Mesocale & Nanoscale Physics

2021-01-14 arXiv Feed

Capillary condensation under atomic-scale confinement

Capillary condensation of water is ubiquitous in nature and technology. Itroutinely occurs in granular and porous media, can strongly alter suchproperties as adhesion, lubrication, friction and corrosion, and is importantin many processes employed by microelectronics, pharmaceutical, food and otherindustries. The century-old Kelvin equation is commonly used to describecondensation phenomena and shown to hold well for liquid menisci with diametersas small as several nm. For even smaller capillaries that are involved incondensation under ambient humidity and, hence, of particular practicalinterest, the Kelvin equation is expected to break down, because the requiredconfinement becomes comparable to the size of water molecules. Here we takeadvantage of van der Waals assembly of two-dimensional crystals to createatomic-scale capillaries and study condensation inside. Our smallestcapillaries are less than 4 angstroms in height and can accommodate just amonolayer of water. Surprisingly, even at this scale, the macroscopic Kelvinequation using the characteristics of bulk water is found to describeaccurately the condensation transition in strongly hydrophilic (mica)capillaries and remains qualitatively valid for weakly hydrophilic (graphene)ones. We show that this agreement is somewhat fortuitous and can be attributedto elastic deformation of capillary walls, which suppresses giant oscillatorybehavior expected due to commensurability between atomic-scale confinement andwater molecules. Our work provides a much-needed basis for understanding ofcapillary effects at the smallest possible scale important in many realisticsituations.

Intrinsic and extrinsic effects on intraband optical conductivity of hot carriers in photoexcited graphene

We present a numerical study on the intraband optical conductivity of hotcarriers at quasiequilibria in photoexcited graphene based on the semiclassicalBoltzmann transport equations (BTE) with the aim of understanding the effectsof intrinsic optical phonon and extrinsic coulomb scattering caused by chargedimpurities at the graphene{substrate interface. We employ iterative solutionsof the BTE and the comprehensive model for the temporal evolutions ofhot-carrier temperature and hot-optical-phonon occupations to reducecomputational costs, instead of using full-BTE solutions. Undoped grapheneexhibited large positive photoconductivity owing to the increase in thermallyexcited carriers and the reduction in charged impurity scattering. Thefrequency dependencies of the photoconductivity in undoped graphene with highconcentrations of charged impurities significantly deviated from that observedin the simple Drude model, which is attributed to temporally varying chargedimpurity scattering during terahertz (THz) probing in the hot-carrier coolingprocess. Heavily doped graphene exhibited small negative photoconductivitysimilar to that of the Drude model. In this case, charged impurity scatteringis substantially suppressed by the carrier-screening effect, and thetemperature dependencies of the Drude weight and optical phonon scatteringgovern negative photoconductivity. In lightly doped graphene, thephotoconductivity changes its sign temporally after the photoexcitation,depending on the carrier and optical phonon temperatures and the pump uence.Moreover, the photoconductivity spectra depend not only on the materialproperty of graphene sample but also on the waveform of the THz-probe pulse.Our approach provides a quantitative understanding of non-Drude behaviors andthe temporal evolution of photoconductivity in graphene.

Analytical solution to the surface states of antiferromagnetic topological insulator MnBi$_2$Te$_4$

Recently, the intrinsic magnetic topological insulator MnBi$_2$Te$_4$ hasattracted great attention. It has an out-of-plane antiferromagnetic order,which is believed to open a sizable energy gap in the surface states. This gap,however, was not always observable in the latest ARPES experiments. To addressthis issue, we analytically derive an effective model for the 2D surface statesby starting from a 3D Hamiltonian for bulk MnBi$_2$Te$_4$ and taking intoaccount the spatial profile of the bulk magnetization. We suggest that the Biantisite defects in the Mn atomic layers in the bulk may be one of the reasonsfor the varied experimental results, since the Bi antisite defects may resultin a much smaller and more localized intralayer ferromagnetic order, leading tothe diminished surface gap. In addition, we calculate the spatial distributionand penetration depth of the surface states, which are mainly embedded in thefirst two septuple layers from the terminating surface. From our analyticalresults, the influence of the bulk parameters on the surface states can befound explicitly. Furthermore, we derive a $\bf{k}\cdot \bf{p}$ model forMnBi$_2$Te$_4$ thin films and show the oscillation of the Chern number betweenodd and even septuple layers. Our results will be helpful for the ongoingexplorations of the MnBi$_x$Te$_y$ family.

Experimental observation of topological exciton-polaritons in transition metal dichalcogenide monolayers

The rise of quantum science and technologies motivates photonics research toseek new platforms with strong light-matter interactions to facilitate quantumbehaviors at moderate light intensities. One promising platform to reach suchstrong light-matter interacting regimes is offered by polaritonic metasurfaces,which represent ultrathin artificial media structured on nano-scale anddesigned to support polaritons - half-light half-matter quasiparticles.Topological polaritons, or 'topolaritons', offer an ideal platform in thiscontext, with unique properties stemming from topological phases of lightstrongly coupled with matter. Here we explore polaritonic metasurfaces based on2D transition metal dichalcogenides (TMDs) supporting in-plane polarizedexciton resonances as a promising platform for topological polaritonics. Weenable a spin-Hall topolaritonic phase by strongly coupling valley polarizedin-plane excitons in a TMD monolayer with a suitably engineered all-dielectrictopological photonic metasurface. We first show that the strong couplingbetween topological photonic bands supported by the metasurface and excitonicbands in MoSe2 yields an effective phase winding and transition to atopolaritonic spin-Hall state. We then experimentally realize this phenomenonand confirm the presence of one-way spin-polarized edge topolaritons. Combinedwith the valley polarization in a MoSe2 monolayer, the proposed system enablesa new approach to engage the photonic angular momentum and valley degree offreedom in TMDs, offering a promising platform for photonic/solid-stateinterfaces for valleytronics and spintronics.

Kondo effects in small bandgap carbon nanotube quantum dots

We study magnetoconductance of the small bandgap carbon nanotube quantum dotsin the presence of spin-orbit coupling in the strong correlations regime. Thefinite-U mean field slave boson approach is used to study many-body effects.Different degeneracies are restored in magnetic field and Kondo effects ofdifferent symmetries arise including SU(3) effects of different types. Fullspin-orbital degeneracy might be recovered for zero field and correspondinglySU(4) Kondo effect sets in. We point out on the possibility of the occurrenceof electron-hole Kondo effects in slanting magnetic fields, which we predictwill occur in the available magnetic fields for orientation of fields close toperpendicular. When the field approaches transverse orientation a crossoverfrom SU(2) or SU(3) symmetry into SU(4) is observed.

Indium gallium nitride quantum dots: Consequence of random alloy fluctuations for polarization entangled photon emission

We analyze the potential of the $c$-plane InGaN/GaN quantum dots forpolarization entangled photon emission by means of an atomistic many-bodyframework. Special attention is paid to the impact of random alloy fluctuationson the excitonic fine structure and the excitonic binding energy. Ourcalculations show that $c$-plane InGaN/GaN quantum dots are ideal candidatesfor high temperature entangled photon emission as long as the underlying$C_{3v}$-symmetry is preserved. However, when assuming random alloyfluctuations in the dot, our atomistic calculations reveal that while the largeexcitonic binding energies are only slightly affected, the $C_{3v}$ symmetry isbasically lost due to the alloy fluctuations. We find that this loss insymmetry significantly impacts the excitonic fine structure. The observedchanges in fine structure and the accompanied light polarizationcharacteristics have a detrimental effect for polarization entangled photonpair emission via the biexciton-exciton cascade. Here, we also discuss possiblealternative schemes that benefit from the large excitonic binding energies, toenable non-classical light emission from $c$-plane InGaN/GaN quantum dots atelevated temperatures.

Confine Electrons in Effective Plane Fractals

As an emerging complex two-dimensional structure, plane fractal has attractedmuch attention due to its novel dimension-related physical properties. In thispaper, we check the feasibility to create an effective Sierpinski carpet (SC),a plane fractal with Hausdorff dimension intermediate between one and two, byapplying an external electric field to a square or a honeycomb lattice. Theelectric field forms a fractal geometry but the atomic structure of theunderlying lattice remains the same. By calculating and comparing variouselectronic properties, we find parts of the electrons can be confinedeffectively in a fractional dimension with a relatively small field, andrepresenting properties very close to these in a real fractal. In particular,compared to the square lattice, the external field required to effectivelyconfine the electron is smaller in the hexagonal lattice, suggesting that agraphene-like system will be an ideal platform to construct an effective SCexperimentally. Our work paves a new way to build fractals from a top-downperspective, and can motivate more studies of fractional dimensions in realsystems.

Floquet generation of Second Order Topological Superconductor

We theoretically investigate the Floquet generation of second-ordertopological superconducting (SOTSC) phase, hosting Majorana corner modes(MCMs), considering a quantum spin Hall insulator (QSHI) with proximity inducedsuperconducting $s$-wave pairing in it. Our dynamical prescription consists ofthe periodic kick in time-reversal symmetry breaking in-plane magnetic fieldand four-fold rotational symmetry breaking mass term while these Floquet MCMsare preserved by anti-unitary particle-hole symmetry. The first drivingprotocol always leads to four zero energy MCMs (i.e. one Majorana per corner)as a sign of a {\it{strong}} SOTSC phase. Interestingly, the second protocolcan result in a {\it{weak}} SOTSC phase, harbouring eight zero energy MCMs (twoMajorana states per corner), in addition to the {\it{strong}} SOTSC phase. Wecharacterize the topological nature of these phases by Floquet quadrupolarmoment and Floquet Wannier spectrum. We believe that relying on the recentexperimental advancement in the driven systems and proximity inducedsuperconductivity, our schemes may be possible to test in the future.

Phonon-assisted Exciton Dissociation in Transition Metal Dichalcogenides

Monolayers of transition metal dichalcogenides (TMDs) have been establishedin the last years as promising materials for novel optoelectronic devices.However, the performance of such devices is often limited by the dissociationof tightly bound excitons into free electrons and holes. While previous studieshave investigated tunneling at large electric fields, we focus in this work onphonon-assisted exciton dissociation that is expected to be the dominantmechanism at small fields. We present a microscopic model based on the densitymatrix formalism providing access to time- and momentum-resolved excitondynamics including phonon-assisted dissociation. We track the pathway ofexcitons from optical excitation via thermalization to dissociation,identifying the main transitions and dissociation channels. Furthermore, wefind intrinsic limits for the quantum efficiency and response time of aTMD-based photodetector and investigate their tunability with externallyaccessible knobs, such as excitation energy, substrate screening, temperatureand strain. Our work provides microscopic insights in fundamental mechanismsbehind exciton dissociation and can serve as a guide for the optimization ofTMD-based optoelectronic devices.

Non-trivial retardation effects in dispersion forces: From anomalous distance dependence to novel traps

In the study of dispersion forces, nonretarded, retarded and thermalasymptotes with their distinct scaling laws are regarded as cornerstone resultsgoverning interactions at different separations. Here, we show that whenparticles interact in a medium, the influence of retardation is qualitativelydifferent, making it necessary to consider the non-monotonous potential infull. We discuss different regimes for several cases and find an anomalousbehaviour of the retarded asymptote. It can change sign, and lead to a trappingpotential.

Transport experiments in semiconductor-superconductor hybrid Majorana devices

As the condensed matter analog of Majorana fermion, Majorana zero-mode iswell known as a building block of fault-tolerant topological quantum computing.In this review, we focus on the recent progress of Majorana experiments,especially experiments about semiconductor-superconductor hybrid devices. Wefirst sketch Majorana zero-mode formation from a bottom-up view, which is moresuitable for beginners and experimentalist. Then, we survey the status ofzero-energy state signatures reported recently, from zero-energy conductancepeaks, the oscillations, the quantization, and the interactions withextra-degrees of freedom. This paper also gives prospects on future experimentsfor advancing one-dimensional semiconductor nanowire-superconductor hybridmaterials and devices.

Josephson effect in graphene bilayers with adjustable relative displacement

The Josephson current is investigated in a superconducting graphene bilayerwhere the pristine graphene sheets can make in-plane or out-of-planedisplacements with respect to each other. The superconductivity can be ofintrinsic nature, or due to a proximity effect. The results demonstrate thatthe supercurrent responds qualitatively differently to relative displacement ifthe superconductivity is due to either intralayer or interlayer spin-singletelectron-electron pairing, thus providing a tool to distinguish between the twomechanisms. Specifically, both the AA and AB stacking orders are studied withantiferromagnetic spin alignment. For the AA stacking order with intralayer andon-site pairing no current reversal is found. In contrast, the supercurrent mayswitch its direction as a function of the in-plane displacement andout-of-plane interlayer coupling for the cases of AA ordering with interlayerpairing and AB ordering with either intralayer or interlayer pairing. Inaddition to sign reversal, the Josephson signal displays many characteristicfingerprints which derive directly from the pairing mechanism. Thus,measurements of the Josephson current as a function of the graphene bilayerdisplacement open up means for achieving a deeper insight of thesuperconducting pairing mechanism.

Effects of Electron-Vibrational Interaction in Exciton-Polariton Luminescence and Relaxation. Time-dependent Polariton Luminescence Spectrum

The non-equilibrium polariton luminescence spectrum is calculated within theframework of the non-Markovian molecular relaxation model. The model accountsfor both the high-frequency (HF) and the low frequency (LF) optically active(OA) molecular vibrations. For the calculation of the compositevibration-polariton operators, we employ the Lang-Firsov transformation for theHFOA vibration mode and account for the classical LFOA vibrationsstochastically. We also propose a polariton fluorescence mechanism in which thespreading of the two-particle polariton expectation value outside thenano-sample is considered as the decay of the composite polariton particle.This opens a way for observation of the hot exciton-polaritons luminescence inorganic-based nano-devices in analogy with the hot luminescence of moleculesand crystals. The theory provides a clear simple physical picture of thepolariton luminescence line-shape relaxation and, as it is demonstrated, agreeswith the experiment.

Optical response of noble metal nanostructures: Quantum surface effects in crystallographic facets

Noble metal nanostructures are ubiquitous elements in nano-optics, supportingplasmon modes that can focus light down to length scales commensurate withnonlocal effects associated with quantum confinement and spatial dispersion inthe underlying electron gas. Nonlocal effects are naturally more prominent forcrystalline noble metals, which potentially offer lower intrinsic loss thantheir amorphous counterparts, and with particular crystal facets giving rise todistinct electronic surface states. Here, we employ a quantum-mechanical modelto describe nonclassical effects impacting the optical response of crystallinenoble-metal films and demonstrate that these can be well-captured using a setof surface-response functions known as Feibelman $d$-parameters. In particular,we characterize the $d$-parameters associated with the (111) and (100) crystalfacets of gold, silver, and copper, emphasizing the importance of surfaceeffects arising due to electron wave function spill-out and thesurface-projected band gap emerging from atomic-layer corrugation. We then showthat the extracted $d$-parameters can be straightforwardly applied to describethe optical response of various nanoscale metal morphologies of interest,including metallic ultra-thin films, graphene-metal heterostructures hostingextremely confined acoustic graphene plasmons, and crystallographic facetedmetallic nanoparticles supporting localized surface plasmons. The tabulated$d$-parameters reported here can circumvent computationally expensivefirst-principles atomistic simulations to describe microscopic nonlocal effectsin the optical response of mesoscopic crystalline metal surfaces, which arebecoming widely available with increasing control over morphology down toatomic length scales for state-of-the-art experiments in nano-optics.

Optimal energy conversion through anti-adiabatic driving breaking time-reversal symmetry

Starting with Carnot engine, the ideal efficiency of a heat engine has beenassociated with quasi-static transformations and vanishingly small outputpower. Here, we exactly calculate the thermodynamic properties of a isothermalheat engine, in which the working medium is a periodically driven underdampedharmonic oscillator, focusing instead on the opposite, anti-adiabatic limit,where the period of a cycle is the fastest time scale in the problem. We showthat in that limit it is possible to approach the ideal energy conversionefficiency $\eta=1$, with finite output power and vanishingly small relativepower fluctuations. The simultaneous realization of all the three desiderata ofa heat engine is possible thanks to the breaking of time-reversal symmetry. Wealso show that non-Markovian dynamics can further improve the power-efficiencytrade-off.

Rotating edge-field driven processing of chiral spin textures in racetrack devices

Topologically distinct magnetic structures like skyrmions, domain walls, andthe uniformly magnetized state have multiple applications in logic devices,sensors, and as bits of information. One of the most promising concepts forapplying these bits is the racetrack architecture controlled by electriccurrents or magnetic driving fields. In state-of-the-art racetracks, thesefields or currents are applied to the whole circuit. Here, we employmicromagnetic and atomistic simulations to establish a concept for racetrackmemories free of global driving forces. Surprisingly, we realize that mixedsequences of topologically distinct objects can be created and propagated overfar distances exclusively by local rotation of magnetization at the sampleboundaries. We reveal the dependence between the chirality of the rotation andthe direction of propagation and define the phase space where the proposedprocedure can be realized. The advantages of this approach are the exclusion ofhigh current and field densities as well as its compatibility with anenergy-efficient three-dimensional design.

Gauge Fixing for Strongly Correlated Electrons coupled to Quantum Light

We discuss the problem of gauge fixing for strongly correlated electronscoupled to quantum light, described by projected low-energy models such asthose obtained within tight-binding methods. Drawing from recent results in thefield of quantum optics, we present a general approach to write down quantumlight-matter Hamiltonian in either dipole or Coulomb gauge which are explicitlyconnected by a unitary transformation, thus ensuring gauge equivalence evenafter projection. The projected dipole gauge Hamiltonian features a linearlight-matter coupling and an instantaneous self-interaction for the electrons,similar to the structure in the full continuum theory. On the other hand, inthe Coulomb gauge the photon field enters in a highly non-linear way, throughphase factors that dress the electronic degrees of freedom. We show that ourapproach generalises the well-known Peierls approximation, to which it reduceswhen local, on-site orbital contributions to light-matter coupling aredisregarded. As an application, we study a two-orbital model of interactingelectrons coupled to a uniform cavity mode, recently studied in the context ofexcitonic superradiance and associated no-go theorems. Using both gauges werecover the absence of superradiant phase in the ground state and show thatexcitations on top of it, described by polariton modes, contain insteadnon-trivial light-matter entanglement. Our results highlight the importance oftreating the non-linear light-matter interaction of the Coulomb gaugenon-perturbatively, to obtain a well-defined ultrastrong coupling limit and tonot spoil gauge equivalence.

Stabilization of NbTe3, VTe3, and TiTe3 via Nanotube Encapsulation

The structure of MX3 transition metal trichalcogenides (TMTs, with M atransition metal and X a chalcogen) is typified by one-dimensional (1D) chainsweakly bound together via van der Waals interactions. This structural motif iscommon across a range of M and X atoms (e.g. NbSe3, HfTe3, TaS3), but not all Mand X combinations are stable. We report here that three new members of the MX3family which are not stable in bulk, specifically NbTe3, VTe3, and TiTe3, canbe synthesized in the few- to single-chain limit via nano-confined growthwithin the stabilizing cavity of multi-walled carbon nanotubes. Transmissionelectron microscopy (TEM) and atomic-resolution scanning transmission electronmicroscopy (STEM) reveal the chain-like nature and the detailed atomicstructure. The synthesized materials exhibit behavior unique to few-chainquasi-1D structures, such as multi-chain spiraling and a trigonalanti-prismatic rocking distortion in the single-chain limit. Density functionaltheory (DFT) calculations provide insight into the crystal structure andstability of the materials, as well as their electronic structure.

Robust metastable skyrmions with tunable size in the chiral magnet FePtMo$_3$N

Synthesis of new materials that can host magnetic skyrmions and theirthorough experimental and theoretical characterization are essential for futuretechnological applications. The $\beta$-Mn-type compound FePtMo$_3$N is onesuch novel material that belongs to the chiral space group $P4_132$, where theantisymmetric Dzyaloshinkii-Moriya interaction is allowed due to the absence ofinversion symmetry. We report the results of small-angle neutron scattering(SANS) measurements of FePtMo$_3$N and demonstrate that its magnetic groundstate is a long-period spin helix with a Curie temperature of 222~K. Themagnetic field-induced redistribution of the SANS intensity showed that thehelical structure transforms to a lattice of skyrmions at $\sim$13~mT attemperatures just below $T_{\text C}$. Our key observation is that the skyrmionstate in FePtMo$_3$N is robust against field cooling down to the lowesttemperatures. Moreover, once the metastable state is prepared by field cooling,the skyrmion lattice exists even in zero field. Furthermore, we show that theskyrmion size in FePtMo$_3$N exhibits high sensitivity to the sampletemperature and can be continuously tuned between 120 and 210~nm. This offersnew prospects in the control of topological properties of chiral magnets.

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