Coherent Transport Research Articles

Novel Bulk Quantum Hall Effect in Nanostructured TaP Macroscopic Crystals.

Bulk quantum Hall effect (QHE), the natural extension of the two-dimensional (2D) QHE, is one of the representative phenomena of coherent electron transport. However, bulk QHE has rarely been reported in real materials with macroscopic sizes. Here, we report a novel bulk QHE in macroscopic millimeter-sized and nanostructured TaP crystals consisting of nanometer-scale lamellae. Specifically, the simultaneous quantum plateaus were observed in both transverse resistivity ρxy and vertical resistivity ρzz. The bulk QHE is attributable to synergetic action between Landau cyclotron movement under magnetic field B and periodically modulated potential due to the nanometer-scaled lamellae. This mechanism would form the fixed number of edge states along B-perpendicular and B-parallel directions respectively, equivalent to stacked 2D-QHE layers, leading to quantized ρxy and ρzz. Our work verifies that microstructure engineering could result in the coherent transport of electrons and generate new quantum phenomena in bulk materials.

Room temperature ballistic transport of exciton-polaritons in a one-dimensional whispering gallery microcavity

In this work, we report the experimental observation of the ballistic transport of condensed exciton-polaritons at room temperature in a one-dimensional whispering-gallery microcavity. Such coherent transport, initiated by the pronounced inter-particle interactions of polaritons, leads to the generation of two symmetric emission beams in the momentum (angular) space. By means of spatially filtered angle-resolved photoluminescence imaging spectroscopy, we were able to identify their origin and successfully rationalize these observations using the potential energy-to-kinetic energy conversion picture. The energy-dependent emission linewidth, as well as TM to TE inter-mode scattering, have also been discussed.

Non-Classical Heat Transfer and Recent Progress

Abstract Heat transfer in solids has traditionally been described by Fourier's law, which assumes local equilibrium and a diffusive transport regime. However, advancements in nanotechnology and the development of novel materials have revealed non-classical heat transfer phenomena that extend beyond this traditional framework. These phenomena, which can be broadly categorized into those governed by kinetic theory and those extending beyond it, include ballistic transport, phonon hydrodynamics, coherent phonon transport, Anderson localization, and glass-like heat transfer, among others. Recent theoretical and experimental studies have focused on characterizing these non-classical behaviors using methods such as the Boltzmann transport equation, molecular dynamics, and advanced spectroscopy techniques. In particular, the dual nature of phonons, exhibiting both particle-like and wave-like characteristics, is fundamental to understanding these phenomena. This review summarizes state-of-the-art findings in the field, highlighting the importance of integrating both particle and wave models to fully capture the complexities of heat transfer in modern materials. The emergence of new research areas, such as chiral and topological phonons, further underscores the potential for advancing phonon engineering. These developments open up exciting opportunities for designing materials with tailored thermal properties and new device mechanisms, potentially leading to applications in thermal management, energy technologies, and quantum science.

Coherent optical spin Hall transport for polaritonics at room temperature.

Spin or valley degrees of freedom hold promise for next-generation spintronics. Nonetheless, the macroscopic coherent spin current formations are still hindered by rapid dephasing due to electron scattering, specifically at room temperature. Exciton polaritons offer excellent platforms for spin-optronic devices via the optical spin Hall effect. However, this effect could neither be unequivocally observed at room temperature nor be exploited for practical spintronic devices due to the presence of strong thermal fluctuations or large linear spin splitting. Here we report the observation of room-temperature optical spin Hall effect of exciton polaritons, with the spin current flow over 60 μm in a formamidinium lead bromide perovskite microcavity. We provide direct evidence of long-range coherence in the flow of polaritons and the spin current carried by them. Leveraging the spin Hall transport of polaritons, we further demonstrate two polaritonic devices, namely, a NOT gate and a spin-polarized beamsplitter, advancing the frontier of room-temperature polaritonics in perovskite microcavities.

Optimally Suppressed Phonon Tunneling in van der Waals Graphene-WS2 Heterostructure with Ultralow Thermal Conductivity.

Van der Waals heterostructures have great potential for realizing ultimately low thermal conductivity because defectless interfaces can be constructed at a length scale smaller than the phonon wavelength, allowing modulation of coherent phonon transport. In this Letter, we demonstrate the mechanism for thermal conductivity reduction at a mode-resolved level. The graphene-WS2 heterostructure with the lowest cross-plane thermal conductivity of 0.048 W/(m·K) is identified from 16,384 candidates by combining Bayesian optimization and molecular dynamics simulations. Then, the angle-resolved phonon transmission is calculated using the mode-resolved atomistic Green's function. The results reveal that the optimal heterostructure nearly completely terminates phonon transport with finite incident angles, owing to the reduced critical incident angle and suppression of phonon tunneling.

Ballistic Electron Source with Magnetically Controlled Valley Polarization in Bilayer Graphene.

The achievement of valley-polarized electron currents is a cornerstone for the realization of valleytronic devices. Here, we report on ballistic coherent transport experiments where two opposite quantum point contacts (QPCs) are defined by electrostatic gating in a bilayer graphene (BLG) channel. By steering the ballistic currents with an out-of-plane magnetic field we observe two current jets, a consequence of valley-dependent trigonal warping. Tuning the BLG carrier density and number of QPC modes (m) with a gate voltage we find that the two jets are present for m=1 and up to m=6, indicating the robustness of the effect. Semiclassical simulations confirm the origin of the signals by quantitatively reproducing the jet separations without fitting parameters. In addition, our model shows that the ballistic current jets have opposite valley polarization. As a consequence, by steering each jet toward the detector using a magnetic field, we achieve full control over the valley polarization of the collected currents, envisioning such devices as ballistic current sources with tunable valley polarization. We also show that collimation experiments are a sensitive probe to the trigonal warping of the Fermi surface.

Interference effects in nonequilibrium quantum transport with long-range interactions

Abstract We investigate how long-range power-law hopping interaction, ∼ 1/r a , affects the characteristics of dissipative quantum transport in a nonequilibrium setting. The model under consideration is a noninteracting bosonic chain subject to thermal baths of differing temperature at its boundaries and dephasing noise which is applied uniformly to all the sites. It is shown that the steady-state current may vary nonmonotonically and has a peak for a finite a depending on the position of the cold bath. This site-specific behaviour stems back to the interference effect caused by the parity of the total sites N and the baths positions. The fractional nature of the system, along with the interplay between coherent and incoherent transport, will affect the steady state current that characterizes the transport.

What can we learn from the experiment of electrostatic conveyor belt for excitons?

Motivated by the experiment of electrostatic conveyor belt for indirect excitons (Winbowet al2011Phys. Rev. Lett.106196806), we studied the exciton patterns for understanding the exciton dynamics. By analyzing the exciton diffusion, we found that the patterns mainly came from the photoluminescence of two kinds of excitons. The patterns near the laser spot came from the hot excitons which can be regarded as the classical particles. However, the patterns far from the laser spot come from the cooled or coherent excitons. Considering the finite lifetime of Bosonic excitons and of the interactions between them, we built a time-dependent nonlinear Schrödinger equation including the non-Hermitian dissipation to describe the coherent exciton dynamics. The real-time and imaginary-time evolutions were used alternately to solve the Schrödinger equation to simulate the exciton diffusion accompanied by the exciton cooling in the moving lattices. By calculating the escape probability, we obtained the transport distances of the coherent excitons in the conveyor, consistent with the experimental data. The cooling speed of excitons was found to be important in coherent exciton transport. Moreover, the plateau in the average transport distance cannot be explained by the dynamical localization-delocalization transition induced by the disorders.

Non‐Monotonic Variation of the Low Lattice Thermal Conductivity with Temperature in Penta‐HgO2 Sheet

AbstractInspired by the experimental synthesis of bulk HgO2 with the potential of exfoliation to form a penta‐HgO2 sheet composed entirely of pentagonal motifs, a detailed theoretical study on the lattice thermal conductivity by using first‐principles calculations combined with the unified theory of thermal transport is performed. It is found that the penta‐HgO2 sheet is semiconducting with an indirect bandgap of 1.18 eV and possesses a low lattice thermal conductivity of 2.07 W m−1 K−1 (3.28 W m−1 K−1) along the x (y)‐direction at 300 K. More interestingly, the variation of its thermal conductivity with temperature is non‐monotonic, different from most cases. The phonon dispersion, phonon scattering, and phonon coherence is further systematically investigated to understand the underlying physics. This results suggest that the strong intrinsic anharmonicity resulting from its unique atomic configuration with the buckled structure and the heavy element of Hg leads to a high scattering rate, resulting in the ultralow particle‐like thermal transport of 0.20 W m−1 K−1 (0.01 W m−1 K−1) in the x (y)‐direction, while the narrow average frequency interval and strong phonon linewidth are responsible for the dominant coherent thermal transport and non‐monotonic variation of the low lattice thermal conductivity of the penta‐HgO2 sheet.

Quantum Dots and Molecular Junctions as Energy Filters for Thermal-to-Electric Energy Conversion

The thermoelectric conversion of heat into electricity requires a form of energy filtering, that allows hot electrons to pass while blocking cold electrons. The energy conversion efficiency is determined by the details of the energy filtering, with an infinitely sharp (delta function) transmission spectrum allowing near ideal (Carnot) efficiency. [1]I will introduce this concept and report on a proof-of-performance experiment based on a quantum dot embedded into a single, semiconductor nanowire. This near-ideal quantum-dot heat engine realized power production at maximum power with Curzon-Ahlborn efficiency as well more than 70% of Carnot efficiency at maximum efficiency settings [2].To further enhance efficiency at maximum power one wishes to shape the transmission function to allow a spectrum of warmer electrons to pass while still completely blocking cold electrons. This may be achieved by exploiting quantum interference phenomena to block low-energy electron transmission by destructive interference, and to thus sculpt the transmission function of quantum dots or molecular junctions.[4] In contrast to quantum dots, molecular junctions have the advantage that they can be used at room temperature and can be produced, in principle, cheaply and scalable. To systematically explore the effect of quantum interferences, we designed and synthesized a new class of porphyrins that are expected to feature destructive quantum interference in single molecule junctions with gold electrodes and may thus show high thermopower, as well as a control that does not. We performed detailed experimental single-molecule break-junction studies of conductance and thermopower on porphyrin molecular junctions. We find a somewhat better thermoelectric performance for the porphyrin where we expect destructive interference. However, the predicted large difference in conductance and thermopower is not supported by our experimental findings.[4]Finally, I will also briefly discuss the potential application of the concepts and systems presented here in hot-carrier photovoltaics, as an approach to increasing photovoltaic energy conversion efficiency beyond current limits.[1] T.E. Humphrey et al., Reversible Quantum Brownian Heat Engines for Electrons. Phys. Rev. Lett., 89, 116801 (2002)[2] M. Josefsson et al. A quantum-dot heat engine operated close to thermodynamic efficiency limits. Nature Nanotechn. 13, 920 (2018)[3] O Karlström et al. Increasing thermoelectric performance using coherent transport. Phys. Rev. B 84, 113415 (2011)[4] H. Xu et al. Electrical Conductance and Thermopower of β-Substituted Porphyrin Molecular Junctions Synthesis and Transport. JACS 145 , 23541 (2023) Figure 1

GPU-Enabled Exascale Quantum Transport Modeling of Carbon Nanotube Devices

The work will describe our efforts to develop an exascale electrostatic-quantum transport framework, Quantum-eXstatic (https://github.com/AMReX-Microelectronics/eXstatic), currently supporting the modeling of carbon nanotube field-effect transistors (CNTFETs). The framework is capable of modeling realistic three-dimensional structures, efficiently, using CPU/GPU heterogeneous architectures of state-of-the-art supercomputers. It is built on top of the AMReX library, which is a GPU-enabled software library containing functionality for writing particle-mesh applications on structured grids. The Quantum-eXstatic framework comprises three major components: the electrostatic module, the quantum transport module, and the part that self-consistently couples the two modules. The electrostatic module using a finite-volume multigrid solver to compute the electrostatic potential induced by charges on the surface of carbon nanotubes, as well as by source, drain, and gate terminals, which can be modeled using an embedded boundary approach to allow for intricate shapes. The quantum transport module uses the Nonequilibrium Green's Function (NEGF) method to model induced charge. Currently, it supports coherent (ballistic) transport, mode-space approximation for subbands of nanotube, contacts modeled as semi-infinite leads, and Hamiltonian representation using the tight-binding approximation. Special features are being included for accurate modeling of long nanotubes, such as adaptive refinement of integration contours to resolve van Hove singularities in long channels. Particular attention is paid to overlap computations with communication to optimize GPU performance. The self-consistency between the two modules is achieved using Broyden's modified second algorithm, which is parallelized using CPUs and GPUs. Preliminary scaling studies show that the code exhibits ~77% weak-scaling efficiency on 512 NVIDIA A100 GPUs of Perlmutter supercomputer for modeling gate-all-around CNTFETs with a channel length of 13.5 microns at equilibrium conditions. This case involved computations of electrostatic potential in 14.5 billion computational cells and Green’s function computation for a Hamiltonian of size 131,072 x 131,072. For problems of this large size, the code takes about 5 min to reach self-consistency. Figure 1

Spectral analysis of coherent and incoherent phonon transport in silicon nanomeshes

Spectral analysis of coherent and incoherent phonon transport in silicon nanomeshes

From stochastic Hamiltonian to quantum simulation: exploring memory effects in exciton dynamics

The unraveling of open quantum system dynamics in terms of stochastic quantum trajectories offers a picture of open system dynamics that consistently considers memory effects stemming from the finite correlation time of environment fluctuations. These fluctuations significantly influence the coherence and energy transport properties of excitonic systems. When their correlation time is comparable to the timescale of the Hamiltonian evolution, it leads to the departure of open system dynamics from the Markovian limit. In this work, we leverage the unraveling of exciton dynamics through stochastic Hamiltonian propagators to design quantum circuits that simulate exciton transport, capturing finite memory effects. In addition to enabling the synthesis of parametrizable quantum circuits, stochastic unitary propagators provide a transparent framework for investigating non-Markovian effects on exciton transport. Our analysis reveals a nuanced relationship between environment correlation time and transport efficiency, identifying a regime of ‘memory-assisted’ quantum transport where time-correlated fluctuations allow the system to reach higher efficiency. However, this property is not universal and can only be realized in conjunction with specific features of the system Hamiltonian.

Topological insulator based axial superconducting quantum interferometer structures

Nanoscale superconducting quantum interference devices are fabricated in-situ from a single Bi0.26Sb1.74Te3 nanoribbon that is defined using selective-area growth and contacted with superconducting Nb electrodes via a shadow mask technique. We present h/(2e) magnetic flux periodic interference in both, fully and non-fully proximitized nanoribbons. The pronounced oscillations are attributed to interference effects of coherent transport through the topological surface states encompassing the cross-section of the nanoribbon.

Half-Metallic Transport and Spin-Polarized Tunneling through the van der Waals Ferromagnet Fe4GeTe2.

We examine the coherent spin-dependent transport properties of the van der Waals (vdW) ferromagnet Fe4GeTe2 using density functional theory combined with the nonequilibrium Green's function method. Our findings reveal that the conductance perpendicular to the layers is half-metallic, meaning that it is almost entirely spin-polarized. This property persists from the bulk to a single layer, even under significant bias voltages and with spin-orbit coupling. Additionally, using dynamical mean field theory for quantum transport, we demonstrate that electron correlations are important for magnetic properties but minimally impact the conductance, preserving almost perfect spin-polarization. Motivated by these results, we then study the tunnel magnetoresistance (TMR) in a magnetic tunnel junction consisting of two Fe4GeTe2 layers with the vdW gap acting as an insulating barrier. We predict a TMR ratio of ∼500%, which can be further enhanced by increasing the number of Fe4GeTe2 layers in the junction.

Coherent excitation transport through ring-shaped networks

Coherent excitation transport through ring-shaped networks

Synthesis of α-Bi/SrTiO3 heterostructure through the Eu-induced reduction of Bi2O2Se/SrTiO3

Thin Bi films, especially noncentrosymmetric α-phase Bi films (α-Bi), have attracted considerable attention in recent years due to their intriguing physical properties such as ferroelectricity, nonlinear optical response, and coherent spin transport. However, the current experimental preparation of α-Bi films still presents substantial challenges, resulting in only isolated α-Bi islands being achieved. In this study, α-Bi/SrTiO3 (α-Bi/STO) was synthesized from a Bi2O2Se/STO (BOS/STO) heterostructure by depositing a Eu layer and reducing a BOS film. The so-formed α-Bi/STO interface features metallic conductivity with electron mobility of 7.7 × 104 cm2/V s and ultralow resistivity of 1 nΩ cm at 2 K, as well as high residual resistance ratios R300 K/R2 K up to 2353. Furthermore, we observed a complex weak antilocalization–weak localization–weak antilocalization crossover with increasing magnetic field at temperatures below 10 K. Our work presents the synthesis of α-Bi films, which could potentially serve as a valuable platform for experimental validation of diverse theoretical predictions associated with α-Bi heterostructures. Furthermore, this special synthesis method provides valuable insights into the preparation of other metastable films.

Atomistic simulations of coherent thermoelectric transport in ultrathin pristine and decorated noble metals nanowires

Nanowires (NWs) are envisioned to be an important component of novel nanoscale electronic devices. Here, we perform atomistic simulations of coherent thermoelectric (TE) transport in free-standing ultrathin NWs of noble metals (viz. Au, Ag, Cu, Pt) cut along the [111] crystallographic direction using a non-equilibrium Greens function based first principles approach. Effect of decorating these NWs with Au or Pt atoms in an fcc environment, is also explored. Results of the calculated electronic structure show that all considered NWs, except the Pt and Pt-decorated NWs, are metallic and non-magnetic. Interestingly, Cu@Pt NW is found to be a magnetic semiconductor with an unequal band gap in ↑− and ↓−spin energy bands, while Pt NW and other Pt-decorated NWs (viz. Ag@Pt and Au@Pt) behave like a magnetic half-metal. To explore the TE potential of as-considered NWs, electrical conductance G, thermal conductance κT, and thermopower S are computed in the linear response regime. Notably, the metallic NWs exhibit a quantized G. Among these, pristine NWs show a quantized value 3G0 up to a temperature of 200 K, while Cu@Au NW preserves its quantized value 5G0 even at T exceeding 300 K. In thermal transport, valence electrons make a dominant contribution to the room-temperature κT of pristine NWs. The phononic contribution κph, however, goes up following decoration, with Pt-decorated NWs showing a pronounced increment (by a factor of ∼2). But, these NWs undergo a simultaneous reduction in the electronic contribution κel owing to half-metallic conduction, resulting in a net suppression of total thermal conductance κT. Moreover, Pt-decorated NWs show a manifold enhancement in zero-bias S, with Ag@Pt exhibiting the highest value ∼119μV/K. However, S can be augmented considerably by adjusting the chemical potential μ of electrodes, and an overall best S∼−890μV/K is found in Au NW for μ=0.575eV. In this way, TE figure of merit ZT is tunable in pristine NWs over the range 1−3.7. Correspondingly, the decorated NWs exhibit somewhat smaller S and ZT. Furthermore, spin-asymmetric electronic transport in Pt and Pt-decorated NWs results in net spin conductance Gs and spin thermopower Ss. It is found that the spin figure of merit ZsT, as high as 3.5, can be attained in Pt NW by controlling μ. Thus, noble metals NWs can be tailored for both thermoelectric and spin-thermoelectric applications. We have also investigated TE transport for the pristine NWs in the [110] direction, and found S and ZT to be comparatively much smaller. Such NWs may be useful as interconnects for low noise applications.

Coherent spin transport in a copper protein.

The coherent electron/spin transport in azurin, a species of copper protein, was calculated based on the Landauer model. The research is motivated by the fast electron transport and spin selectivity/polarization in azurin, which have been reported in relation to the chiral-induced spin selectivity of the peptide structure. The calculated spin polarization of copper proteins was large. This phenomenon was strongly influenced by the spin density of the atoms in the ligand group, whereas the contribution of copper was negligible. The results suggest that spin polarization in copper proteins is enhanced by that of the ligand groups. The predicted spin polarization aligns primarily with the scanning tunneling microscope-based break-junction technique to study the electronic properties of single-molecule junctions. Computational techniques employed in this study are nonequilibrium Green's functions (NEGF) and density functional theory (DFT) based on the Landauer model, implemented using the QuantumATK software (Synopsys Inc.). The Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional was adopted for spin-polarized generalized gradient approximation (SGGA). The valence atomic orbitals were constructed using the wavefunctions of the SIESTA package, which was based on the norm-conserving Troullier-Martins relativistic pseudopotentials for describing core electrons. The mesh used for real-space integration was 150 Ha.

Sub-100-fs energy transfer in coenzyme NADH is a coherent process assisted by a charge-transfer state

Excitation energy transfer (EET) is a key photoinduced process in biological chromophoric assemblies. Here we investigate the factors which can drive EET into efficient ultrafast sub-ps regimes. We demonstrate how a coherent transport of electronic population could facilitate this in water solvated NADH coenzyme and uncover the role of an intermediate dark charge-transfer state. High temporal resolution ultrafast optical spectroscopy gives a 54±11 fs time constant for the EET process. Nonadiabatic quantum dynamical simulations computed through the time-evolution of multidimensional wavepackets suggest that the population transfer is mediated by photoexcited molecular vibrations due to strong coupling between the electronic states. The polar aqueous solvent environment leads to the active participation of a dark charge transfer state, accelerating the vibronically coherent EET process in favorably stacked conformers and solvent cavities. Our work demonstrates how the interplay of structural and environmental factors leads to diverse pathways for the EET process in flexible heterodimers and provides general insights relevant for coherent EET processes in stacked multichromophoric aggregates like DNA strands.