Electron-electron Interaction Effects Research Articles

Unconventional large linear magnetoresistance in Cu2−xTe

We report a large linear magnetoresistance in Cu2−xTe, reaching Δρ/ρ(0) = 250% at 2 K in a 9 T field, for samples with x = 0.13 to 0.22. These results are comparable to those for Ag2X materials, though for Cu2−xTe the carrier densities are considerably larger. Examining the magnitudes and the crossover from quadratic to high-field linear behavior, we show that models based on classical transport behavior best explain the observed results. The effects are traced to the misdirection of currents in high mobility transport channels, likely due to behavior at grain boundaries such as topological surface states or a high mobility interface phase. The resistivity also exhibits a T2 dependence in the temperature range where the large linear MR appears, an indicator of electron-electron interaction effects within the high mobility states. Thus this is an example of a system in which electron-electron interactions dominate the low-temperature linear magnetoresistance.

Nonthermal switching of charge order: Dynamical slowing down and optimal control

We investigate the laser-induced dynamics of electronically driven charge-density-wave order. A comprehensive mean-field analysis of the attractive Hubbard model in the weak-coupling regime reveals ultrafast switching and ultrafast melting of the order via a nonthermal pathway. The resulting nonequilibrium phase diagram exhibits multiple dynamical phase transitions with increasing field strength. Using an intuitive pseudospin picture, we show that the laser can be regarded as a external (pseudo) magnetic field, and that the distinct dynamical regimes can be connected to the spin precession angle. We furthermore study the effects of electron-electron interactions beyond mean-field to show that the main features of the phase diagram are robust against scattering or thermalization processes. For example, the nonthermal state with switched order is characterized by a particularly slow relaxation. We also demonstrate how these nonthermal phases can be stabilized by tailoring the pulse shape with optimal control theory.

Unexpected metal-insulator transition in thick Ca1-xSrxVO3 film on SrTiO3 (100) single crystal

Epitaxial Ca1-xSrxVO3 (0 ≦ x ≦ 1) thin films were grown on (100)-oriented SrTiO3 substrates by using the pulsed laser deposition technique. In contrast to the previous report that metal-insulator transition (MIT) in Ca1-xSrxVO3 (CSVO) was achieved only for extremely thin films (several nm thick), MIT was observed at 39, 72, and 113 K for films with a thickness of 50 nm. The electronic structure was investigated by hard and soft X-ray photoemission spectroscopy (HX-PES and SX-PES). The difference between these PES results was significant due to the variation in an escape depth of photoelectrons of PES. While HX-PES showed that the V 2p3/2 spectra consisted of four peaks (V5+, V4+, V3+, and V2+/1+), SX-PES showed only three peaks (V5+, V4+, and V3+). This difference can be caused by a strain from the substrate, which leads to the chemical disorder (V5+, V4+, V3+, and V2+/1+). The thin film near the substrate is affected by the strain. The positive magnetoresistance is attributed to the effect of electron-electron interactions in the disorder system. Therefore, the emergence of MIT can be explained by the electron-electron interactions from the chemical disorder due to the strain.

Numerically exact full counting statistics of the nonequilibrium Anderson impurity model

The time-dependent full counting statistics of charge transport through an interacting quantum junction is evaluated from its generating function, controllably computed with the inchworm Monte Carlo method. Exact noninteracting results are reproduced; then, we continue to explore the effect of electron-electron interactions on the time-dependent charge cumulants, first-passage time distributions, and $n$-electron transfer distributions. We observe a crossover in the noise from Coulomb blockade to Kondo-dominated physics as the temperature is decreased. In addition, we uncover long-tailed spin distributions in the Kondo regime and analyze queuing behavior caused by correlations between single-electron transfer events.

Topological confinement effects of electron-electron interactions in biased zigzag-edge bilayer graphene nanoribbons

We have investigated topological confinement effects of electron-electron interactions on gapless edge states in zigzag-edge bilayer graphene nanoribbons under a voltage bias between the layers by using a tight-binding model with on-site Coulomb interactions. Spin-dependent potentials at the four edge sites resulting from the electron-electron interactions not only determine the spin configurations but also can result in topological confinement effects. In the magnetic phases with interlayer antiferromagnetic spin configurations, the edge potentials do not produce gapless edge states, and only gapless edge states can exist due to the edge shape. In the magnetic phases with interlayer ferromagnetic spin configurations, various gapless edge states due to the edge potential and edge shape are produced. Gapless edge states corresponding to the quantum spin-valley Hall phase and the layered antiferromagnetic phase can be described by topological confinement effects alone. The half-metallic quantum valley Hall phase can be described by topological confinement effects at inequivalent edge sites.

Majorana Kramers pairs in Rashba double nanowires with interactions and disorder

We analyze the effects of electron-electron interactions and disorder on a Rashba double-nanowire setup coupled to an s-wave superconductor, which has been recently proposed as a versatile platform to generate Kramers pairs of Majorana bound states in the absence of magnetic fields. We identify the regime of parameters for which these Kramers pairs are stable against interaction and disorder effects. We use bosonization, perturbative renormalization group, and replica techniques to derive the flow equations for various parameters of the model and evaluate the corresponding phase diagram with topological and disorder-dominated phases. We confirm aforementioned results by considering a more microscopic approach which starts from the tunneling Hamiltonian between the three-dimensional s-wave superconductor and the nanowires. We find again that the interaction drives the system into the topological phase and, as the strength of the source term coming from the tunneling Hamiltonian increases, strong electron-electron interactions are required to reach the topological phase.

Effect of electron-electron interaction on scattering process of oppositely charged polarons in conjugated polymers

By using one-dimensional tight-binding Su-Schrieffer-Heeger model modified to include electron-electric field interaction and electron-electron interaction, we simulate the scattering process of oppositely charged polarons with spin parallel and antiparallel, respectively. It is found that at weak field, the electron-electron interaction is dominant. Under the effect of strong Coulomb attraction, the polarons with spin antiparallel could not be separated after collision, and ultimately recombine into the singlet exciton. While in case of spin parallel, the strong Coulomb repulsion makes no collision occur unless strengthening the electric field. At moderate field, the electron-electron interaction could facilitate the recombination of polarons into triplet exciton rather than singlet. It is obtained that there exists competition between electron-electric field interaction and electron-electron interaction, a suitable strength of which could promote the formation of exciton in conjugated polymers.

Modifications of the Lifshitz-Kosevich formula in two-dimensional Dirac systems

Starting from the Luttinger-Ward functional we derive an expression for the oscillatory part of the grand potential of a two dimensional Dirac system in a magnetic field. We perform the computation for the clean and the disordered system, and we study the effect of electron-electron interactions on the oscillations. Unlike in the two dimensional electron gas (2DEG), a finite temperature and impurity scattering also affects the oscillation frequency. Furthermore, we find that in graphene, compared to the 2DEG, additional interaction induced damping effects occur: to two-loop order electron-electron interactions do lead to an additional damping factor in the amplitude of the Lifshitz-Kosevich-formula.

Simultaneous effects of electron-electron interactions, Rashba spin-orbit interaction and magnetic field on susceptibility of quantum dots

In this work, we have theoretically studied susceptibility of an InAs quantum dot in the presence of Rashba spin–orbit interaction (SOI), electron-electron (e−-e−) interaction, and magnetic field. We have used two potential models, shifted parabolic and inverse lateral shifted parabolic potentials. We have first solved the Schrodinger equation to obtain energy levels and then obtain the susceptibility using canonical ensemble. It is found that the susceptibility at low magnetic field is negative and then it becomes positive with increasing the magnetic field. This transition occurs at a particular magnetic field. The transition diamagnetism to paramagnetism occurs at lower magnetic field when the temperature increases. The susceptibility reduces with increasing temperature without considering the Rashba SOI. With considering e−-e− interaction, the peak position of susceptibility shifts toward lower magnetic fields. Also, the temperature has an effect on width and height of susceptibility.

Charge Density Waves and the Hidden Nesting of Purple Bronze K_{0.9}Mo_{6}O_{17}.

We introduce the first multiorbital effective tight-binding model to describe the effect of electron-electron interactions in this system. Upon fixing all the effective hopping parameters in the normal state against an abinitio band structure, and with only the overall scale of the interactions as the sole adjustable parameter, we find that a self-consistent Hartree-Fock solution reproduces extremely well the experimental behavior of the charge density wave (CDW) order parameter in the full range 0<T<T_{c}, as well as the precise reciprocal space locations of the partial gap opening and Fermi arc development. The interaction strengths extracted from fitting to the experimental CDW gap are consistent with those derived from an independent Stoner-type analysis.

Effect of interactions on quantum-limited detectors

We consider the effect of electron-electron interactions on a voltage biased quantum point contact in the tunneling regime used as a detector of a nearby qubit. We model the leads of the quantum point contact as Luttinger liquids, incorporate the effects of finite temperature and analyze the detection-induced decoherence rate and the detector efficiency, $Q$. We find that interactions generically reduce the induced decoherence along with the detector's efficiency, and strongly affect the relative strength of the decoherence induced by tunneling and that induced by interactions with the local density. With increasing interaction strength, the regime of quantum-limited detection ($Q=1$) is shifted to increasingly lower temperatures or higher bias voltages respectively. For small to moderate interaction strengths, $Q$ is a monotonously decreasing function of temperature as in the non-interacting case. Surprisingly, for sufficiently strong interactions we identify an intermediate temperature regime where the efficiency of the detector increases with rising temperature.

Floquet Majorana fermions and parafermions in driven Rashba nanowires

We study a periodically driven nanowire with Rashba-like conduction and valence bands in the presence of a magnetic field. We identify topological regimes in which the system hosts zero-energy Majorana fermions. We further investigate the effect of strong electron-electron interactions that give rise to parafermion zero energy modes hosted at the nanowire ends. The first setup we consider allows for topological phases by applying only static magnetic fields without the need of superconductivity. The second setup involves both superconductivity and time-dependent magnetic fields and allows one to generate topological phases without fine-tuning of the chemical potential. Promising candidate materials are graphene nanoribbons due to their intrinsic particle-hole symmetry.

Quantized edge modes in atomic-scale point contacts in graphene

The zigzag edges of single- or few-layer graphene are perfect one-dimensional conductors owing to a set of gapless states that are topologically protected against backscattering. Direct experimental evidence of these states has been limited so far to their local thermodynamic and magnetic properties, determined by the competing effects of edge topology and electron-electron interaction. However, experimental signatures of edge-bound electrical conduction have remained elusive, primarily due to the lack of graphitic nanostructures with low structural and/or chemical edge disorder. Here, we report the experimental detection of edge-mode electrical transport in suspended atomic-scale constrictions of single and multilayer graphene created during nanomechanical exfoliation of highly oriented pyrolytic graphite. The edge-mode transport leads to the observed quantization of conductance close to multiples of G0 = 2e2/h. At the same time, conductance plateaux at G0/2 and a split zero-bias anomaly in non-equilibrium transport suggest conduction via spin-polarized states in the presence of an electron-electron interaction.

Bond patterns and charge-order amplitude in quarter-filled charge-transfer solids

Metal-insulator transition accompanied by charge-ordering has been widely investigated in quasi-one-dimensional conductors, including in particular organic charge-transfer solids. Among such materials the 1/4-filled band charge-transfer solids are of strong interest, because of the commensurate nature of the charge-ordering in these systems. The period-four charge-order pattern ...1100... here is accompanied by two distinct bond distortion patterns, giving rise to bond-charge-density waves (BCDW) of types 1 and 2. Using quantum Monte Carlo methods, we determine the phase diagram within the extended Hubbard Hamiltonian that gives both types 1 and 2 BCDW in the thermodynamic limit. We further investigate the effect of electron-electron and electron-phonon interactions on the amount of charge disproportionation. Our results show that between these two bond patterns, one (BCDW2) in general coexists with a large magnitude charge order, which is highly sensitive to electron-phonon interactions, while the other (BCDW1) is characterized by weak charge order. We discuss the relevance of our work to experiments on several 1/4-filled conductors, focusing in particular on the materials (EDO-TTF)_2X and (DMEDO-TTF)_2X with large amplitude charge-order.

Dependence of the 0.5 × (2e2/h) conductance plateau on the aspect ratio of InAs quantum point contacts with in-plane side gates

The observation of a 0.5 × (2e2/h) conductance plateau in asymmetrically biased quantum point contacts (QPCs) with in-plane side gates (SGs) has been attributed to the onset of spin-polarized current through these structures. For InAs QPCs with the same width but a longer channel length, there is roughly a fourfold increase in the range of common sweep voltage applied to the SGs over which the 0.5 × (2e2/h) plateau is observed when the QPC aspect ratio (ratio of length over the width of the narrow portion of the structure) is increased by a factor 3. Non-equilibrium Green's function simulations indicate that the increase in the size of the 0.5 × (2e2/h) plateau is due to an increased importance, over a larger range of common sweep voltage, of the effects of electron-electron interactions in QPC devices with a larger aspect ratio. The use of asymmetrically biased QPCs with in-plane SGs and large aspect ratio could therefore pave the way to build robust spin injectors and detectors for the successful implementation of spin field effect transistors.

Half-metallic quantum valley Hall effect in biased zigzag-edge bilayer graphene nanoribbons

We have investigated electron-electron interaction effects on gapless edge states in the antiferromagnetic phase of zigzag-edge bilayer graphene nanoribbons under a voltage bias between the layers by using a tight-binding model with on-site Coulomb interactions. We found that a zigzag-edge bilayer graphene nanoribbon can have gapless edge states corresponding to peculiar topologically nontrivial insulator phases, such as a half-metallic quantum valley Hall phase. The half-metallicity was found to be due to the on-site Coulomb interactions through which excess charges produced by the voltage bias between the layers give rise to the potential difference between the opposite edges. A topological phase diagram in the antiferromagnetic phase of a zigzag-edge bilayer graphene nanoribbon was determined as a function of the bias voltage and the ribbon width. The quantum confinement effect, which was found to be graphene-like for narrow ribbons and to be bilayer graphene-like for wide ribbons, was also confirmed to play an important role in determining the topologically nontrivial insulator phases.

Interplay between Rashba interaction and electromagnetic field in the edge states of a two-dimensional topological insulator

The author investigates in a nonperturbative way the effects of Rashba interaction and electromagnetic field on the edge states of a two-dimensional topological insulator. He shows that the electron dynamics is equivalent to a problem of massless Dirac fermions propagating with an inhomogeneous velocity, unveiling interesting consequences. The Rashba coupling always enhances the electron velocity and, despite the inelastic and time-reversal breaking processes induced by the electromagnetic field, no backscattering occurs without interaction. Furthermore, when the photoexcitation occurs far from the Rashba region, the latter effectively acts as a ``superluminal gate'', boosting the photoexcited wavepacket outside the light-cone determined by the bare Fermi velocity ${v}_{F}$. In contrast, for an electric pulse overlapping the Rashba region, the emerging wave packets are squeezed in a manner that depends on the overlap area. The author also discusses the effects of electron-electron interaction, for both intraspin and interspin density-density coupling. The results suggest that Rashba interaction, often considered as an unwanted disorder effect, may be exploited to tailor the shape and the propagation time of photoexcited spin-polarized wave packets.

Theory of interaction-induced renormalization of Drude weight and plasmon frequency in chiral multilayer graphene

We develop a theory for the optical conductivity of doped multilayer graphene including the effects of electron-electron interactions. Applying the quantum kinetic formalism, we formulate a set of pseudospin Bloch equations that governs the dynamics of the nonequilibrium density matrix driven by an external \emph{a.c.} electric field under the influence of Coulomb interactions. These equations reveal a dynamical mechanism that couples the Drude and interband responses arising from the chirality of pseudospin textures in multilayer graphene systems. We demonstrate that this results in an interaction-induced enhancement of the Drude weight and plasmon frequency strongly dependent on the pseudospin winding number. Using bilayer graphene as an example, we also study the influence of higher-energy bands and find that they contribute considerable renormalization effects not captured by a low-energy two-band description. We argue that this enhancement of Drude weight and plasmon frequency occurs generally in materials characterized by electronic chirality.

Effects of long-range disorder and electronic interactions on the optical properties of graphene quantum dots

We theoretically investigate the effects of long-range disorder and electron-electron interactions on the optical properties of hexagonal armchair graphene quantum dots consisting of up to 10806 atoms. The numerical calculations are performed using a combination of tight-binding, mean-field Hubbard and configuration interaction methods. Imperfections in the graphene quantum dots are modelled as a long-range random potential landscape, giving rise to electron-hole puddles. We show that, when the electron-hole puddles are present, tight-binding method gives a poor description of the low-energy absorption spectra compared to meanfield and configuration interaction calculation results. As the size of the graphene quantum dot is increased, the universal optical conductivity limit can be observed in the absorption spectrum. When disorder is present, calculated absorption spectrum approaches the experimental results for isolated monolayer of graphene sheet.

Quantum Monte Carlo simulation of topological phase transitions

We study the electron-electron interaction effects on topological phase transitions by the ab-initio quantum Monte Carlo simulation. We analyze two-dimensional class A topological insulators and three-dimensional Weyl semimetals with the long-range Coulomb interaction. The direct computation of the Chern number shows the electron-electron interaction modifies or extinguishes topological phase transitions.