arXiv subjects

Jyoti Katoch

Publications and source records attributed to Jyoti Katoch.

33 records · Page 2Linked to original sources

arXiv2019

Imaging microscopic electronic contrasts at the interface of single-layer WS$_2$ with oxide and boron nitride substrates

The electronic properties of devices based on two-dimensional materials are significantly influenced by interactions with substrate and electrode materials. Here, we use photoemission electron microscopy to investigate the real- and momentum-space electronic structures of electrically contacted single-layer WS$_2$ stacked on hBN, SiO$_2$ and TiO$_2$ substrates. Using work function and X-ray absorption imaging we single-out clean microscopic regions of each interface type and collect the valence band dispersion. We infer the alignments of the electronic band gaps and electron affinities from the measured valence band offsets of WS$_2$ and the three substrate materials using a simple electron affinity rule and discuss the implications for vertical band structure engineering using mixed three- and two-dimensional materials.

cond-mat.mtrl-sci↗

arXiv2018

Transport Spectroscopy of Sublattice-Resolved Resonant Scattering in Hydrogen-Doped Bilayer Graphene

We report the experimental observation of sublattice-resolved resonant scattering in bilayer graphene by performing simultaneous cryogenic atomic hydrogen doping and electron transport measurements in ultrahigh vacuum. This allows us to monitor the hydrogen adsorption on the different sublattices of bilayer graphene without atomic-scale microscopy. Specifically, we detect two distinct resonant scattering peaks in the gate-dependent resistance, which evolve as a function of atomic hydrogen dosage. Theoretical calculations show that one of the peaks originates from resonant scattering by hydrogen adatoms on the α-sublattice (dimer site) while the other originates from hydrogen adatoms on the \b{eta}-sublattice (non-dimer site), thereby enabling a method for characterizing the relative sublattice occupancy via transport measurements. Utilizing this new capability, we investigate the adsorption and thermal desorption of hydrogen adatoms via controlled annealing and conclude that hydrogen adsorption on the \b{eta}-sublattice is energetically favored. Through site-selective desorption from the α-sublattice, we realize hydrogen doping with adatoms primarily on a single sublattice, which is highly desired for generating ferromagnetism.

cond-mat.mes-hall↗

arXiv2018

Electronic Structure of Exfoliated and Epitaxial Hexagonal Boron Nitride

Hexagonal boron nitride (hBN) is an essential component in van der Waals heterostructures as it provides high quality and weakly interacting interfaces that preserve the electronic properties of adjacent materials. While exfoliated flakes of hBN have been extensively studied using electron transport and optical probes, detailed experimental measurements of the energy- and momentum- dependent electronic excitation spectrum are lacking. Here, we directly determine the full valence band (VB) electronic structure of micron-sized exfoliated flakes of hBN using angle-resolved photoemission spectroscopy with micrometer spatial resolution. We identify the π- and σ-band dispersions, the hBN stacking order and determine a total VB bandwidth of 19.4 eV. We compare these results with electronic structure data for epitaxial hBN on graphene on silicon carbide grown in situ using a borazine precursor. The epitaxial growth and electronic properties are investigated using photoe- mission electron microscopy. Our measurements show that the fundamental electronic properties of hBN are highly dependent on the fabrication strategy.

cond-mat.mtrl-sci↗

arXiv2018

Probing Tunneling Spin Injection into Graphene via Bias Dependence

The bias dependence of spin injection in graphene lateral spin valves is systematically studied to determine the factors affecting the tunneling spin injection efficiency. Three types of junctions are investigated, including MgO and hexagonal boron nitride (hBN) tunnel barriers and direct contacts. A DC bias current applied to the injector electrode induces a strong nonlinear bias dependence of the nonlocal spin signal for both MgO and hBN tunnel barriers. Furthermore, this signal reverses its sign at a negative DC bias for both kinds of tunnel barriers. The analysis of the bias dependence for injector electrodes with a wide range of contact resistances suggests that the sign reversal correlates with bias voltage rather than current. We consider different mechanisms for nonlinear bias dependence and conclude that the energy-dependent spin-polarized electronic structure of the ferromagnetic electrodes, rather than the electrical field-induced spin drift effect or spin filtering effect of the tunnel barrier, is the most likely explanation of the experimental observations.

cond-mat.mes-hall↗

arXiv2018

Spin inversion in graphene spin valves by gate-tunable magnetic proximity effect at one-dimensional contacts

Graphene has remarkable opportunities for spintronics due to its high mobility and long spin diffusion length, especially when encapsulated in hexagonal boron nitride (h-BN). Here, for the first time, we demonstrate gate-tunable spin transport in such encapsulated graphene-based spin valves with one-dimensional (1D) ferromagnetic edge contacts. An electrostatic backgate tunes the Fermi level of graphene to probe different energy levels of the spin-polarized density of states (DOS) of the 1D ferromagnetic contact, which interact through a magnetic proximity effect (MPE) that induces ferromagnetism in graphene. In contrast to conventional spin valves, where switching between high- and low-resistance configuration requires magnetization reversal by an applied magnetic field or a high-density spin-polarized current, we provide an alternative path with the gate-controlled spin inversion in graphene. The resulting tunable MPE employing a simple ferromagnetic metal holds promise for spintronic devices and to realize exotic topological states, from quantum spin Hall and quantum anomalous Hall effects, to Majorana fermions and skyrmions.

cond-mat.mes-hall↗

arXiv2018

Giant spin-splitting and gap renormalization driven by trions in single-layer WS$_2$/h-BN heterostructures

In two-dimensional (2D) semiconducting transition metal dichalcogenides (TMDs), new electronic phenomena such as tunable band gaps and strongly bound excitons and trions emerge from strong many-body effects, beyond spin-orbit coupling- and lattice symmetry-induced spin and valley degrees of freedom. Combining single-layer (SL) TMDs with other 2D materials in van der Waals heterostructures offers an intriguing means of controlling the electronic properties through these many-body effects via engineered interlayer interactions. Here, we employ micro-focused angle-resolved photoemission spectroscopy (microARPES) and in-situ surface doping to manipulate the electronic structure of SL WS$_2$ on hexagonal boron nitride (WS$_2$/h-BN). Upon electron doping, we observe an unexpected giant renormalization of the SL WS$_2$ valence band (VB) spin-orbit splitting from 430~meV to 660~meV, together with a band gap reduction of at least 325~meV, attributed to the formation of trionic quasiparticles. These findings suggest that the electronic, spintronic and excitonic properties are widely tunable in 2D TMD/h-BN heterostructures, as these are intimately linked to the quasiparticle dynamics of the materials.

cond-mat.mtrl-sci↗

arXiv2017

NaSn2As2: An Exfoliatable Layered van der Waals Zintl Phase

The discovery of new families of exfoliatable 2D crystals that have diverse sets of electronic, optical, and spin-orbit coupling properties, enables the realization of unique physical phenomena in these few-atom thick building blocks and in proximity to other materials. Herein, using NaSn2As2 as a model system, we demonstrate that layered Zintl phases having the stoichiometry ATt2Pn2 (A = Group 1 or 2 element, Tt = Group 14 tetrel element and Pn = Group 15 pnictogen element) and feature networks separated by van der Waals gaps can be readily exfoliated with both mechanical and liquid-phase methods. We identified the symmetries of the Raman active modes of the bulk crystals via polarized Raman spectroscopy. The bulk and mechanically exfoliated NaSn2As2 samples are resistant towards oxidation, with only the top surface oxidizing in ambient conditions over a couple of days, while the liquid-exfoliated samples oxidize much more quickly in ambient conditions. Employing angle-resolved photoemission spectroscopy (ARPES), density functional theory (DFT), and transport on bulk and exfoliated samples, we show that NaSn2As2 is a highly conducting 2D semimetal, with resistivities on the order of 10-6 Ω m. Due to peculiarities in the band structure, the dominating p-type carriers at low temperature are nearly compensated by the opening of n-type conduction channels as temperature increases. This work further expands the family of exfoliatable 2D materials to layered van der Waals Zintl phases, opening up opportunities in electronics and spintronics.

cond-mat.mtrl-sci↗

arXiv2017

Strong modulation of spin currents in bilayer graphene by static and fluctuating proximity exchange fields

Two dimensional (2D) materials provide a unique platform to explore the full potential of magnetic proximity driven phenomena, which can be further used for applications in next generation spintronic devices. Of particular interest is to understand and control spin currents in graphene by the magnetic exchange field of a nearby ferromagnetic material in graphene/ferromagnetic-insulator (FMI) heterostructures. Here, we present the experimental study showing the strong modulation of spin currents in graphene layers by controlling the direction of the exchange field due to FMI magnetization. Owing to clean interfaces, a strong magnetic exchange coupling leads to the experimental observation of complete spin modulation at low externally applied magnetic fields in short graphene channels. Additionally, we discover that the graphene spin current can be fully dephased by randomly fluctuating exchange fields. This is manifested as an unusually strong temperature dependence of the non-local spin signals in graphene, which is due to spin relaxation by thermally-induced transverse fluctuations of the FMI magnetization.

cond-mat.mes-hall↗

arXiv2016

Spatially Resolved Electronic Properties of Single-Layer WS$_2$ on Transition Metal Oxides

There is a substantial interest in the heterostructures of semiconducting transition metal dichalcogenides (TMDCs) amongst each other or with arbitrary materials, through which the control of the chemical, structural, electronic, spintronic, and optical properties can lead to a change in device paradigms. A critical need is to understand the interface between TMDCs and insulating substrates, for example high-$κ$ dielectrics, which can strongly impact the electronic properties such as the optical gap. Here we show that the chemical and electronic properties of the single-layer (SL) TMDC, WS$_2$, can be transferred onto high-$κ$ transition metal oxide substrates TiO$_2$ and SrTiO$_3$. The resulting samples are much more suitable for measuring their electronic and chemical structures with angle-resolved photoemission than their native-grown SiO$_2$ substrates. We probe the WS$_2$ on the micron scale across 100-micron flakes, and find that the occupied electronic structure is exactly as predicted for freestanding SL WS$_2$ with a strong spin-orbit splitting of 420~meV and a direct band gap at the valence band maximum. Our results suggest that TMDCs can be combined with arbitrary multi-functional oxides, which may introduce alternative means of controlling the optoelectronic properties of such materials.

cond-mat.mtrl-sci↗

arXiv2016

Nanosecond spin relaxation times in single layer graphene spin valves with hexagonal boron nitride tunnel barriers

We present an experimental study of spin transport in single layer graphene using atomic sheets of hexagonal boron nitride (h-BN) as a tunnel barrier for spin injection. While h-BN is expected to be favorable for spin injection, previous experimental studies have been unable to achieve spin relaxation times in the nanosecond regime, suggesting potential problems originating from the contacts. Here, we investigate spin relaxation in graphene spin valves with h-BN barriers and observe room temperature spin lifetimes in excess of a nanosecond, which provides experimental confirmation that h-BN is indeed a good barrier material for spin injection into graphene. By carrying out measurements with different thicknesses of h-BN, we show that few layer h-BN is a better choice than monolayer for achieving high non-local spin signals and longer spin relaxation times in graphene.

cond-mat.mes-hall↗

arXiv2015

Strength of the dominant scatterer in graphene on silicon oxide

A large variability of carrier mobility of graphene-based field effect transistors hampers graphene science and technology. We determine the scattering strength of the dominant scatterer responsible for the variability of graphene-based transistors on silicon oxide. The strength of the scatterer is found to be more consistent with charged impurities than with resonant impurities.

cond-mat.mes-hall↗

arXiv2011

Impact of Calcium on Transport Property of Graphene

We have measured the impact of calcium adsorbates on the transport property of graphene. Although calcium renders conductivity linearly dependent on the carrier density of graphene as predicted, our experimental results diverge from the existing theoretical calculations. Our data expose the inadequacy of any existing theory to describe the minimum conductivity of graphene and indicate that a more complete testing of the impurity scattering calculations will require improving the experimental capabilities by minimizing the contribution from the substrate-bound charged impurities and developing an ability to count the number of adsorbates while measuring transport.

cond-mat.mes-hall↗

arXiv2011

Multiphonon Raman Scattering in Graphene

We report multiphonon Raman scattering in graphene samples. Higher order combination modes involving 3 phonons and 4 phonons are observed in single-layer (SLG), bi-layer (BLG), and few layer (FLG) graphene samples prepared by mechanical exfoliation. The intensity of the higher order phonon modes (relative to the G peak) is highest in SLG and decreases with increasing layers. In addition, all higher order modes are observed to upshift in frequency almost linearly with increasing graphene layers, betraying the underlying interlayer van der Waals interactions.

cond-mat.mes-hall↗

arXiv2011

Effects of Layer Stacking on the Combination Raman modes in Graphene

We have observed new combination modes in the range from 1650 - 2300 cm-1 in single-(SLG), bi-, few-layer and incommensurate bilayer graphene (IBLG) on silicon dioxide substrates. The M band at ~1750 cm-1 is suppressed for both SLG and IBLG. A peak at ~1860 cm-1 (iTALO-) is observed due to a combination of the iTA and LO phonons. The intensity of this peak decreases with increasing number of layers and this peak is absent in bulk graphite. Two previously unidentified modes at ~1880 cm-1 (iTALO+) and ~2220 cm-1 (iTOTA) in SLG are tentatively assigned as combination modes around the K point of the graphene Brillouin zone. The peak frequencies of the iTALO+ (iTOTA) modes are observed to increase (decrease) linearly with increasing graphene layers.

cond-mat.mes-hall↗

arXiv2010

Uncovering the Dominant Scatterer in Graphene Sheets on SiO2

We have measured the impact of atomic hydrogen adsorption on the electronic transport properties of graphene sheets as a function of hydrogen coverage and initial, pre-hydrogenation field-effect mobility. Our results are compatible with hydrogen adsorbates inducing intervalley mixing by exerting a short-range scattering potential. The saturation coverages for different devices are found to be proportional to their initial mobility, indicating that the number of native scatterers is proportional to the saturation coverage of hydrogen. By extrapolating this proportionality, we show that the field-effect mobility can reach $1.5 \times 10^4$ cm$^2$/V sec in the absence of the hydrogen-adsorbing sites. This affinity to hydrogen is the signature of the most dominant type of native scatterers in graphene-based field-effect transistors on SiO$_2$.

cond-mat.mes-hall↗