arXiv subjects

J. Hone

Publications and source records attributed to J. Hone.

At least 19 recordsLinked to original sources

arXiv2023

Evidence for a Superfluid-to-solid Transition of Bilayer Excitons

One of the most spectacular properties associated with Bose-Einstein condensation (BEC) is superfluidity in which the system exhibits zero viscosity and flows without dissipation. The superfluid phase has been observed in wide ranging Bosonic systems spanning naturally occurring quantum fluids, such as liquid helium, to engineered platforms such as bilayer excitons and cold atom systems. Theoretical works have proposed that interactions could drive the BEC ground state into another exotic phase that simultaneously exhibits properties of both a crystalline solid and a superfluid - termed a supersolid. Identifying a material system, however, that hosts the predicted BEC solid phase, driven purely by interactions and without imposing an external lattice potential, has remained elusive. Here we report observation of a superfluid to insulator transition in the layer-imbalanced regime of bilayer magneto-excitons. Mapping the transport behavior of the bilayer condensate as a function of density and temperature, suggests that the insulating phase is an ordered state of dilute excitons, stabilized by dipole interactions. The insulator melts into a recovered superfluid upon increasing the temperature, which could indicate that the low temperature solid is also a quantum coherent phase.

cond-mat.mes-hall↗

arXiv2023

Observation of Wigner cusps in a metallic carbon nanotube

Previous gate-dependent conductance measurements of metallic carbon nanotubes have revealed unexplainable conductance suppressions, occurring at two different gate voltages. These were previously attributed to the gate-dependency of contact resistance. Our gate-dependent conductivity measurements on a metallic nanotube with known chirality show that these bimodal conductance suppressions are the manifestations of Wigner cusps, often seen in atomic and nuclear physics experiments.

cond-mat.mes-hall↗

arXiv2022

Negative refraction in hyperbolic hetero-bicrystals

We visualized negative refraction of phonon polaritons, which occurs at the interface between two natural crystals. The polaritons - hybrids of infrared photons and lattice vibrations - form collimated rays that display negative refraction when passing through a planar interface between the two hyperbolic van der Waals materials: molybdenum oxide ($MoO_3$) and isotopically pure hexagonal boron nitride ($h^{11}BN$). At a special frequency $\omega_0$, these rays can circulate along closed diamond-shaped trajectories. We have shown that polariton eigenmodes display regions of both positive and negative dispersion interrupted by multiple gaps that result from polaritonic level repulsion and strong coupling.

cond-mat.mes-hall↗

arXiv2022

Infrared Plasmons Propagate through a Hyperbolic Nodal Metal

Metals are canonical plasmonic media at infrared and optical wavelengths, allowing one to guide and manipulate light at the nano-scale. A special form of optical waveguiding is afforded by highly anisotropic crystals revealing the opposite signs of the dielectric functions along orthogonal directions. These media are classified as hyperbolic and include crystalline insulators, semiconductors and artificial metamaterials. Layered anisotropic metals are also anticipated to support hyperbolic waveguiding. Yet this behavior remains elusive, primarily because interband losses arrest the propagation of infrared modes. Here, we report on the observation of propagating hyperbolic waves in a prototypical layered nodal-line semimetal ZrSiSe. The observed waveguiding originates from polaritonic hybridization between near-infrared light and nodal-line plasmons. Unique nodal electronic structures simultaneously suppress interband loss and boost the plasmonic response, ultimately enabling the propagation of infrared modes through the bulk of the crystal.

cond-mat.mes-hall↗

arXiv2020

Pressure induced topological quantum phase transition in Weyl semimetal T_d-MoTe_2

We report the pressure (p_max = 1.5 GPa) evolution of the crystal structure of the Weyl semimetal T_d-MoTe_2 by means of neutron diffraction experiments. We find that the fundamental non-centrosymmetric structure T_d is fully suppressed and transforms into a centrosymmertic 1T' structure at a critical pressure of p_cr = 1.2 GPa. This is strong evidence for a pressure induced quantum phase transition (QPT) between topological to a trivial electronic state. Although the topological QPT has strong effect on magnetoresistance, it is interesting that the superconducting critical temperature T_c, the superfluid density, and the SC gap all change smoothly and continuously across p_cr and no sudden effects are seen concomitantly with the suppression of the T_d structure. This implies that the T_c, and thus the SC pairing strength, is unaffected by the topological QPT. However, the QPT requires the change in the SC gap symmetry from non-trivial s+- to a trivial s++ state, which we discuss in this work. Our systematic characterizations of the structure and superconducting properties associated with the topological QPT provide deep insight into the pressure induced phase diagram in this topological quantum material.

cond-mat.mtrl-sci↗

arXiv2019

Seeing moir\'e superlattices

Moir\'e superlattices in van der Waals (vdW) heterostructures have given rise to a number of emergent electronic phenomena due to the interplay between atomic structure and electron correlations. A lack of a simple way to characterize moir\'e superlattices has impeded progress in the field. In this work we outline a simple, room-temperature, ambient method to visualize real-space moir\'e superlattices with sub-5 nm spatial resolution in a variety of twisted vdW heterostructures including but not limited to conducting graphene, insulating boron nitride and semiconducting transition metal dichalcogenides. Our method utilizes piezoresponse force microscopy, an atomic force microscope modality which locally measures electromechanical surface deformation. We find that all moir\'e superlattices, regardless of whether the constituent layers have inversion symmetry, exhibit a mechanical response to out-of-plane electric fields. This response is closely tied to flexoelectricity wherein electric polarization and electromechanical response is induced through strain gradients present within moir\'e superlattices. Moir\'e superlattices of 2D materials thus represent an interlinked network of polarized domain walls in a non-polar background matrix.

cond-mat.mes-hall↗

arXiv2019

Absence of dissipationless transport in clean 2D superconductors

Dissipationless charge transport is one of the defining properties of superconductors (SC). The interplay between dimensionality and disorder in determining the onset of dissipation in SCs remains an open theoretical and experimental problem. In this work, we present measurements of the dissipation phase diagrams of SCs in the two dimensional (2D) limit, layer by layer, down to a monolayer in the presence of temperature (T), magnetic field (B), and current (I) in 2H-NbSe2. Our results show that the phase-diagram strongly depends on the SC thickness even in the 2D limit. At four layers we can define a finite region in the I-B phase diagram where dissipationless transport exists at T=0. At even smaller thicknesses, this region shrinks in area. In a monolayer, we find that the region of dissipationless transport shrinks towards a single point, defined by T=B=I=0. In applied field, we show that time-dependent-Ginzburg-Landau (TDGL) simulations that describe dissipation by vortex motion, qualitatively reproduce our experimental I-B phase diagram. Last, we show that by using non-local transport and TDGL calculations that we can engineer charge flow and create phase boundaries between dissipative and dissipationless transport regions in a single sample, demonstrating control over non-equilibrium states of matter.

cond-mat.mes-hall↗

arXiv2019

Evidence for pairing states of composite fermions in double-layer graphene

Pairing interaction between fermionic particles leads to composite Bosons that condense at low temperature. Such condensate gives rise to long range order and phase coherence in superconductivity, superfluidity, and other exotic states of matter in the quantum limit. In graphene double-layers separated by an ultra-thin insulator, strong interlayer Coulomb interaction introduces electron-hole pairing across the two layers, resulting in a unique superfluid phase of interlayer excitons. In this work, we report a series of emergent fractional quantum Hall ground states in a graphene double-layer structure, which is compared to an expanded composite fermion model with two-component correlation. The ground state hierarchy from bulk conductance measurement and Hall resistance plateau from Coulomb drag measurement provide strong experimental evidence for a sequence of effective integer quantum Hall effect states for the novel two-component composite fermions (CFs), where CFs fill integer number of effective LLs (Lambda-level). Most remarkably, a sequence of incompressible states with interlayer correlation are observed at half-filled Lambda-levels, which represents a new type of order involving pairing states of CFs that is unique to graphene double-layer structure and beyond the conventional CF model.

cond-mat.mes-hall↗

arXiv2018

High quality magnetotransport in graphene using the edge-free Corbino geometry

We report fabrication of graphene devices in a Corbino geometry consisting of concentric circular electrodes with no physical edge connecting the inner and outer electrodes. High device mobility is realized using boron nitride encapsulation together with a dual-graphite gate structure. Bulk conductance measurement in the quantum Hall effect (QHE) regime outperforms previously reported Hall bar measurements, with improved resolution observed for both the integer and fractional QHE states. We identify apparent phase transitions in the fractional sequence in both the lowest and first excited Landau levels (LLs) and observed features consistent with electron solid phases in higher LLs.

cond-mat.mes-hall↗

arXiv2018

THz nanofocusing with cantilevered THz-resonant antenna tips

We developed THz-resonant scanning probe tips, yielding strongly enhanced and nanoscale confined THz near fields at their tip apex. The tips with length in the order of the THz wavelength ({\lambda} = 96.5 {\mu}m) were fabricated by focused ion beam (FIB) machining and attached to standard atomic force microscopy (AFM) cantilevers. Measurements of the near-field intensity at the very tip apex (25 nm radius) as a function of tip length, via graphene-based (thermoelectric) near-field detection, indicate their first and second order geometrical antenna resonances for tip length of 33 and 78 {\mu}m, respectively. On resonance, we find that the near-field intensity is enhanced by one order of magnitude compared to tips of 17 {\mu}m length (standard AFM tip length), which is corroborated by numerical simulations that further predict remarkable intensity enhancements of about 107 relative to the incident field. Because of the strong field enhancement and standard AFM operation of our tips, we envision manifold and straightforward future application in scattering-type THz near-field nanoscopy and THz photocurrent nanoimaging, nanoscale nonlinear THz imaging, or nanoscale control and manipulation of matter employing ultrastrong and ultrashort THz pulses.

physics.optics↗

arXiv2018

Hundredfold Enhancement of Light Emission via Defect Control in Monolayer Transition-Metal Dichalcogenides

Two dimensional (2D) transition-metal dichalcogenide (TMD) based semiconductors have generated intense recent interest due to their novel optical and electronic properties, and potential for applications. In this work, we characterize the atomic and electronic nature of intrinsic point defects found in single crystals of these materials synthesized by two different methods - chemical vapor transport and self-flux growth. Using a combination of scanning tunneling microscopy (STM) and scanning transmission electron microscopy (STEM), we show that the two major intrinsic defects in these materials are metal vacancies and chalcogen antisites. We show that by control of the synthetic conditions, we can reduce the defect concentration from above $10^{13} /cm^2$ to below $10^{11} /cm^2$. Because these point defects act as centers for non-radiative recombination of excitons, this improvement in material quality leads to a hundred-fold increase in the radiative recombination efficiency.

cond-mat.mtrl-sci↗

arXiv2018

Extreme Sensitivity of the Superconducting State in Thin Films

All non-interacting two-dimensional electronic systems are expected to exhibit an insulating ground state. This conspicuous absence of the metallic phase has been challenged only in the case of low-disorder, low density, semiconducting systems where strong interactions dominate the electronic state. Unexpectedly, over the last two decades, there have been multiple reports on the observation of a state with metallic characteristics on a variety of thin-film superconductors. To date, no theoretical explanation has been able to fully capture the existence of such a state for the large variety of superconductors exhibiting it. Here we show that for two very different thin-film superconductors, amorphous indium-oxide and a single-crystal of 2H-NbSe2, this metallic state can be eliminated by filtering external radiation. Our results show that these superconducting films are extremely sensitive to external perturbations leading to the suppression of superconductivity and the appearance of temperature independent, metallic like, transport at low temperatures. We relate the extreme sensitivity to the theoretical observation that, in two-dimensions, superconductivity is only marginally stable.

cond-mat.supr-con↗

arXiv2017

Fast thermal relaxation in cavity-coupled graphene bolometers with a Johnson noise read-out

Since the invention of the bolometer, its main design principles relied on efficient light absorption into a low-heat-capacity material and its exceptional thermal isolation from the environment. While the reduced thermal coupling to its surroundings allows for an enhanced thermal response, it in turn strongly reduces the thermal time constant and dramatically lowers the detector's bandwidth. With its unique combination of a record small electronic heat capacity and a weak electron-phonon coupling, graphene has emerged as an extreme bolometric medium that allows for both, high sensitivity and high bandwidths. Here, we introduce a hot-electron bolometer based on a novel Johnson noise readout of the electron gas in graphene, which is critically coupled to incident radiation through a photonic nanocavity. This proof-of-concept operates in the telecom spectrum, achieves an enhanced bolometric response at charge neutrality with a noise equivalent power NEP < 5pW/ Sqrt(Hz), a thermal relaxation time of {\tau} < 34ps, an improved light absorption by a factor ~3, and an operation temperature up to T=300K.

cond-mat.mes-hall↗

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.

cond-mat.mes-hall↗

arXiv2017

Even denominator fractional quantum Hall state in bilayer graphene

The multi-component nature of bilayer graphene (BLG), together with the ability to controllably tune between the various ground state orders, makes it a rich system in which to explore interaction driven phenomena. In the fractional quantum Hall effect (FQHE) regime, the unique Landau level spectrum of BLG is anticipated to support a non-Abelian even-denominator state that is tunable by both electric and magnetic fields. However, observation of this state, which is anticipated to be stronger than in conventional systems, has been conspicuously difficult. Here we report transport measurements of a robust even denominator FQHE in high-mobility, dual gated BLG devices. We confirm that the stability of the energy gap can be sensitively tuned and map the phase diagram. Our results establish BLG as a dynamic new platform to study topological ground states with possible non-Abelian excitations.

cond-mat.mes-hall↗

arXiv2016

Excitonic superfluid phase in Double Bilayer Graphene

Spatially indirect excitons can be created when an electron and a hole, confined to separate layers of a double quantum well system, bind to form a composite Boson. Because there is no recombination pathway such excitons are long lived making them accessible to transport studies. Moreover, the ability to independently tune both the intralayer charge density and interlayer electron-hole separation provides the capability to reach the low-density, strongly interacting regime where a BEC-like phase transition into a superfluid ground state is anticipated. To date, transport signatures of the superfluid condensate phase have been seen only in quantum Hall bilayers composed of double well GaAs heterostructures. Here we report observation of the exciton condensate in the quantum Hall effect regime of double layer structures of bilayer graphene. Correlation between the layers is identified by quantized Hall drag appearing at matched layer densities, and the dissipationless nature of the phase is confirmed in the counterflow geometry. Independent tuning of the layer densities and interlayer bias reveals a selection rule involving both the orbital and valley quantum number between the symmetry-broken states of bilayer graphene and the condensate phase, while tuning the layer imbalance stabilizes the condensate to temperatures in excess of 4K. Our results establish bilayer graphene quantum wells as an ideal system in which to study the rich phase diagram of strongly interacting Bosonic particles in the solid state.

cond-mat.mes-hall↗

arXiv2016

Engineering the structural and electronic phases of MoTe2 through W substitution

MoTe$_2$ is an exfoliable transition metal dichalcogenide (TMD) which crystallizes in three symmetries, the semiconducting trigonal-prismatic $2H-$phase, the semimetallic $1T^{\prime}$ monoclinic phase, and the semimetallic orthorhombic $T_d$ structure. The $2H-$phase displays a band gap of $\sim 1$ eV making it appealing for flexible and transparent optoelectronics. The $T_d-$phase is predicted to possess unique topological properties which might lead to topologically protected non-dissipative transport channels. Recently, it was argued that it is possible to locally induce phase-transformations in TMDs, through chemical doping, local heating, or electric-field to achieve ohmic contacts or to induce useful functionalities such as electronic phase-change memory elements. The combination of semiconducting and topological elements based upon the same compound, might produce a new generation of high performance, low dissipation optoelectronic elements. Here, we show that it is possible to engineer the phases of MoTe$_2$ through W substitution by unveiling the phase-diagram of the Mo$_{1-x}$W$_x$Te$_2$ solid solution which displays a semiconducting to semimetallic transition as a function of $x$. We find that only $\sim 8$ \% of W stabilizes the $T_d-$phase at room temperature. Photoemission spectroscopy, indicates that this phase possesses a Fermi surface akin to that of WTe$_2$.

cond-mat.mes-hall↗

arXiv2016

Second-harmonic imaging microscopy for time-resolved investigations of transition metal dichalcogenides

Two-dimensional transition metal dichalcogenides (TMDC) have shown promise for various applications in optoelectronics and so-called valleytronics. Their operation and performance strongly depend on the stacking of individual layers. Here, optical second-harmonic generation (SHG) in imaging mode is shown to be a versatile tool for systematic time-resolved investigations of TMDC monolayers and heterostructures in consideration of the material's structure. Large sample areas can be probed without the need of any mapping or scanning. By means of polarization dependent measurements, the crystalline orientation of monolayers or the stacking angles of heterostructures can be evaluated for the whole field of view. Pump-probe experiments then allow to correlate observed transient changes of the second-harmonic response with the underlying structure. The corresponding time-resolution is virtually limited by the pulse duration of the used laser. As an example, polarization dependent and time-resolved measurements on mono- and multilayer MoS$_2$ flakes grown on a SiO$_2$/Si(001) substrate are presented.

cond-mat.mtrl-sci↗