arXiv subjects

R. C. Ashoori

Publications and source records attributed to R. C. Ashoori.

At least 19 recordsLinked to original sources

arXiv2023

Time, momentum, and energy resolved pump-probe tunneling spectroscopy of two-dimensional electron systems

Real-time probing of electrons can uncover intricate relaxation mechanisms and many-body interactions hidden in strongly correlated materials. While experimenters have used ultrafast optical pump-probe methods in bulk materials, laser heating and insensitivity below the surface prevent their application to encapsulated low-dimensional electron systems at millikelvin temperatures, home to numerous intriguing electronic phases. Here, we introduce time, momentum, and energy resolved pump-probe tunneling spectroscopy (Tr-MERTS). The method allows the injection of electrons at particular energies and observation of their subsequent decay in energy-momentum space. Using Tr-MERTS, we visualize electronic decay processes in Landau levels with lifetimes up to tens of microseconds. Although most observed features agree with simple energy-relaxation, we discover an unexpected splitting in the nonequilibrium energy spectrum in the vicinity of a ferromagnetic state. An exact diagonalization study of the system suggests that the splitting arises from a maximally spin-polarized higher energy state, distinct from a conventional equilibrium skyrmion. Furthermore, we observe time-dependent relaxation of the splitting, which we attribute to single-flipped spins forming topological spin textures. These results establish Tr-MERTS as a powerful tool for studying the dynamics and properties of a two-dimensional electronic system beyond equilibrium.

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arXiv2019

Complete spin phase diagram of the fractional quantum Hall liquid

Study of the ground-state electronic spin-polarization can permit discovery and identification of novel correlated phases in the quantum Hall (QH) system. It can thus determine the potential usefulness of QH states for quantum computing. However, prior measurements involving optical and NMR techniques may perturb the system away from delicate ground-states. Here, we present spin-resolved pulsed tunneling (SRPT) that precisely determines the complete phase diagram of the ground-state spin-polarization as a function of magnetic field (B) and filling factor (ν). We observe fully-polarized ν = 5/2 and 8/3 states at very small B, suggesting strong deviation from the weakly-interacting composite-fermion model that largely successfully describes our phase diagram for the lowest Landau level. The results establish SRPT as a powerful technique for investigating correlated electron phenomena.

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arXiv2019

Electronic Compressibility of Magic-Angle Graphene Superlattices

We report the first electronic compressibility measurements of magic-angle twisted bilayer graphene. The evolution of the compressibility with carrier density offers insights into the interaction-driven ground state that have not been accessible in prior transport and tunneling studies. From capacitance measurements, we determine chemical potential as a function of carrier density and find the widths of the energy gaps at fractional filling of the moiré lattice. In the electron-doped regime, we observe unexpectedly large gaps at quarter- and half-filling and strong electron-hole asymmetry. Moreover, we measure a $\mathord{\sim}35\,\textrm{meV}$ mini-bandwidth that is much wider than most theoretical estimates. Finally, we explore the field dependence up to the quantum Hall regime and observe significant differences from transport measurements.

cond-mat.mes-hall↗

arXiv2018

Correlated Insulator Behaviour at Half-Filling in Magic Angle Graphene Superlattices

Van der Waals (vdW) heterostructures are an emergent class of metamaterials comprised of vertically stacked two-dimensional (2D) building blocks, which provide us with a vast tool set to engineer their properties on top of the already rich tunability of 2D materials. One of the knobs, the twist angle between different layers, plays a crucial role in the ultimate electronic properties of a vdW heterostructure and does not have a direct analog in other systems such as MBE-grown semiconductor heterostructures. For small twist angles, the moiré pattern produced by the lattice misorientation creates a long-range modulation. So far, the study of the effect of twist angles in vdW heterostructures has been mostly concentrated in graphene/hexagonal boron nitride (h-BN) twisted structures, which exhibit relatively weak interlayer interaction due to the presence of a large bandgap in h-BN. Here we show that when two graphene sheets are twisted by an angle close to the theoretically predicted 'magic angle', the resulting flat band structure near charge neutrality gives rise to a strongly-correlated electronic system. These flat bands exhibit half-filling insulating phases at zero magnetic field, which we show to be a Mott-like insulator arising from electrons localized in the moiré superlattice. These unique properties of magic-angle twisted bilayer graphene (TwBLG) open up a new playground for exotic many-body quantum phases in a 2D platform made of pure carbon and without magnetic field. The easy accessibility of the flat bands, the electrical tunability, and the bandwidth tunability though twist angle may pave the way towards more exotic correlated systems, such as unconventional superconductors or quantum spin liquids.

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arXiv2017

Direct measurement of discrete valley and orbital quantum numbers in a multicomponent quantum Hall system

Strongly interacting two dimensional electron systems (2DESs) host a complex landscape of broken symmetry states. The possible ground states are further expanded by internal degrees of freedom such as spin or valley-isospin. While direct probes of spin in 2DESs were demonstrated two decades ago, the valley quantum number has only been probed indirectly in semiconductor quantum wells, graphene mono- and bilayers, and transition-metal dichalcogenides. Here, we present the first direct experimental measurement of valley polarization in a two dimensional electron system, effected via the direct mapping of the valley quantum number onto the layer polarization in bilayer graphene at high magnetic fields. We find that the layer polarization evolves in discrete steps across 32 electric field-tuned phase transitions between states of different valley, spin, and orbital polarization. Our data can be fit by a model that captures both single particle and interaction induced orbital, valley, and spin anisotropies, providing the most complete model of this complex system to date. Among the newly discovered phases are theoretically unanticipated orbitally polarized states stabilized by skew interlayer hopping. The resulting roadmap to symmetry breaking in bilayer graphene paves the way for deterministic engineering of fractional quantum Hall states, while our layer-resolved technique is readily extendable to other two dimensional materials where layer polarization maps to the valley or spin quantum numbers, providing an essential direct probe that is a prerequisite for manipulating these new quantum degrees of freedom.

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arXiv2016

Observation of Helical Edge States and Fractional Quantum Hall Effect in a Graphene Electron-hole Bilayer

A quantum Hall edge state provides a rich foundation to study electrons in 1-dimension (1d) but is limited to chiral propagation along a single direction. Here, we demonstrate a versatile platform to realize new 1d systems made by combining quantum Hall edge states of opposite chiralities in a graphene electron-hole bilayer. Using this approach, we engineer helical 1d edge conductors where the counterpropagating modes are localized in separate electron and hole layers by a tunable electric field. These helical conductors exhibit strong nonlocal transport signals and suppressed backscattering due to the opposite spin polarizations of the counterpropagating modes. Moreover, we investigate these electron-hole bilayers in the fractional quantum Hall regime, where we observe conduction through fractional and integer edge states of opposite chiralities, paving the way towards the realization of 1d helical systems with fractional quantum statistics.

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arXiv2014

Torque magnetometry of an amorphous-alumina/strontium-titanate interface

We report torque magnetometry measurements of an oxide heterostructure consisting of an amorphous Al$_2$O$_3$ thin film grown on a crystalline SrTiO$_3$ substrate ($a$-AO/STO) by atomic layer deposition. We find a torque response that resembles previous studies of crystalline LaAlO$_3$/SrTiO$_3$ (LAO/STO) heterointerfaces, consistent with strongly anisotropic magnetic ordering in the plane of the interface. Unlike crystalline LAO, amorphous Al$_2$O$_3$ is nonpolar, indicating that planar magnetism at an oxide interface is possible without the strong internal electric fields generated within the polarization catastrophe model. We discuss our results in the context of current theoretical efforts to explain magnetism in crystalline LAO/STO.

cond-mat.str-el↗

arXiv2013

Tunable symmetry breaking and helical edge transport in a graphene quantum spin Hall state

Low-dimensional electronic systems have traditionally been obtained by electrostatically confining electrons, either in heterostructures or in intrinsically nanoscale materials such as single molecules, nanowires, and graphene. Recently, a new paradigm has emerged with the advent of symmetry-protected surface states on the boundary of topological insulators, enabling the creation of electronic systems with novel properties. For example, time reversal symmetry (TRS) endows the massless charge carriers on the surface of a three-dimensional topological insulator with helicity, locking the orientation of their spin relative to their momentum. Weakly breaking this symmetry generates a gap on the surface, resulting in charge carriers with finite effective mass and exotic spin textures. Analogous manipulations of the one-dimensional boundary states of a two-dimensional topological insulator are also possible, but have yet to be observed in the leading candidate materials. Here, we demonstrate experimentally that charge neutral monolayer graphene displays a new type of quantum spin Hall (QSH) effect, previously thought to exist only in TRS topological insulators, when it is subjected to a very large magnetic field angled with respect to the graphene plane. Unlike in the TRS case, the QSH presented here is protected by a spin-rotation symmetry that emerges as electron spins in a half-filled Landau level are polarized by the large in-plane magnetic field. The properties of the resulting helical edge states can be modulated by balancing the applied field against an intrinsic antiferromagnetic instability, which tends to spontaneously break the spin-rotation symmetry. In the resulting canted antiferromagnetic (CAF) state, we observe transport signatures of gapped edge states, which constitute a new kind of one-dimensional electronic system with tunable band gap and associated spin-texture.

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arXiv2013

Massive Dirac fermions and Hofstadter butterfly in a van der Waals heterostructure

Van der Waals heterostructures comprise a new class of artificial materials formed by stacking atomically-thin planar crystals. Here, we demonstrate band structure engineering of a van der Waals heterostructure composed of a monolayer graphene flake coupled to a rotationally-aligned hexagonal boron nitride substrate. The spatially-varying interlayer atomic registry results both in a local breaking of the carbon sublattice symmetry and a long-range moiré superlattice potential in the graphene. This interplay between short- and long-wavelength effects results in a band structure described by isolated superlattice minibands and an unexpectedly large band gap at charge neutrality, both of which can be tuned by varying the interlayer alignment. Magnetocapacitance measurements reveal previously unobserved fractional quantum Hall states reflecting the massive Dirac dispersion that results from broken sublattice symmetry. At ultra-high fields, integer conductance plateaus are observed at non-integer filling factors due to the emergence of the Hofstadter butterfly in a symmetry-broken Landau level.

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arXiv2011

Coexistence of Magnetic Order and Two-dimensional Superconductivity at LaAlO$_3$/SrTiO$_3$ Interfaces

A two dimensional electronic system with novel electronic properties forms at the interface between the insulators LaAlO$_3$ and SrTiO$_3$. Samples fabricated until now have been found to be either magnetic or superconducting, depending on growth conditions. We combine transport measurements with high-resolution magnetic torque magnetometry and report here evidence of magnetic ordering of the two-dimensional electron liquid at the interface. The magnetic ordering exists from well below the superconducting transition to up to 200 K, and is characterized by an in-plane magnetic moment. Our results suggest that there is either phase separation or coexistence between magnetic and superconducting states. The coexistence scenario would point to an unconventional superconducting phase in the ground state.

cond-mat.mtrl-sci↗

arXiv2010

Observations of Plasmarons in a System of Massive Electrons

Calculations of the single particle density of states (SPDOS) of electron liquids have long predicted that there exist two distinct charged excitations: the usual quasiparticle consisting of an electron or hole screened by a correlation hole, and a "plasmaron" consisting of a hole resonantly bound to real plasmons in the Fermi sea(1,2). Using tunneling spectroscopy to measure the SPDOS of a two-dimensional electronic system, we demonstrate the first detection of the plasmaron in a system in which electrons have mass. We monitor the evolution of the plasmaron with applied magnetic field and discover unpredicted "magnetoplasmarons" which appear in spectra as negative index Landau levels. These sharp features corresponding to long-lived quasiparticles appear at high energies where SPDOS structure is ordinarily broadened by electron-electron interactions.

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arXiv2010

Large capacitance enhancement and negative compressibility of two-dimensional electronic systems at LaAlO$_3$/SrTiO$_3$ interfaces

Novel electronic systems forming at oxide interfaces comprise a class of new materials with a wide array of potential applications. A high mobility electron system forms at the LaAlO$_3$/SrTiO$_3$ interface and, strikingly, both superconducts and displays indications of hysteretic magnetoresistance. An essential step for device applications is establishing the ability to vary the electronic conductivity of the electron system by means of a gate. We have fabricated metallic top gates above a conductive interface to vary the electron density at the interface. By monitoring capacitance and electric field penetration, we are able to tune the charge carrier density and establish that we can completely deplete the metallic interface with small voltages. Moreover, at low carrier densities, the capacitance is significantly enhanced beyond the geometric capacitance for the structure. In the same low density region, the metallic interface overscreens an external electric field. We attribute these observations to a negative compressibility of the electronic system at the interface. Similar phenomena have been observed previously in semiconducting two-dimensional electronic systems. The observed compressibility result is consistent with the interface containing a system of mobile electrons in two dimensions.

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arXiv2010

Anomalous structure in the single particle spectrum of the fractional quantum Hall effect

The two-dimensional electron system (2DES) is a unique laboratory for the physics of interacting particles. Application of a large magnetic field produces massively degenerate quantum levels known as Landau levels. Within a Landau level the kinetic energy of the electrons is suppressed, and electron-electron interactions set the only energy scale. Coulomb interactions break the degeneracy of the Landau levels and can cause the electrons to order into complex ground states. In the high energy single particle spectrum of this system, we observe salient and unexpected structure that extends across a wide range of Landau level filling fractions. The structure appears only when the 2DES is cooled to very low temperature, indicating that it arises from delicate ground state correlations. We characterize this structure by its evolution with changing electron density and applied magnetic field. We present two possible models for understanding these observations. Some of the energies of the features agree qualitatively with what might be expected for composite Fermions, which have proven effective for interpreting other experiments in this regime. At the same time, a simple model with electrons localized on ordered lattice sites also generates structure similar to those observed in the experiment. Neither of these models alone is sufficient to explain the observations across the entire range of densities measured. The discovery of this unexpected prominent structure in the single particle spectrum of an otherwise thoroughly studied system suggests that there exist core features of the 2DES that have yet to be understood.

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arXiv2007

High Resolution Spectroscopy of Two-Dimensional Electron Systems

Spectroscopic methods involving the sudden injection or ejection of electrons in materials are a powerful probe of electronic structure and interactions. These techniques, such as photoemission and tunneling, yield measurements of the "single particle" density of states (SPDOS) spectrum of a system. The SPDOS is proportional to the probability of successfully injecting or ejecting an electron in these experiments. It is equal to the number of electronic states in the system able to accept an injected electron as a function of its energy and is among the most fundamental and directly calculable quantities in theories of highly interacting systems. However, the two-dimensional electron system (2DES), host to remarkable correlated electron states such as the fractional quantum Hall effect, has proven difficult to probe spectroscopically. Here we present an improved version of time domain capacitance spectroscopy (TDCS) that now allows us to measure the SPDOS of a 2DES with unprecedented fidelity and resolution. Using TDCS, we perform measurements of a cold 2DES, providing the first direct measurements of the single-particle exchange-enhanced spin gap and single particle lifetimes in the quantum Hall system, as well as the first observations of exchange splitting of Landau levels not at the Fermi surface. The measurements reveal the difficult to reach and beautiful structure present in this highly correlated system far from the Fermi surface.

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arXiv2006

The Density of States in the Two-Dimensional Electron Gas and Quantum Dots (Ph.D. thesis, Cornell University, January 1991)

This thesis describes capacitance and tunneling experiments performed on two-dimensional electron gas (2DEG) and quantum dot systems. It develops a system of equations that allow determination, by means of capacitance measurements, of the electronic density of states, the electron density, and the chemical potential in a 2DEG. The thesis describes the use of these techniques in the observation of a magnetic field induced energy gap to tunneling in the 2DEG and the single electron addition spectrum in arrays of quantum dots.

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arXiv2005

Imaging Transport Resonances in the Quantum Hall Effect

We use a scanning capacitance probe to image transport in the quantum Hall system. Applying a DC bias voltage to the tip induces a ring-shaped incompressible strip (IS) in the 2D electron system (2DES) that moves with the tip. At certain tip positions, short-range disorder in the 2DES creates a quantum dot island in the IS. These islands enable resonant tunneling across the IS, enhancing its conductance by more than four orders of magnitude. The images provide a quantitative measure of disorder and suggest resonant tunneling as the primary mechanism for transport across ISs.

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arXiv2002

Modeling Subsurface Charge Accumulation Images of a Quantum Hall Liquid

Subsurface Charge Accumulation imaging is a cryogenic scanning probe technique that has recently been used to spatially probe incompressible strips formed in a two-dimensional electron system (2DES) at high magnetic fields. In this paper, we present detailed numerical modeling of these data. At a basic level, the method produces results that agree well with the predictions of models based on simple circuit elements. Moreover, the modeling method is sufficiently advanced to simulate the spatially resolved measurements. By comparing directly the simulations to the experimentally measured data, we can extract quantitatively local electronic features of the 2DES. In particular, we deduce the electron density of states inside the incompressible strips and electrical resistance across them.

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arXiv2001

Direct observation of the charging of a 2D electron gas through an incompressible strip in the quantum Hall regime

Using charge accumulation imaging, we measure the charge flow across an incompressible strip and follow its evolution with magnetic field. The strip runs parallel to the edge of a gate deposited on the sample and forms at positions where an exact number of integer Landau levels is filled. An RC model of charging fits the data well and enables us to determine the longitudinal resistance of the strip. Surprisingly, we find that the strip becomes more resistive as its width decreases.

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