Using a novel structure, consisting of two, independently contacted graphene single layers separated by an ultra-thin dielectric, we experimentally measure the Coulomb drag of massless fermions in graphene. At temperatures higher than 50 K, the Coulomb drag follows a temperature and carrier density dependence consistent with the Fermi liquid regime. As the temperature is reduced, the Coulomb drag exhibits giant fluctuations with an increasing amplitude, thanks to the interplay between coherent transport in the graphene layer and interaction between the two layers.
We show that Coulomb drag in hydrodynamic bilayer systems leads to additional viscosity terms in the hydrodynamic equations, i.e., the drag and drag-Hall viscosities, besides the well-known kinematic and Hall viscosities. These new viscosity terms arise from a change of the stress tensor due to the interlayer Coulomb interactions. All four viscosity terms are tunable by varying the applied magnetic field and the electron densities in the two layers. At certain ratios between the electron densities in the two layers, the drag viscosity dramatically changes the longitudinal transport resulting i…
We present a theory of negative Coulomb drag in capacitively coupled quantum wires based on the commensurability of the electron density and the long-range nature of the Coulomb interaction in the Tomonaga-Luttinger liquid. The commensurability introduces a notion of doped particles and holes. We point out that the long-range interaction allows a particle-hole pairing over the wires and that the particle-hole pairing brings about the positive drag of holes, that is, the negative drag.
We report Coulomb drag measurements between vertically-integrated quantum wires separated by a barrier only 15 nm wide. The temperature dependence of the drag resistance is measured in the true one-dimensional (1D) regime where both wires have less than one 1D subband occupied. As a function of temperature, an upturn in the drag resistance is observed in three distinct devices at a temperature $T^* \sim 1.6$ K. This crossover in Coulomb drag behaviour is consistent with Tomonaga-Luttinger liquid models for the 1D-1D drag between quantum wires.
We theoretically study Coulomb drag between two helical edges with broken spin-rotational symmetry, such as would occur in two capacitively coupled quantum spin Hall insulators. For the helical edges, Coulomb drag is particularly interesting because it specifically probes the inelastic interactions that break the conductance quantization for a single edge. Using the kinetic equation formalism, supplemented by bosonization, we find that the drag resistivity $ρ_D$ exhibits a nonmonotonic dependence on the temperature $T$. In the limit of low $T$, $ρ_D$ vanishes with decreasing $T$ as a power law…
Recent years have seen a surge of interest in studies of hydrodynamic transport in electronic systems. We investigate the electron viscosity of metals and find a new component that is closely related to Coulomb drag. Using the linear response theory, viscosity, a transport coefficient for momentum, can be extracted from the retarded correlation function of the momentum flux, i.e., the stress tensor. There exists a previously overlooked contribution to the shear viscosity from the interacting part of the stress tensor which accounts for the momentum flow induced by interactions. This contributi…
Electron interactions in and between wires become increasingly complex and important as circuits are scaled to nanometre sizes, or employ reduced-dimensional conductors like carbon nanotubes, nanowires and gated high mobility 2D electron systems. This is because the screening of the long-range Coulomb potential of individual carriers is weakened in these systems, which can lead to phenomenon such as Coulomb drag: a current in one wire induces a voltage in a second wire through Coulomb interactions alone. Previous experiments have observed electron drag in wires separated by a soft electrostati…
We develop a kinetic equation description of Coulomb drag between ballistic one-dimensional electron systems, which enables us to demonstrate that equilibration processes between right- and left-moving electrons are crucially important for establishing dc drag. In one-dimensional geometry, this type of equilibration requires either backscattering near the Fermi level or scattering with small momentum transfer near the bottom of the electron spectrum. Importantly, pairwise forward scattering in the vicinity of the Fermi surface alone is not sufficient to produce a nonzero dc drag resistivity $ρ…
We investigate the influence of the chemical potential mismatch $Δ$ (different electron densities) on Coulomb drag between two parallel ballistic quantum wires. For pair collisions, the drag resistivity $ρ_{\rm D}(Δ)$ shows a peculiar anomaly at $Δ=0$ with $ρ_{\rm D}$ being finite at $Δ=0$ and vanishing at any nonzero $Δ$. The "bodyless" resonance in $ρ_{\rm D}(Δ)$ at zero $Δ$ is only broadened by processes of multi-particle scattering. We analyze Coulomb drag for finite $Δ$ in the presence of both two- and three-particle scattering within the kinetic equation framework, focusing on a Fokker-P…
We investigate interaction effects in transport phenomena in bilayer graphene (BLG). For the minimal conductivity in pristine BLG, we find that the conductivity assumes a constant value in the limit $T\to 0$, with the first correction being $\propto \sqrt{T}$. This has to be contrasted from the standard $1/T^2$ in Fermi liquids (neglecting additional logarithms and above all disorder). We furthermore study the Coulomb drag resistivity between two BLGs in the whole range from deep within the Fermi liquid regime all the way to the charge neutrality (CN) point. We find that in the Fermi liquid re…
Nonreciprocal perfect Coulomb drag in electron-hole bilayers: coherent exciton superflow as a diode
Tsung-Dao Lee Institute & School of Physics and Astronomy, Shanghai Jiao Tong University（上海交大李政道研究所与物理与天文学院）；Shanghai Branch, Hefei National Laboratory（合肥国家实验室上海分部）
Coulomb drag between two spin incoherent Luttinger liquids
在足够低密度的一维电子气中，相邻电子之间的磁（自旋）交换能$J$相对于特征电荷能（费米能$E_F$）被指数抑制。在非零温度$T$下，可以达到能级$J \ll T \ll E_F$，我们称之为自旋非相干卢廷格液态。我们讨论了两个平行的量子线之间的库仑阻力在自旋非相干制度，以及交叉到这个状态从低温制度，通过使用一个模型的波动维格纳固体。对于固定的电子密度，当温度从零升高到高于$J$时，密度-密度关联中的$2k_F$振荡消失。其结果是，库仑阻力的温度依赖性显着改变，并可能导致非单调的依赖性。讨论了等密度和不等密度丝之间的阻力，以及丝中弱无序的影响。我们推测，弱无序可能发挥重要作用，在提取信息的量子线在真正的拖动实验。
Coulomb drag is a process whereby the repulsive interactions between electrons in spatially separated conductors enable a current flowing in one of the conductors to induce a voltage drop in the other. If the second conductor is part of a closed circuit, a net current will flow in that circuit. The drag current is typically much smaller than the drive current owing to the heavy screening of the Coulomb interaction. There are, however, rare situations in which strong electronic correlations exist between the two conductors. For example, bilayer two-dimensional electron systems can support an ex…
In Coulomb drag, a current flowing in one conductor can induce a voltage across an adjacent conductor via the Coulomb interaction. The mechanisms yielding drag effects are not always understood, even though drag effects are sufficiently general to be seen in many low-dimensional systems. In this Letter, we observe Coulomb drag in a Coulomb-coupled double quantum dot (CC-DQD) and, through both experimental and theoretical arguments, identify cotunneling as essential to obtaining a correct qualitative understanding of the drag behavior.
We show that Coulomb drag in ultra-clean graphene double layers can be used for controlling the on/off ratio for current flow by tunning the external gate voltage. Hence, although graphene remains semi-metallic, the double layer graphene system can be tuned from conductive to a highly resistive state. We show that our results explain previous data of Coulomb drag in double layer graphene samples in disordered SiO2 substrates.
Interaction between local magnetization and conduction electrons is responsible for a variety of phenomena in magnetic materials. We have shown that the spin-dependent motive force induced by magnetization dynamics in a conducting ferromagnet lead to the spin Coulomb drag effect. The spin Coulomb drag an intrinsic friction mechanism which operates whenever the average velocities of up-spin and down-spin electrons differ.
We study theoretically Coulomb drag in capacitively coupled quantum dots (CQDs) -- a biasdriven dot coupled to an unbiased dot where transport is due to Coulomb mediated energy transfer drag. To this end, we introduce a master-equation approach which accounts for higher-order tunneling (cotunneling) processes as well as energy-dependent lead couplings, and identify a mesoscopic Coulomb drag mechanism driven by nonlocal multi-electron cotunneling processes. Our theory establishes the conditions for a nonzero drag as well as the direction of the drag current in terms of microscopic system parame…
Coulomb interaction between two closely spaced parallel layers of electron system can generate the frictional drag effect by interlayer Coulomb scattering. Employing graphene double layers separated by few layer hexagonal boron nitride (hBN), we investigate density tunable magneto- and Hall-drag under strong magnetic fields. The observed large magneto-drag and Hall-drag signals can be related with Laudau level (LL) filling status of the drive and drag layers. We find that the sign and magnitude of the magneto- and Hall-drag resistivity tensor can be quantitatively correlated to the variation o…
Tunable reciprocal and nonreciprocal contributions to 1D Coulomb Drag
University of Florida（佛罗里达大学）；National High Magnetic Field Laboratory（国家高磁场实验室）；Center for Integrated Nanotechnologies, Sandia National Laboratories（桑迪亚国家实验室集成纳米技术中心）
We develop a theory of Coulomb drag due to momentum transfer between graphene layers in a strong magnetic field. The theory is intended to apply in systems with disorder that is weak compared to Landau level separation, so that Landau level mixing is weak, but strong compared to correlation energies within a single Landau level, so that fractional quantum Hall physics is not relevant. We find that in contrast to the zero-field limit, the longitudinal magneto-Coulomb drag is finite, and in fact attains a maximum at the simultaneous charge neutrality point (CNP) of both layers. Our theory also p…
We present the first experimental study of mesoscopic fluctuations of Coulomb drag in a system with two layers of composite fermions, which are seen when either the magnetic field or carrier concentration are varied. These fluctuations cause an alternating sign of the average drag. We study these fluctuations at different temperatures to establish the dominant dephasing mechanism of composite fermions.
Coulomb drag as a signature of the paired quantum Hall state
受M. P. Lilly et.在朗道能级填充因子为$ν=1/2$的情况下，研究了两层系统中的库仑阻力。我们发现，在不可压缩的成对量子霍尔态在零温度下的拖曳电导率可以是有限的。在$T_c$以上，拖曳电导率也大大增强，在该位置处，弱耦合可压缩液体与成对量子霍尔液体之间发生转变。我们讨论了我们的结果对最近的实验的影响。
The Coulomb drag phenomenon in a Coulomb-coupled double quantum dot system is revisited with a simple model that highlights the importance of simultaneous tunneling of electrons. Previously, cotunneling effects on the drag current in mesoscopic setups have been reported both theoretically and experimentally. However, in both cases the sequential tunneling contribution to the drag current was always present unless the drag level position were too far away from resonance. Here, we consider the case of very large Coulomb interaction between the dots, whereby the drag current needs to be assisted …
We study Coulomb drag in a system consisting of a carbon nanotube (CNT) and monolayer graphene. Within the Fermi liquid theory we calculate the drag resistivity and find that the dimensional mismatch of the system components leads to a dependence of the drag rate on the carrier density, temperature, and spacing, which is substantially different from what is known for graphene double layers. Due to the competing effects of forward and backward scattering, we identify new features of the drag dependence on the electron density, which allows us to control their relative contribution to the drag r…
We develop a Boltzmann-Langevin description of Coulomb drag effect in clean double-layer systems with large interlayer separation $d$ as compared to the average interelectron distance $λ_F$. Coulomb drag arises from density fluctuations with spatial scales of order $d$. At low temperatures, their characteristic frequencies exceed the intralayer equilibration rate of the electron liquid, and Coulomb drag may be treated in the collisionless approximation. As temperature is raised, the electron mean free path becomes short due to electron-electron scattering. This leads to local equilibration of …
We review the fabrication and key transport properties of graphene double layers, consisting of two graphene monolayers placed in close proximity, independently contacted, and separated by an ultra-thin dielectric. We outline a simple band structure model relating the layer densities to the applied gate and inter-layer biases, and show that calculations and experimental results are in excellent agreement both at zero and in high magnetic fields. Coulomb drag measurements, which probe the electron-electron scattering between the two layers reveal two distinct regime: (i) diffusive drag at eleva…
Anomalous Hall effect (AHE) and spin Hall effect (SHE) are fundamental phenomena, and their potential for application is great. However, we understand the interaction effects unsatisfactorily, and should have clarified issues about the roles of the Fermi sea term and Fermi surface term of the conductivity of the intrinsic AHE or SHE of an interacting multiorbital metal and about the effects of spin-Coulomb drag on the intrinsic SHE. Here we resolve the first issue and provide the first step about the second issue by developing a general formalism in the linear response theory with appropriate …
Coulomb drag between nanoscale conductors is of both fundamental and practical interest. Here, we theoretically study drag in a double quantum-dot (QD) system consisting of a biased drive QD and an unbiased drag QD coupled via a direct interdot Coulomb interaction. We demonstrate that the Coulomb drag is driven by the charge fluctuations in the drive QD, and show how the properties of the associated quantum noise allow to distinguish it from, e.g., shot-noise driven drag in circuits of weakly interacting quantum conductors. In the strong-interaction regime exhibiting an orbital ("pseudospin") …
Spin Coulomb drag beyond the random phase approximation
在无规相近似下研究了准二维电子气中的自旋库仑阻力。我们发现，有限的横向宽度的电子气的自旋库仑阻力的显着减少。然而，这种减少在很大程度上被RPA之外的多体局域场效应的增强所补偿，从而恢复了与C. P. Weber \textit{et al.}，Nature，\textbf{437}，1330（2005）.
Recently, novel Coulomb drag mechanisms in capacitively coupled double quantum dots were uncovered by the T-matrix based master equation (TME). The TME is so far the primary approach to studying Coulomb drag in the weak-coupling regime; however, its accuracy and reliability remain unexplored. Here, we evaluate the performance of the TME for Coulomb drag via a comparison with numerically exact results obtained by the hierarchical equation-of-motion approach. We find that the TME can capture qualitative current evolutions versus dot levels, temperature, and effective coupling strengths, but only…
Recent measurements revealed an anomalous Coulomb drag in graphene, hinting at new physics at charge neutrality. The anomalous drag is explained by a new mechanism based on energy transport, which involves interlayer energy transfer, coupled to charge flow via lateral heat currents and thermopower. The old and new drag mechanisms are governed by distinct physical effects, resulting in starkly different behavior, in particular for drag magnitude and sign near charge neutrality. The new mechanism explains the giant enhancement of drag near charge neutrality, as well as its sign and anomalous sen…
