In band insulators, where the Fermi surface is absent, adiabatic transport is allowed only due to the geometry of the Hilbert space. By driving the system at a small but finite frequency $ω$, transport is still expected to depend sensitively on the quantum geometry. Here we show that this expectation is correct and can be made precise by expressing the Kubo formula for conductivity as the variation of the \emph{time-dependent polarization} with respect to the applied field. In particular, a little appreciated effect is that at linear order in frequency, the longitudinal conductivity results from an intrinsic capacitance, determined by the ratio of the quantum metric and the spectral gap. We demonstrate that this intrinsic capacitance has a measurable effect in a wide range of insulators with non-negligible metric, including the electron gas in a quantizing magnetic field, the gapped bands of hBN-aligned twisted bilayer graphene, and obstructed atomic insulators such as diamond whose large refractive index has a topological origin. We also discuss the influence of quantum geometry on the dielectric constant.
A Metal-dielectric-topological insulator capacitor device based on hBN-encapsulated CVD grown Bi2Se3 is realized and investigated in the radio frequency regime. The RF quantum capacitance and device resistance are extracted for frequencies as a high as 10 GHz, and studied as a function of the applied gate voltage. The combination of the superior quality hBN dielectric gate with the optimized transport characteristics of CVD grown Bi2Se3 (n~10^18cm-3 in 8 nm) allow us to attain a bulk depleted regime by dielectric gating. A quantum capacitance minimum is observed revealing a purely Dirac regime…
BiSbTeSe$_2$ is a 3D topological insulator (3D-TI) with Dirac type surface states and low bulk carrier density, as donors and acceptors compensate each other. Dominating low temperature surface transport in this material is heralded by Shubnikov-de Haas oscillations and the quantum Hall effect. Here, we experimentally probe and model the electronic density of states (DOS) in thin layers of BiSbTeSe$_2$ by capacitance experiments both without and in quantizing magnetic fields. By probing the lowest Landau levels, we show that a large fraction of the electrons filled via field effect into the sy…
Quantum capacitance of an ultrathin topological insulator film in a magnetic field
本文研究了外磁场作用下双拓扑绝缘体薄膜的量子磁电容。本研究探讨塞曼作用与薄膜上下表面间杂化的相互作用。确定态密度，我们发现，电子空穴对称性被打破时，塞曼和杂化能量相对于彼此变化。这导致电荷中性点处的磁电容特性的变化。我们进一步表明，在塞曼相互作用和杂化的存在下，磁电容表现出拍在低和分裂的Shubnikov de Haas振荡在高垂直磁场。此外，我们解决了交叉从垂直到平行的磁场，并找到最近的实验数据的一致性。
The discovery of two-dimensional gapless Dirac fermions in graphene and topological insulators (TI) has sparked extensive ongoing research toward applications of their unique electronic properties. The gapless surface states in three-dimensional insulators indicate a distinct topological phase of matter with a non-trivial Z2 invariant that can be verified by angle-resolved photoemission spectroscopy or magnetoresistance quantum oscillation. In TI nanowires, the gapless surface states exhibit Aharonov-Bohm (AB) oscillations in conductance, with this quantum interference effect accompanying a ch…
In this paper, we demonstrate the principle of operation of a metal-insulator-semiconductor (MIS) capacitor based on undoped organic semiconductor. In spite of low charge concentration within the semiconductor, this device exhibits a capacitance variation with respect to applied gate voltage $V_g$, resembling the capacitance-voltage $C$-$V$ characteristics of a traditional doped semiconductor based MIS capacitor. A physics based model is developed to derive charge concentration, surface potential $ψ_s$ and the capacitance of organic MIS capacitor. The model is validated with TCAD simulation re…
We study the zero-temperature phase diagram of a dissipationless and disorder-free Josephson junction chain. Namely, we determine the critical Josephson energy below which the chain becomes insulating, as a function of the ratio of two capacitances: the capacitance of each Josephson junction and the capacitance between each superconducting island and the ground. We develop an imaginary-time path integral Quantum Monte-Carlo algorithm in the charge representation, which enables us to efficiently handle the electrostatic part of the chain Hamiltonian. We find that a large part of the phase diagr…
Spectroscopy of discrete Landau levels (LLs) in bulk-insulating three-dimensional topological insulators (3D TIs) in perpendicular magnetic field characterizes the Dirac nature of their surface states. Despite a number of studies demonstrating the quantum Hall effect (QHE) of topological surface states, quantitative evaluation of the LL energies, which serve as fundamental electronic quantities for study of the quantum states, is still limited. In this work, we explore the density of states of LLs by measuring quantum capacitance (CQ) in a truly bulk insulating 3D TI via a van der Waals hetero…
We analyze the distributions of electric potential and field, polarization and charge, and the differential capacitance of a silicon metal-oxide-ferroelectric field effect transistor (MOSFET), in which a gate insulator consists of thin layers of dielectric SiO2 and weak ferroelectric HfO2. It appeared possible to achieve a quasi-steady-state negative capacitance of the HfO2 layer, C_(HfO_2 )<0, if the layer thickness is close to the critical thickness of the size-induced ferroelectric-paraelectric phase transition. The quasi-steady-state negative capacitance, being a very slow-varying transien…
We investigate superconductor-insulator quantum phase transitions in ultrathin capacitively coupled superconducting nanowires with proliferating quantum phase slips. We derive a set of coupled Berezinskii-Kosterlitz-Thouless-like renormalization group equations demonstrating that interaction between quantum phase slips in one of the wires gets modified due to the effect of plasma modes propagating in another wire. As a result, the superconductor-insulator phase transition in each of the wires is controlled not only by its own parameters but also by those of the neighboring wire as well as by m…
When transistor gate insulators have nanometer-scale equivalent oxide thickness (EOT), the gate capacitance ($C_\textrm{G}$) becomes smaller than the oxide capacitance ($C_\textrm{ox}$) due to the quantum capacitance and charge centroid capacitance of the channel. Here, we study the capacitance of monolayer MoS$_\textrm{2}$ as a prototypical two-dimensional (2D) channel while considering spatial variations in the potential, charge density, and density of states. At 0.5 nm EOT, the monolayer MoS$_\textrm{2}$ capacitance is smaller than its quantum capacitance, limiting the single-gated $C_\text…
Transformation of an energy spectrum and wave functions in the crossover from two- to three-dimensional topological insulator in HgTe quantum wells: long and thorny way
Electrostatic Coupling between Two Surfaces of a Topological Insulator Nanodevice
我们报道了低载流子密度拓扑绝缘体Bi 1.5 Sb 0.5 Te 1.7 Se 1.3的双栅纳米器件的电子输运测量。在所有器件中，上下表面态都可以通过顶部和底部栅电极独立地调谐到狄拉克点。在薄器件中，发现电场穿透体，表明表面状态之间的有限电容耦合。充电模型允许我们使用穿透电场作为表面间电容$C_{TI}$和表面态能量密度关系$μ$（n）的测量，这被发现与独立的ARPES测量一致。在高磁场下，观察到通过表面态的场渗透增加，强烈暗示由于时间反演对称性破坏而导致的表面态带隙的开口。
Integrating ferroelectric negative capacitance (NC) into the field-effect transistor (FET) promises to break fundamental limits of power dissipation known as Boltzmann tyranny. However, realizing the stable static negative capacitance in the non-transient non-hysteretic regime remains a daunting task. The problem stems from the lack of understanding of how the fundamental origin of the NC due to the emergence of the domain state can be put in use for implementing the NC FET. Here we put forth an ingenious design for the ferroelectric domain-based field-effect transistor with the stable reversi…
The paired top and bottom Dirac surface states, each associated with a half-integer quantum Hall (QH) effect, and a resultant integer QH conductance (νe2/h), are hallmarks of a three-dimensional (3D) topological insulator (TI). In a dual-gated system, chemical potentials of the paired surface states are controlled through separate gates. In this work, we establish tunable capacitive coupling between the surface states of a bulk-insulating TI BiSbTeSe2 and study the effect of this coupling on QH plateaus and Landau level (LL) fan diagram via dual-gate control. We observe non-linear QH transitio…
Probing the Quantum Capacitance of Rydberg Transitions of Surface Electrons on Liquid Helium via Microwave Frequency Modulation
RIKEN Center for Quantum Computing（理化学研究所量子计算中心）；Institute of Physics, Kazan Federal University（喀山联邦大学物理研究所）；B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine（乌克兰国家科学院B.维尔金低温物理与工程研究所）；McGill University（麦吉尔大学）；RIKEN Cluster for Pioneering Research（理化学所前沿研究集群）
Topological-Insulator Heterophase Gate Stacks for Transistor Electrostatics
Gwangju Institute of Science and Technology（光州科学技术院）；Ulsan National Institute of Science and Technology（蔚山科学技术院）；Aalto University（阿尔托大学）；VTT Technical Research Centre of Finland Ltd.（芬兰技术研究中心有限公司）；Sungkyunkwan University（成均馆大学）；Korea Research Institute of Chemical Technology（韩国化学技术研究所）；OtaNano-Nanomicroscopy Center, Aalto University（阿尔托大学奥塔纳米显微镜中心）；Yonsei University（延世大学）；Beijing Institute of Technology（北京理工大学）；Nanyang Technological University（南洋理工大学）；Arizona State University（亚利桑那州立大学）；Seoul National University（首尔国立大学）；University of Cambridge（剑桥大学）
Two-dimensional electron and hole gases separated by a few nm from each other are produced in p-i-n diodes based upon MBE-grown GaAs/AlGaAs heterostructures. At such interlayer distances, the exciton formation and possibly Bose-Einstein condensation (BEC) is expected. We measure the capacitance between the layers and find it to oscillate as a function of the bias voltage. The peak values exceed the geometric capacitance by up to a factor of two. The surprisingly regular periods of the oscillations are of the order of 10 to 30 mV and scale linearly with the inverse of the thickness, between 60 …
We report on transport and capacitance spectroscopy study of two kinds of quantum wells, namely Cd$_{0.02}$Hg$_{0.98}$Te and Cd$_{0.06}$Hg$_{0.94}$Te with the thicknesses of 7.4 and 11.5 nm, accordingly. The fraction of Cd was chosen in a way that the both quantum wells are expected to have gapless band structure typical for a Dirac fermions system. We have established that the first quantum well exhibits a massless Dirac fermions system with a quality slightly better then in conventional HgTe quantum wells of critical thickness. Second quantum well exhibits a high-quality two-dimensional topo…
Towards the understanding of the origin of charge-current-induced spin voltage signals in the topological insulator Bi$_2$Se$_3$
拓扑绝缘体提供了一个新的平台，自旋电子学，由于自旋纹理的表面状态，拓扑鲁棒的弹性背散射。在这里，我们报告的调查所测得的电压从努力电探针自旋动量锁定在拓扑绝缘体Bi $_2$ Se $_3$使用铁磁接触。在反转铁磁接触的磁化时，我们发现测量电压的反转。考虑到磁化相对于电流的方向，对该电压的偏置和温度依赖性进行了广泛的分析。我们的研究结果表明，所测量的电压可以出现由于边缘场引起的霍尔电压，不同于电流引起的表面态载流子的自旋极化，最近报道。理解测量电压的非平凡起源对于实现具有拓扑绝缘体的自旋电子器件是重要的。
The giant spin Hall effect (SHE) at room temperature is one of the most attractive feature of topological insulators (TIs) for applications to nano-scale spin devices. Its origin, however, remains a controversial problem. Here, we identify the origin of the giant SHE in BiSb thin films by measuring the spin Hall angle $θ_{SH}$ under controllable contribution of surface and bulk conduction. We found that $θ_{SH}$ of a Bi$_{0.6}$Sb$_{0.4}$ TI thin film takes colossal values (450 - 530 at 8 K, and 38 at 300 K), and is almost governed by contribution from the topological surface states. Meanwhile,…
Motivated by recent experiments observing a large antidamping spin-orbit torque (SOT) on the surface of a three-dimensional topological insulator, we investigate the origin of the current-induced SOT beyond linear-response theory. We find that a strong antidamping SOT arises from intraband transitions in non-linear response, and does not require interband transitions as is the case in linear transport mechanisms. The joint effect of warping and an in-plane magnetization generates a non-linear antidamping SOT which can exceed the intrinsic one by several orders of magnitude, depending on warpin…
We compare thermal-gradient-driven transverse voltages in ferrimagnetic-insulator/heavy-metal bilayers (Tm3Fe5O12/W and Tm3Fe5O12/Pt) to corresponding electrically-driven transverse resistances at and above room temperature. We find for Tm3Fe5O12/W that the thermal and electrical effects can be explained by a common spin-current detection mechanism, the physics underlying spin Hall magnetoresistance (SMR). However, for Tm3Fe5O12/Pt the ratio of the electrically-driven transverse voltages (planar Hall signal/anomalous Hall signal) is much larger than the ratio of corresponding thermal-gradient …
Ionic liquid gated field-effect transistors (FETs) based on semiconducting transition metal dichalcogenides (TMDs) are used to study a rich variety of extremely interesting physical phenomena, but important aspects of how charge carriers are accumulated in these systems are not understood. We address these issues by means of a systematic experimental study of transport in monolayer MoSe$_2$ and WSe$_2$ as a function of magnetic field and gate voltage, exploring accumulated densities of carriers ranging from approximately 10$^{14}$ cm$^{-2}$ holes in the valence band to 4x10$^{14}$ cm$^{-2}$ el…
