We report Coulomb drag measurements on GaAs-AlGaAs electron-hole bilayers. The two layers are separated by a 10 or 25nm barrier. Below T$\approx$1K we find two features that a Fermi-liquid picture cannot explain. First, the drag on the hole layer shows an upturn, which may be followed by a downturn. Second, the effect is either absent or much weaker in the electron layer, even though the measurements are within the linear response regime. Correlated phases have been anticipated in these, but surprisingly, the experimental results appear to contradict Onsager's reciprocity theorem.
We study the frictional drag between carriers in two bilayer graphene flakes separated by a 2 $-$ 5 nm thick hexagonal boron nitride dielectric. At temperatures ($T$) lower than $\sim$ 10 K, we observe a large anomalous negative drag emerging predominantly near the drag layer charge neutrality. The anomalous drag resistivity increases dramatically with reducing {\it T}, and becomes comparable to the layer resistivity at the lowest {\it T} = 1.5 K. At low $T$ the drag resistivity exhibits a breakdown of layer reciprocity. A comparison of the drag resistivity and the drag layer Peltier coefficie…
We develop a theory for describing frictional drag in bilayer systems with in-plane periodic potential modulations, and use it to investigate the drag between bilayer systems in which one of the layers is modulated in one direction. At low temperatures, as the density of carriers in the modulated layer is changed, we show that the transresistivity component in the direction of modulation can change its sign. We also give a physical explanation for this behavior.
Coupled 2D sheets of electrons and holes are predicted to support novel quantum phases. Two experiments of Coulomb drag in electron-hole (e-h) double bilayer graphene (DBLG) have reported an unexplained and puzzling sign reversal of the drag signal. However, we show that this effect is due to the multiband character of DBLG. Our multiband Fermi liquid theory produces excellent agreement and captures the key features of the experimental drag resistance for all temperatures. This demonstrates the importance of multiband effects in DBLG: they have a strong effect not only on superfluidity, but al…
Coulomb drag experiments can give us information about the interaction state of double-layer systems. Here, we demonstrate anomalous Coulomb drag behaviours in a two-dimensional electron-electron bilayer system constructed by stacking atomically thin MoS2 on opposite sides of thin dielectric layers of boron nitride. In the low temperature regime, the measured drag resistance does not follow the behaviour predicted by the Coulomb drag models of exchanging momenta and energies with the particles in Fermi-liquid bilayer systems. Instead, it shows an upturn to higher and higher values. We investig…
The mechanisms of optical activity and quantum transport of twisted bilayer graphene are studied. The formation of unique electron states in the bilayer systems is studied using an effective continuum model. Such states are shown to support the correlation of transverse motions of electrons in two graphene layers. Because of the chiral structure of the atomic lattices, the contribution of such drag correlations is incompletely cancelled, thus resulting in a drag term of the optical conductivity tensor. We show that the drag term of the conductivity is the manifestation of the spatial dispersio…
Coulomb drag between parallel quantum wells provides a uniquely sensitive measurement of electron correlations since the drag response depends on interactions only. Recently it has been demonstrated that a new regime of strong interactions can be accessed for devices consisting of two monlolayer graphene (MLG) crystals, separated by few layer hexagonal boron-nitride. Here we report measurement of Coulomb drag in a double bilayer graphene (BLG) stucture, where the interaction potential is anticipated to be yet further enhanced compared to MLG. At low temperatures and intermediate densities a ne…
We investigate the effect of exchange and correlation (xc) in low-density electron bilayers. Along with the direct Coulomb interaction, the effective electron-electron interaction mediated by the exchange of virtual polar optical (PO) phonons is considered. We find that the introduction of xc corrections results in a significant enhancement of the transresistivity and qualitative changes in its temperature dependence. The virtual PO-phonon contribution behaves similarly to the Coulomb drag and reduces noticeably the total drag thereby resulting in a better agreement with the recent experimenta…
Quantized spin waves, or magnons, in a magnetic insulator are assumed to interact weakly with the surroundings, and to flow with little dissipation or drag, producing exceptionally long diffusion lengths and relaxation times. In analogy to Coulomb drag in bilayer two dimensional electron gases, in which the contribution of the Coulomb interaction to the electric resistivity is studied by measuring the interlayer resistivity (transresistivity), we predict a nonlocal drag of magnons in a ferromagnetic bilayer structure based on semiclassical Boltzmann equations. Nonlocal magnon drag depends on m…
Several recent experiments have reported an anomalous temperature dependence of the Coulomb drag effect in electron-hole bilayers. Motivated by these puzzling data, we study theoretically a low-density electron-hole bilayer, where electrons and holes avoid quantum degeneracy by forming excitonic molecules. We describe the ionization-recombination crossover between the electron-hole plasma and exciton gas and calculate both the intralayer and drag resistivity as a function of temperature. The latter exhibits a minimum followed by a sharp upturn at low temperatures in a qualitative agreement wit…
We present measurements of Coulomb drag in an ambipolar GaAs/AlGaAs double quantum well structure that can be configured as both an electron-hole bilayer and a hole-hole bilayer, with an insulating barrier of only 10 nm between the two quantum wells. The Coulomb drag resistivity is a direct measure of the strength of the interlayer particle-particle interactions. We explore the strongly interacting regime of low carrier densities (2D interaction parameter $r_s$ up to 14). Our ambipolar device design allows comparison between the effects of the attractive electron-hole and repulsive hole-hole i…
Employing diagrammatic perturbation theory, we calculate the (charge) Coulomb drag resistivity $ρ_D$ and spin Coulomb drag resistivity $ρ_{\uparrow\downarrow}$ in the presence of Rashba spin-orbit coupling. Analytical expressions for $ρ_D$ and $ρ_{\uparrow\downarrow}$ are derived, and it is found that spin-orbit interaction produces a small enhancement to $ρ_D$ and $ρ_{\uparrow\downarrow}$ in the ballistic regime while $ρ_D$ is unchanged in the diffusive regime. This enhancement in the ballistic regime is attributed to the enhancement of the nonlinear susceptibility (i.e. current produced thro…
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 theoretically calculate the interaction-induced frictional Coulomb drag resistivity between two graphene monolayers as well as between two graphene bilayers, which are spatially separated by a distance "$d$". We show that the drag resistivity between graphene monolayers can be significantly affected by the intralayer momentum-relaxation mechanism. For energy independent intralayer scattering, the frictional drag induced by inter-layer electron-electron interaction goes asymptotically as $ρ_D \sim T^2/n^4d^6$ and $ρ_D \sim T^2/n^2d^2$ in the high-density ($k_F d \gg 1$) and low-density ($k_F…
Interaction between local magnetization and conduction electrons is responsible for a variety of phenomena in magnetic materials. We have shown that the spin-dependent motive force induced by magnetization dynamics in a conducting ferromagnet lead to the spin Coulomb drag effect. The spin Coulomb drag an intrinsic friction mechanism which operates whenever the average velocities of up-spin and down-spin electrons differ.
We study theoretically Coulomb drag in capacitively coupled quantum dots (CQDs) -- a biasdriven dot coupled to an unbiased dot where transport is due to Coulomb mediated energy transfer drag. To this end, we introduce a master-equation approach which accounts for higher-order tunneling (cotunneling) processes as well as energy-dependent lead couplings, and identify a mesoscopic Coulomb drag mechanism driven by nonlocal multi-electron cotunneling processes. Our theory establishes the conditions for a nonzero drag as well as the direction of the drag current in terms of microscopic system parame…
In this work we implement the self-consistent Thomas-Fermi model that also incorporates a local conductivity model to an electron-electron bilayer system, in order to describe novel magneto-transport properties such as the Drag Phenomenon. The model can successfully account for the poor screening of the potential within the incompressible strips and its impact on the interlayer Coulomb interaction. An externally applied current in the active layer results in the tilting of the Landau levels and built-up of a Hall potential across the layer, which, in turn, induces a tilted potential profile in…
We have fabricated bilayer-graphene double layers separated by a thin ($\sim$20 nm) boron nitride layer and performed Coulomb drag and counterflow thermoelectric transport measurements. The measured Coulomb drag resistivity is nearly three orders smaller in magnitude than the intralayer resistivities. The counterflow Seebeck coefficient is found to be well approximated by the difference between Seebeck coefficients of individual layers and exhibit a peak in the regime where two layers have opposite sign of charge carriers. The measured maximum counterflow power factor is $\sim$ 700 $μ$W/K$^2$c…
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…
We report electrical transport measurements on GaAs/AlGaAs based electron-hole bilayers. These systems are expected to make a transition from a pair of weakly coupled two-dimensional systems to a strongly coupled exciton system as the barrier between the layers is reduced. Once excitons form, phenomena such as Bose-Einstein condensation of excitons could be observed. In our devices, electrons and holes are confined in double quantum wells, and carriers in the devices are induced with top and bottom gates leading to variable density in each layer. Separate contact to each layer allows Coulomb d…
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 study a bilayer structure consisting of an antiferromagnetic insulator and a normal metal. An electron current is driven in the normal metal with direction parallel to the interface between the materials. Due to interfacial exchange coupling between the localized spins in the antiferromagnet and the itinerant electrons in the normal metal, a magnon current can then be induced in the antiferromagnet. Using an uncompensated antiferromagnetic interface, creating an asymmetry in the interfacial coupling to the two degenerate magnon modes, we find that it is possible to generate a magnon spin-cu…
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.
Motivated by the recent Coulomb drag experiment of M. P. Lilly et. al, we study the Coulomb drag in a two-layer system with Landau level filling factor $ν=1/2$. We find that the drag conductivity in the incompressible paired quantum Hall state at zero temperature can be finite. The drag conductivity is also greatly enhanced above $T_c$, at which the transition between the weakly coupled compressible liquids and the paired quantum Hall liquid takes place. We discuss the implications of our results for the recent experiment.
Measurements and calculations of Coulomb drag between two low density, closely spaced, two-dimensional electron systems are reported. The experimentally measured drag exceeds that calculated in the random phase approximation by a significant, and density dependent, factor. Studies of the dependence of the measured drag on the difference in density between the two layers clearly demonstrate that previously ignored q=2k_F scattering processes can be very important to the drag at low densities and small layer separations.
We study the Coulomb drag transresistivity between graphene layers employing the finite temperature density response function. We analyze the Coulomb coupling between the two layers and show that a universal scaling behavior, independent of the interlayer distance can be deduced. We argue that this universal behavior may be experimentally observable in a system of two carbon nanotubes of large radius.
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 the way back-scattering electron--electron interaction generates Coulomb drag between quantum wires with different densities. At low temperature $T$ the system can undergo a commensurate-- incommensurate transition as the potential difference $|W|$ between the two wires passes a critical value $Δ$, and this transition is reflected in a marked change in the dependence of drag resistivity on $W$ and $T$. At high temperature a density difference between the wires suppresses Coulomb drag induced by back scattering, and we use the Tomonaga--Luttinger model to study this suppression in deta…
We work out a theory of the Coulomb drag current created under the ballistic transport regime in a one-dimensional nanowire by a ballistic non-Ohmic current in a nearby parallel nanowire. As in the Ohmic case, we predict sharp oscillation of the drag current as a function of gate voltage or the chemical potential of electrons. We study also dependence of the drag current on the voltage V across the driving wire. For relatively large values of V the drag current is proportional to V^2.
I suggest to use the effect of Coulomb drag between two closely positioned graphite monolayers (graphene sheets) for experimental measurement of the strength of weak non-linearities of the spectrum in graphene. I consider trigonal warping as a representative mechanism responsible for the drag effect. Since graphene is relatively defect-free, I evaluate the drag conductivity in the ballistic regime and find that it is proportional to the fourth power of the warping strength.
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 present a theory for Coulomb drag between two mesoscopic systems. Our formalism expresses the drag in terms of scattering matrices and wave functions, and its range of validity covers both ballistic and disordered systems. The consequences can be worked out either by analytic means, such as the random matrix theory, or by numerical simulations. We show that Coulomb drag is sensitive to localized states, which usual transport measurements do not probe. For chaotic 2D-systems we find a vanishing average drag, with a nonzero variance. Disordered 1D-wires show a finite drag, with a large varian…
We demonstrate that in a wide range of temperatures Coulomb drag between two weakly coupled quantum wires is dominated by processes with a small interwire momentum transfer. Such processes, not accounted for in the conventional Luttinger liquid theory, cause drag only because the electron dispersion relation is not linear. The corresponding contribution to the drag resistance scales with temperature as T^2 if the wires are identical, and as T^5 if the wires are different.
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 study drag effect in a system of two electrically isolated quantum point contacts (QPC), coupled by Coulomb interactions. Drag current exhibits maxima as a function of QPC gate voltages when the latter are tuned to the transitions between quantized conductance plateaus. In the linear regime this behavior is due to enhanced electron-hole asymmetry near an opening of a new conductance channel. In the non-linear regime the drag current is proportional to the shot noise of the driving circuit, suggesting that the Coulomb drag experiments may be a convenient way to measure the quantum shot noise…
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…
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") …
Coulomb drag between two quantum wires is exponentially sensitive to the mismatch of their electronic densities. The application of a magnetic field can compensate this mismatch for electrons of opposite spin directions in different wires. The resulting enhanced momentum transfer leads to the conversion of the charge current in the active wire to the spin current in the passive wire.
We study the spin Coulomb drag in a quasi-two-dimensional electron gas beyond the random phase approximation (RPA). We find that the finite transverse width of the electron gas causes a significant reduction of the spin Coulomb drag. This reduction, however, is largely compensated by the enhancement coming from the inclusion of many-body local field effects beyond the RPA, thereby restoring good agreement with the experimental observations by C. P. Weber \textit{et al.}, Nature, \textbf{437}, 1330 (2005).
The influence of a longitudinal magnetic field on the Coulomb drag current created in the ballistic transport regime in a quantum well by a ballistic current in a nearby parallel quantum well is investigated. We consider the case where the magnetic field is so strong that the Larmour radius is smaller than the width of the well. Both in Ohmic and non-Ohmic case, sharp oscillations of the drag current as a function of the gate voltage or chemical potential are predicted. We also study dependence of the drag current on the voltage $V$ across the driving wire, as well as on the magnetic field $B$…
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…
Dirac fermions are actively investigated, and the discovery of the quantized anomalous Hall effect of massive Dirac fermions has spurred the promise of low-energy electronics. Some materials hosting Dirac fermions are natural platforms for interlayer coherence effects such as Coulomb drag and exciton condensation. Here we determine the role played by the anomalous Hall effect in Coulomb drag in massive Dirac fermion systems. We focus on topological insulator films with out-of plane magnetizations in both the active and passive layers. The transverse response of the active layer is dominated by…
We investigate transport and Coulomb drag properties of semiconductor-based electron-hole bilayer systems. Our calculations are motivated by recent experiments in undoped electron-hole bilayer structures based on GaAs-AlGaAs gated double quantum well systems. Our results indicate that the background charged impurity scattering is the most dominant resistive scattering mechanism in the high-mobility bilyers. We also find that the drag transresistivity is significantly enhanced when the electron-hole layer separation is small due to the exchange induced renormalization of the single layer compre…
We report the observation of Coulomb drag between a two-dimensional (2D) electron gas in graphene and a one-dimensional (1D) wire composed of a carbon nanotube. We find that drag occurs when the bulk of graphene is conducting, but is strongly suppressed in the quantum Hall regime when magnetic field confines conducting electrons to the edges of graphene and far from the nanotube. Out-of-equilibrium and non-linear drag measurements show intriguing interplay between 1D and 2D conductors. These hybrid electronic devices of novel geometry could lead to potential applications for Van der Waals elec…
We consider a number of strongly-correlated quantum Hall states which are likely to be realized in bilayer quantum Hall systems at total Landau level filling fraction ${ν_T}=1$. One state, the $(3,3,-1)$ state, can occur as an instability of a compressible state in the large $d/l_B$ limit, where $d$ and $l_B$ are the interlayer distance and magnetic length, respectively. This state has a hierarchical descendent which is interlayer coherent. Another interlayer coherent state, which is expected in the small $d/l_B$ limit is the well-known Halperin $(1,1,1)$ state. Using the concept of composite …
We evaluate the Coulomb drag current in two finite-length Tomonaga-Luttinger-liquid wires coupled by an electrostatic backscattering interaction. The drag current in one wire shows oscillations as a function of the bias voltage applied to the other wire, reflecting interferences of the plasmon standing waves in the interacting wires. In agreement with this picture, the amplitude of the current oscillations is reduced with increasing temperature. This is a clear signature of non-Fermi-liquid physics because for coupled Fermi liquids the drag resistance is always expected to increase as the temp…
The frictional drag between parallel two-dimensional electron systems has been measured in a regime of strong interlayer correlations. When the bilayer system enters the excitonic quantized Hall state at total Landau level filling factor ν_T=1 the longitudinal component of the drag vanishes but a strong Hall component develops. The Hall drag resistance is observed to be accurately quantized at h/e^2.
We study the Coulomb drag between two single graphene sheets in intrinsic and extrinsic graphene systems with no interlayer tunneling. The general expression for the nonlinear susceptibility appropriate for single-layer graphene systems is derived using the diagrammatic perturbation theory, and the corresponding exact zero-temperature expression is obtained analytically. We find that, despite the existence of a non-zero conductivity in an intrinsic graphene layer, the Coulomb drag between intrinsic graphene layers vanishes at all temperatures. In extrinsic systems, we obtain numerical results …
Two-dimensional (2D) heterointerfaces often provide extraordinary carrier transport as exemplified by superconductivity or excitonic superfluidity. Recently, double-layer graphene separated by few-layered boron nitride demonstrated the Coulomb drag phenomenon: carriers in the active layer drag the carriers in the passive layer. Here, we propose a new switching device operating via Coulomb drag interaction at a graphene/MoS2 (GM) heterointerface. The ideal van der Waals distance allows strong coupling of the interlayer electron-hole pairs, whose recombination is prevented by the Schottky barrie…
We develop a Luttinger liquid theory of the Coulomb drag of persistent currents flowing in concentric mesoscopic rings, by incorporating non-linear corrections to the electron dispersion relation. We demonstrate that at low temperatures, interactions between electrons in different rings generate an additional phase and thus alter the period of Aharonov-Bohm oscillations. The resulting nondissipative drag depends strongly on the relative parity of the electron numbers. We also show that interactions set a new temperature scale below which the linear response theory does not apply at certain val…
We show that the Coulomb drag effect exhibits saturation at small temperatures, when calculated to the third order in the interlayer interactions. The zero-temperature transresistance is inversely proportional to the third power of the dimensionless sheet conductance. The effect is therefore the strongest in low mobility samples. This behavior should be contrasted with the conventional (second order) prediction that the transresistance scales as a certain power of temperature and is almost mobility-independent. The result demonstrates that the zero-temperature drag is not an unambiguous signat…
The onset of exciton condensation in a topological insulator thin film was recently predicted. We calculate the critical temperature for this transition, taking into account screening effects. Furthermore, we show that the proximity to this transition can be probed by measuring the Coulomb drag resistivity between the surfaces of the thin film as a function of temperature. This resistivity shows an upturn upon approaching the exciton-condensed state.
