies $η\geq 1$ % extracted from bulk crystals or exfoliated monolayers. Here, we evaluate the potential of these compounds by studying the photovoltaic response of electrostatically generated PN-junctions composed of approximately ten atomic-layers of MoSe$_2$ stacked onto the dielectric $h$-BN. In addition to ideal diode-like response, we find that these junctions can yield, under AM-1.5 illumination, photovoltaic efficiencies $η$ exceeding 14 %, with fill-factors of ~ 70 %. Given the available strategies for increasing $η$ such as gap tuning, improving the quality of the electrical contacts, or the fabrication of tandem cells, our study suggests a remarkable potential for photovoltaic applications based on TMDs.

cond-mat.mes-hall↗

arXiv2015

Metal to insulator quantum-phase transition in few-layered ReS$_2$

In ReS$_2$ a layer-independent direct band-gap of 1.5 eV implies a potential for its use in optoelectronic applications. ReS$_2$ crystallizes in the 1T$^{\prime}$-structure which leads to anisotropic physical properties and whose concomitant electronic structure might host a non-trivial topology. Here, we report an overall evaluation of the anisotropic Raman response and the transport properties of few-layered ReS$_2$ field-effect transistors. We find that ReS$_2$ exfoliated on SiO$_2$ behaves as an $n$-type semiconductor with an intrinsic carrier mobility surpassing $μ_i$ ~30 cm$^2$/Vs at $T = 300$ K which increases up to ~350 cm$^2$/Vs at 2 K. Semiconducting behavior is observed at low electron densities $n$, but at high values of n the resistivity decreases by a factor > 7 upon cooling to 2 K and displays a metallic $T^2$-dependence. This indicates that the band structure of 1T$^{\prime}$-ReS$_2$ is quite susceptible to an electric field applied perpendicularly to the layers. The electric-field induced metallic state observed in transition metal dichalcogenides was recently claimed to result from a percolation type of transition. Instead, through a scaling analysis of the conductivity as a function of $T$ and $n$, we find that the metallic state of ReS$_2$ results from a second-order metal to insulator transition driven by electronic correlations. This gate-induced metallic state offers an alternative to phase engineering for producing ohmic contacts and metallic interconnects in devices based on transition metal dichalcogenides.

cond-mat.mes-hall↗

arXiv2015

Optoelectronic switch based on intrinsic dual Schottky diodes in ambipolar MoSe$_2$ field-effect transistors

Here, we report the observation of a hitherto unreported optoelectronic effect, namely a light-induced diode-like response in multi-layered MoSe$_2$ field-effect transistors whose sense of current rectification is controllable through a gate voltage. We argue, through numerical simulations, that this behavior results from the difference in the size of the Schottky barriers between drain and source metal contacts. Each barrier can be modeled as a Schottky diode but with opposite senses of current rectification between them, with the diode response resulting from the light induced promotion of photo-generated carriers across the smaller barrier. The back gate voltage controls the sense of current rectification by modulating the relative amplitude between them. This effect, which gives rise to a novel type of optoelectronic switch, also yields a photovoltaic response. Hence, it could provide an alternative to PN-junctions when harvesting photovoltaic currents from transition metal dichalcogenides. We argue that the photovoltaic efficiency associated to this effect could be increased by just increasing the relative asymmetry between both Schottky barriers. We also suggest that this new electro-optical effect has potential for technological applications.

cond-mat.mes-hall↗

arXiv2015

High Photoresponsivity and Short Photo Response Times in Few-Layered WSe$_2$ Transistors

Here, we report the photoconducting response of field-effect transistors based on three atomic layers of chemical vapor transport grown WSe$_2$ crystals mechanically exfoliated onto SiO$_2$. We find that tri-layered WSe$_2$ field-effect transistors, built with the simplest possible architecture, can display high hole mobilities ranging from 350 cm$^2$/Vs at room temperature (saturating at a value of ~500 cm$^2$/Vs below 50 K) displaying a strong photocurrent response which leads to exceptionally high photo responsivities up to 7 A/W under white light illumination of the entire channel for power densities p < 10$^2$ W/m$^2$. Under a fixed wavelength of $λ$ = 532 nm and a laser spot size smaller than the conducting channel area we extract photo responsitivities approaching 100 mA/W with concomitantly high external quantum efficiencies up to ~ 40 % at room temperature. These values surpass values recently reported from more complex architectures, such as graphene and transition metal dichalcogenides based heterostructures. Also, tri-layered WSe$_2$ photo-transistors display photo response times in the order of 10 microseconds. Our results indicate that the addition of a few atomic layers considerably decreases the photo response times, probably by minimizing the interaction with the substrates, while maintaining a very high photo-responsivity.

cond-mat.mes-hall↗

arXiv2014

Valley Splitting and Polarization by the Zeeman Effect in Monolayer MoSe2

We have measured circularly polarized photoluminescence in monolayer MoSe2 under perpendicular magnetic fields up to 10 T. At low doping densities, the neutral and charged excitons shift linearly with field strength at a rate of $\mp$ 0.12 meV/T for emission arising, respectively, from the K and K' valleys. The opposite sign for emission from different valleys demonstrates lifting of the valley degeneracy. The magnitude of the Zeeman shift agrees with predicted magnetic moments for carriers in the conduction and valence bands. The relative intensity of neutral and charged exciton emission is modified by the magnetic field, reflecting the creation of field-induced valley polarization. At high doping levels, the Zeeman shift of the charged exciton increases to $\mp$ 0.18 meV/T. This enhancement is attributed to many-body effects on the binding energy of the charged excitons.

cond-mat.mes-hall↗

arXiv2014

Distinct Competing Ordered ν=2 States in Bilayer Graphene

Because of its large density-of-states and the 2π Berry phase near its low-energy band-contact points, neutral bilayer graphene (BLG) at zero magnetic field (B) is susceptible to chiral-symmetry breaking, leading to a variety of gapped spontaneous quantum Hall states distinguished by valley and spin-dependent quantized Hall conductivities. Among these, the layer antiferromagnetic state, which has quantum valley Hall (QVH) effects of opposite sign for opposite spins, appears to be the thermodynamic ground state. Though other gapped states have not been observed experimentally at B=0, they can be explored by exploiting their adiabatic connection to quantum Hall states with the same total Hall conductivity σH. In this paper, by using a magnetic field to select filling factor ν=2 states with σH=2e^2/h, we demonstrate the presence of a quantum anomalous Hall (QAH) state for the majority spin, and a Kekulé state with spontaneous valley coherence and a quantum valley Hall state for the minority spin in BLG. By providing the first spectroscopic mapping of spontaneous Hall states at ν=2, our results shed further light on the rich set of competing ordered states in BLG.

cond-mat.mes-hall↗

arXiv2012

Broken Symmetry Quantum Hall States in Dual Gated ABA Trilayer Graphene

We present low temperature transport measurements on dual-gated suspended trilayer graphene in the quantum Hall (QH) regime. We observe QH plateaus at filling factors ν=-8, -2, 2, 6, and 10, in agreement with the full-parameter tight binding calculations. In high magnetic fields, odd-integer plateaus are also resolved, indicating almost complete lifting of the 12-fold degeneracy of the lowest Landau levels (LL). Under an out-of-plane electric field E, we observe degeneracy breaking and transitions between QH plateaus. Interestingly, depending on its direction, E selectively breaks the LL degeneracies in the electron-doped or hole-doped regimes. Our results underscore the rich interaction-induced phenomena in trilayer graphene.

cond-mat.mes-hall↗

arXiv2012

Minimum Conductivity and Evidence for Phase Transitions in Ultra-clean Bilayer Graphene

Bilayer graphene (BLG) at the charge neutrality point (CNP) is strongly susceptible to electronic interactions, and expected to undergo a phase transition into a state with spontaneous broken symmetries. By systematically investigating a large number of singly- and doubly-gated bilayer graphene (BLG) devices, we show that an insulating state appears only in devices with high mobility and low extrinsic doping. This insulating state has an associated transition temperature Tc~5K and an energy gap of ~3 meV, thus strongly suggesting a gapped broken symmetry state that is destroyed by very weak disorder. The transition to the intrinsic broken symmetry state can be tuned by disorder, out-of-plane electric field, or carrier density.

cond-mat.mes-hall↗

arXiv2010

Magnetoconductance Oscillations in High-Mobility Suspended Bilayer and Trilayer Graphene

We report pronounced magnetoconductance oscillations observed on suspended bilayer and trilayer graphene devices with mobilities up to 270,000 cm2/Vs. For bilayer devices, we observe conductance minima at all integer filling factors nu between 0 and -8, as well as a small plateau at ν=1/3. For trilayer devices, we observe features at nu=-1, -2, -3 and -4, and at ν~0.5 that persist to 4.5K at B=8T. All of these features persist for all accessible values of Vg and B, and could suggest the onset of symmetry breaking of the first few Landau (LL) levels and fractional quantum Hall states.

cond-mat.mes-hall↗

arXiv2007

Magnetically Induced Field Effect in Carbon Nanotube Devices

Three-terminal devices with conduction channels formed by quasi-metallic carbon nanotubes (CNT) are shown to operate as nanotube-based field-effect transistors under strong magnetic fields. The off-state conductance of the devices varies exponentially with the magnetic flux intensity. We extract the quasi-metallic CNT chirality as well as the characteristics of the Schottky barriers formed at the metal-nanotube contacts from temperature-dependent magnetoconductance measurements.

cond-mat.mes-hall↗