arXiv subjects

Gil-Ho Lee

Publications and source records attributed to Gil-Ho Lee.

At least 37 records · Page 2Linked to original sources

arXiv2021

Steady Floquet-Andreev States Probed by Tunnelling Spectroscopy

Engineering quantum states through light-matter interaction has created a new paradigm in condensed matter physics. A representative example is the Floquet-Bloch state, which is generated by time-periodically driving the Bloch wavefunctions in crystals. Previous attempts to realise such states in condensed matter systems have been limited by the transient nature of the Floquet states produced by optical pulses, which masks the universal properties of non-equilibrium physics. Here, we report the generation of steady Floquet Andreev (F-A) states in graphene Josephson junctions by continuous microwave application and direct measurement of their spectra by superconducting tunnelling spectroscopy. We present quantitative analysis of the spectral characteristics of the F-A states while varying the phase difference of superconductors, temperature, microwave frequency and power. The oscillations of the F-A state spectrum with phase difference agreed with our theoretical calculations. Moreover, we confirmed the steady nature of the F-A states by establishing a sum rule of tunnelling conductance, and analysed the spectral density of Floquet states depending on Floquet interaction strength. This study provides a basis for understanding and engineering non-equilibrium quantum states in nano-devices.

cond-mat.mes-hall↗

arXiv2021

Characterization of Shapiro steps in the presence of a 4π-periodic Josephson current

The Majorana zero-energy modes (MZMs) residing at the boundary of topological superconductors have attracted a great deal of interest recently, as they provide a platform to explore fundamental physics such as non-Abelian statistics, as well as fault-tolerant quantum computation. Period doubling of Shapiro steps in a Josephson junction under microwave irradiation has been regarded as strong evidence for the emergence of the MZMs at the junction edges. However, questions remain as to how the Shapiro steps respond to the presence of a 4π-periodic Josephson current. In this study, we investigated the characteristic features of Shapiro steps with respect to the ratio (α) of the 4π-periodic current to the topologically trivial 2π-periodic one, as well as the reduced microwave frequency (Ω) and McCumber parameter (β) of the junction. Our analysis reproduced Shapiro steps similar to those observed experimentally for specific parameter sets of α,Ω ({\lesssim 0.1}), and β ({gtrsim 1.0}). Full suppression of the first lobe of the n=1 step guarantees the presence of a 4π-periodic Josephson current.In addition, we discuss the range of Ω and β needed for full suppression of the first lobe of the {n=1} step, even for small α ({<0.1}). To observe "period-doubled Shapiro steps", even with a small α, the junction should have a large {I_c}{R_N} product and sufficiently large junction capacitance.

cond-mat.supr-con↗

arXiv2021

Spin-orbit Torque Switching in an All-Van der Waals Heterostructure

Current-induced control of magnetization in ferromagnets using spin-orbit torque (SOT) has drawn attention as a new mechanism for fast and energy efficient magnetic memory devices. Energy-efficient spintronic devices require a spin-current source with a large SOT efficiency ($ξ$) and electrical conductivity ($σ$), and an efficient spin injection across a transparent interface. Herein, we use single crystals of the van der Waals (vdW) topological semimetal WTe$_2$ and vdW ferromagnet Fe$_3$GeTe$_2$ to satisfy the requirements in their all-vdW-heterostructure with an atomically sharp interface. The results exhibit values of $ξ{\approx}4.6$ and $σ{\approx}2.25{\times}10^5 Ω^{-1} m^{-1}$ for WTe$_2$. Moreover, we obtain the significantly reduced switching current density of $3.90{\times}10^6 A/cm^2$ at 150 K, which is an order of magnitude smaller than those of conventional heavy-metal/ ferromagnet thin films. These findings highlight that engineering vdW-type topological materials and magnets offers a promising route to energy-efficient magnetization control in SOT-based spintronics.

cond-mat.mes-hall↗

arXiv2021

Angstrom-wide conductive channels in black phosphorus by Cu intercalation

Intercalation is an effective method to improve and modulate properties of two-dimensional materials. Even so, spatially controlled intercalation at atomic scale, which is important to introduce and modulated properties, has not been successful due to difficulties in controlling the diffusion of intercalants. Here, we show formation of angstrom-wide conductive channels (~4.3 A) in black phosphorus by Cu intercalation. The atomic structure, resultant microstructural effects, intercalation mechanism, and local variations of electronic properties modulated in black phosphorus by Cu intercalation were investigated extensively by transmission electron microscopy including in situ observation, DFT calculation, and conductive atomic force microscopy.

cond-mat.mtrl-sci↗

arXiv2020

Graphene-based Josephson junction microwave bolometer

Sensitive microwave detectors are critical instruments in radioastronomy, dark matter axion searches, and superconducting quantum information science. The conventional strategy towards higher-sensitivity bolometry is to nanofabricate an ever-smaller device to augment the thermal response. However, this direction is increasingly more difficult to obtain efficient photon coupling and maintain the material properties in a device with a large surface-to-volume ratio. Here we advance this concept to an ultimately thin bolometric sensor based on monolayer graphene. To utilize its minute electronic specific heat and thermal conductivity, we develop a superconductor-graphene-superconductor (SGS) Josephson junction bolometer embedded in a microwave resonator of resonant frequency 7.9 GHz with over 99\% coupling efficiency. From the dependence of the Josephson switching current on the operating temperature, charge density, input power, and frequency, we demonstrate a noise equivalent power (NEP) of 7 $\times 10^{-19}$ W/Hz$^{1/2}$, corresponding to an energy resolution of one single photon at 32 GHz and reaching the fundamental limit imposed by intrinsic thermal fluctuation at 0.19 K.

cond-mat.mes-hall↗

arXiv2020

Josephson-junction infrared single-photon detector

Josephson junctions (JJs) are ubiquitous superconducting devices, enabling high sensitivity magnetometers and voltage amplifiers, as well as forming the basis of high performance cryogenic computer and superconducting quantum computers. While JJ performance can be degraded by quasiparticles (QPs) formed from broken Cooper pairs, this phenomenon also opens opportunities to sensitively detect electromagnetic radiation. Here we demonstrate single near-infrared photon detection by coupling photons to the localized surface plasmons of a graphene-based JJ. Using the photon-induced switching statistics of the current-biased JJ, we reveal the critical role of QPs generated by the absorbed photon in the detection mechanism. The photon-sensitive JJ will enable a high-speed, low-power optical interconnect for future JJ-based computing architectures.

quant-ph↗

arXiv2020

Strongly-bound excitons and trions in anisotropic 2D semiconductors

Monolayer and few-layer phosphorene are anisotropic quasi-two-dimensional (quasi-2D) van der Waals (vdW) semiconductors with a linear-dichroic light-matter interaction and a widely-tunable direct-band gap in the infrared frequency range. Despite recent theoretical predictions of strongly-bound excitons with unique properties, it remains experimentally challenging to probe the excitonic quasiparticles due to the severe oxidation during device fabrication. In this study, we report observation of strongly-bound excitons and trions with highly-anisotropic optical properties in intrinsic bilayer phosphorene, which are protected from oxidation by encapsulation with hexagonal boron nitride (hBN), in a field-effect transistor (FET) geometry. Reflection contrast and photoluminescence spectroscopy clearly reveal the linear-dichroic optical spectra from anisotropic excitons and trions in the hBN-encapsulated bilayer phosphorene. The optical resonances from the exciton Rydberg series indicate that the neutral exciton binding energy is over 100 meV even with the dielectric screening from hBN. The electrostatic injection of free holes enables an additional optical resonance from a positive trion (charged exciton) ~ 30 meV below the optical bandgap of the charge-neutral system. Our work shows exciting possibilities for monolayer and few-layer phosphorene as a platform to explore many-body physics and novel photonics and optoelectronics based on strongly-bound excitons with two-fold anisotropy.

cond-mat.mes-hall↗

arXiv2020

Lattice dynamics and spin-phonon interaction in strained NiO films

NiO thin films with various strains were grown on SrTiO3 (STO) and MgO substrates using a pulsed laser deposition technique. The films were characterized using an x-ray diffractometer, atomic force microscopy, and infrared reflectance spectroscopy. The films grown on STO (001) substrate show a compressive in-plane strain which increases as the film thickness is reduced, resulting in an increase of the NiO phonon frequency. On the other hand, a tensile strain was detected in the NiO film grown on MgO (001) substrate which induces a softening of the phonon frequency. Overall, the variation of in-plane strain from -0.36% to +0.48% yields the decrease of the phonon frequency from 409.6 cm-1 to 377.5 cm-1 which occurs due to the ~1% change of the inter-atomic distances. The magnetic exchange -driven phonon splitting Delta(W) in three different sample, with relaxed (i.e. zero) strain, 0.36% compressive and 0.48% tensile strain was measured as a function of temperature. The Delta(W) increases on cooling in NiO relaxed film as in the previously published work on a bulk crystal. The splitting increases on cooling also in 0.48% tensile strained film, but Delta(W) is systematically 3-4 cm-1 smaller than in relaxed film. Since the phonon splitting is proportional to the non-dominant magnetic exchange interaction J1, the reduction of phonon splitting in tensile-strained film was explained by a diminishing J1 with lattice expansion. Increase of Delta(W) on cooling can be also explained by rising of J1 with reduced temperature.

cond-mat.str-el↗

arXiv2019

Imaging the Flow of Holes from a Collimating Contact in Graphene

A beam of holes formed in graphene by a collimating contact is imaged using a liquid-He cooled scanning probe microscope (SPM). The mean free path of holes is greater than the device dimensions. A zigzag shaped pattern on both sides of the collimating contact absorb holes that enter at large angles. The image charge beneath the SPM tip defects holes, and the pattern of flow is imaged by displaying the change in conductance between contacts on opposite sides, as the tip is raster scanned across the sample. Collimation is confirmed by bending hole trajectories away from the receiving contact with an applied magnetic field. The SPM images agree well with ray-tracing simulations.

cond-mat.mes-hall↗

arXiv2019

Evidence of Higher Order Topology in Multilayer WTe$_2$ from Josephson Coupling through Anisotropic Hinge States

The noncentrosymmetric Td-WTe$_2$, previously known as a type-II Weyl semimetal, is expected to have higher order topological phases with topologically protected, helical one-dimensional (1D) hinge states when their scarcely separated Weyl points get annihilated. However, the detection of these hinge states is difficult in the presence of the semimetallic behaviour of the bulk. Here, we spatially resolved the hinge states by analysing the magnetic field interference of supercurrent in Nb-WTe$_2$-Nb proximity Josephson junctions. The Josephson current along the a-axis of the WTe$_2$ crystal, but not along the b-axis, showed sharp enhancements at the edges of the junction; the amount of enhanced Josephson current was comparable to the upper limits of a single 1D conduction channel. Our experimental observations provide evidence of the higher order topological phase in WTe$_2$ and its corresponding anisotropic topological hinge states, in good agreement with theoretical calculations. Our work paves the way for hinge transport studies on topological semimetals in superconducting heterostructures, including their topological superconductivity.

cond-mat.mes-hall↗

arXiv2019

Strain-induced increase of dielectric constant in EuO thin film

Recently, lattice dynamics of the highly strained europium monoxide, as a promising candidate for strong ferroelectric-ferromagnet material, applied in the next-generation storage devices, attracted huge attention in the solid-state electronics. Here, the authors investigate the effect of tensile strain on dielectric properties of pulsed laser deposited EuO thin films from 10 to 200 K. A nearly 3% out-of-plane lattice compression is observed as the film thickness was reduced to 8 nm, which could originate from the lattice mismatch between film and a LaAlO3 substrate. The temperature and frequency dependence of capacitance and loss factor of the films reveals the dominant role of electronic and ionic polarization below 100 K. The interdigitated capacitor fabricated on strained film shows almost 50% increase of capacitance compared to the relaxed one, which corresponds to a considerable increase of dielectric constant induced by in-plane tensile strain. Softening of the in-plane polarized transverse phonon modes of EuO lattice due to tensile strain might have the major contribution to this behaviour according to the Lyddane-Sachs-Teller relation.

cond-mat.mtrl-sci↗

arXiv2019

Spin-phonon interaction increased by compressive strain in antiferromagnetic MnO thin films

MnO thin films with various thicknesses and strains were grown on MgO substrates by pulsed laser deposition, then characterized using x-ray diffraction and infrared reflectance spectroscopy. Films grown on (001)-oriented MgO substrates exhibit homogenous biaxial compressive strain which increases as the film thickness is reduced. For that reason, the frequency of doubly-degenerate phonon increases with the strain, and splits below Néel temperature TN due to the magnetic-exchange interaction. Films grown on (110)-oriented MgO substrates exhibit a huge phonon splitting already at room temperature due to the anisotropic in-plane compressive strain. Below TN, additional phonon is activated in the IR spectra; this trend is evidence for a spin-order-induced structural phase transition from tetragonal to monoclinic phase. Total phonon splitting is 55 cm-1 in (110)-oriented MnO film, which is more than twice the value in bulk MnO. This result is evidence that the nearest neighbor exchange interaction, which is responsible for the magnetically driven phonon splitting, is greatly increased in compressively strained films.

cond-mat.str-el↗

arXiv2018

Impact of geometry and non-idealities on electron 'optics' based graphene p-n junction devices

We articulate the challenges and opportunities of unconventional devices using the photon like flow of electrons in graphene, such as Graphene Klein Tunnel (GKT) transistors. The underlying physics is the employment of momentum rather than energy filtering to engineer a gate tunable transport gap in a 2D Dirac cone bandstructure. In the ballistic limit, we get a clean tunable gap that implies subthermal switching voltages below the Boltzmann limit, while maintaining a high saturating current in the output characteristic. In realistic structures, detailed numerical simulations and experiments show that momentum scattering, especially from the edges, bleeds leakage paths into the transport gap and turns it into a pseudogap. We quantify the importance of reducing edge roughness and overall geometry on the low-bias transfer characteristics of GKT transistors and benchmark against experimental data. We find that geometry plays a critical role in determining the performance of electron optics based devices that utilize angular resolution of electrons.

cond-mat.mes-hall↗

arXiv2018

Graphene Transistor Based on Tunable Dirac-Fermion-Optics

The linear energy-momentum dispersion, coupled with pseudo-spinors, makes graphene an ideal solid-state material platform to realize an electronic device based on Dirac-Fermionic relativistic quantum mechanics. Employing local gate control, several examples of electronic devices based on Dirac fermion dynamics have been demonstrated, including Klein tunneling, negative refraction and specular Andreev reflection. In this work, we present a quantum switch based on analogous Dirac-fermion-optics (DFO), in which the angle dependence of Klein tunneling is explicitly utilized to build tunable collimators and reflectors for the quantum wave function of Dirac fermions. We employ a novel dual-source design with a single flat reflector, which minimizes diffusive edge scattering and suppresses the background incoherent transmission. Our gate-tunable collimator-reflector device design enables measurement of the net DFO contribution in the switching device operation. We measure a full set of transmission coefficients of DFO wavefunction between multiple leads of the device, separating the classical contribution from that of any disorder in the channel. Since the DFO quantum switch demonstrated in this work requires no explicit energy gap, the switching operation is expected to be robust against thermal fluctuations and inhomogeneity length scales comparable to the Fermi wavelength. We find our quantum switch works at an elevated temperature up to 230 K and large bias current density up to 102 A/m, over a wide range of carrier densities. The tunable collimator-reflector coupled with the conjugated source electrodes developed in this work provides an additional component to build more efficient DFO electronic devices.

cond-mat.mes-hall↗

arXiv2018

Logarithmic singularities and quantum oscillations in magnetically doped topological insulators

We report magnetotransport measurements on magnetically doped (Bi,Sb)$_2$Te$_3$ films grown by molecular beam epitaxy. In Hallbar devices, logarithmic dependence on temperature and bias voltage are obseved in both the longitudinal and anomalous Hall resistance. The interplay of disorder and electron-electron interactions is found to explain quantitatively the observed logarithmic singularities and is a dominant scattering mechanism in these samples. Submicron scale devices exhibit intriguing quantum oscillations at high magnetic fields with dependence on bias voltage. The observed quantum oscillations can be attributed to bulk and surface transport.

cond-mat.str-el↗

arXiv2017

Short Ballistic Josephson Coupling in Planar Graphene Junctions with Inhomogeneous Carrier Doping

We report on short ballistic (SB) Josephson coupling in junctions embedded in a planar heterostructure of graphene. Ballistic Josephson coupling is confirmed by the Fabry-Perot-type interference of the junction critical current $I_c$. An exceptionally large $I_cR_N$ product close to $2Δ_0/e$ ($R_N$; normal-state junction resistance, $Δ_0$ ; zero-temperature superconducting energy gap) is an indication of SB strong Josephson coupling. $I_c$ shows a temperature dependence inconsistent with the conventional short-junction-like behavior; $I_c(T)$ curves deviate systematically from the standard Kulik-Omel'yanchuk prediction. We argue that this feature stems from the planar nature of graphene junction, which is susceptible to the effects of inhomogeneous carrier doping as well in graphene near the superconducting contacts.

cond-mat.mes-hall↗

arXiv2017

Imaging electron flow from collimating contacts in graphene

The ballistic motion of electrons in graphene encapsulated in hexagonal boron nitride (hBN) promises exciting opportunities for electron-optics devices. A narrow electron beam is desired, with both the mean free path and coherence length exceeding the device size. One can form a collimating contact in graphene by adding zigzag contacts on either side of the electron emitter that absorb stray electrons to form a collimated electron beam [23]. Here we provide images of electron flow from a collimating contact that directly show the width and shape of the electron beam, obtained using a Scanning Gate Microscope (SGM) cooled to 4.2 K. The device is a hBN-encapsulated graphene hall bar with narrow side contacts on either side of the channel that have an electron emitter at the end and absorbing zig-zag contacts at both side. To form an image of electron flow, the SGM tip is raster scanned at a constant height above the sample surface while the transmission to a receiving contact on opposite sides of the channel is measured. By displaying the change ΔT vs. tip position, an image of ballistic flow is obtained. The angular width of the electron beam leaving the collimating contact is found by applying a perpendicular magnetic field B that bends electron paths into cyclotron orbits. SGM images reveal that electron flow from a collimating contact disappears quickly at B = 0.05T while the flow from a non-collimating contact persists up to B = 0.19 T. Ray tracing simulations agree well with the experimental images over a range of B and electron density n. By fitting the half-width at half-max (HWHM) of the magnitude of electron flow in the experimental SGM images, we find a narrow half angular width Δθ = 9.2° for the electron flow from the collimating contact, compared with a wide flow Δθ = 54° from the non-collimating contact.

cond-mat.mes-hall↗

arXiv2017

Proximity coupling in superconductor-graphene heterostructures

This review discusses the electronic properties and the prospective research directions of superconductor-graphene heterostructures. The basic electronic properties of graphene are introduced to highlight the unique possibility of combining two seemingly unrelated physics, superconductivity and relativity. We then focus on graphene-based Josephson junctions, one of the most versatile superconducting quantum devices. The various theoretical methods that have been developed to describe graphene Josephson junctions are examined, together with their advantages and limitations, followed by a discussion on the advances in device fabrication and the relevant length scales. The phase-sensitive properties and phase-particle dynamics of graphene Josephson junctions are examined to provide an understanding of the underlying mechanisms of Josephson coupling via graphene. Thereafter, microscopic transport of correlated quasiparticles produced by Andreev reflections at superconducting interfaces and their phase-coherent behaviors are discussed. Quantum phase transitions studied with graphene as an electrostatically tunable two-dimensional platform are reviewed. The interplay between proximity-induced superconductivity and the quantum-Hall phase is discussed as a possible route to study topological superconductivity and non-Abelian physics. Finally, a brief summary on the prospective future research directions is given.

cond-mat.mes-hall↗