Strong Electron-electron Interactions Research Articles

Hydrodynamic Coulomb drag and bounds on diffusion

We study Coulomb drag between an active layer with a clean electron liquid and a passive layer with a pinned electron lattice in the regime of fast intralayer equilibration. Such a two-fluid system offers an experimentally realizable way to disentangle the fast rate of intralayer electron-electron interactions from the much slower rate of momentum transfer between both layers. We identify an intermediate temperature range above the Fermi energy of the electron fluid but below the Debye energy of the electronic crystal where the hydrodynamic drag resistivity is directly proportional to a fast electron-electron scattering rate. The results are compatible with the conjectured scenario for strong electron-electron interactions which poses that a linear temperature dependence of resistivity originates from a "Planckian" electron relaxation time $\tau_{eq}\sim \hbar/k_BT$. We compare this to the better known semiclassical case, where the diffusion constant is found to be not proportional to the microscopic timescale.

Fractional topological superconductivity and parafermion corner states

We consider a system of weakly coupled Rashba nanowires in the strong spin-orbit interaction (SOI) regime. The nanowires are arranged into two tunnel-coupled layers proximitized by a top and bottom superconductor such that the superconducting phase difference between them is $\pi$. We show that in such a system strong electron-electron interactions can stabilize a helical topological superconducting phase hosting Kramers partners of $\mathbb{Z}_{2m}$ parafermion edge modes, where $m$ is an odd integer determined by the position of the chemical potential. Furthermore, upon turning on a weak in-plane magnetic field, the system is driven into a second-order topological superconducting phase hosting zero-energy $\mathbb{Z}_{2m}$ parafermion bound states localized at two opposite corners of a rectangular sample. As a special case, zero-energy Majorana corner states emerge in the non-interacting limit $m=1$, where the chemical potential is tuned to the SOI energy of the single nanowires.

Properties of the donor impurity band in mixed valence insulators

In traditional semiconductors with large effective Bohr radius, an electron donor creates a hydrogen-like bound state just below the conduction band edge. The properties of the impurity band arising from such hydrogenic impurities have been studied extensively during the last 70 years. In this paper we consider whether a similar bound state and a similar impurity band can exist in mixed valence insulators, where the gap arises at low temperature due to strong electron-electron interactions. We find that the structure of the hybridized conduction band leads to an unusual bound state that can be described using the physics of the one-dimensional hydrogen atom. The properties of the resulting impurity band are also modified in a number of ways relative to the traditional semiconductor case; most notably, the impurity band can hold a much larger concentration without inducing an insulator-to-metal transition. We estimate the critical doping associated with this transition, and then proceed to calculate the DC and AC conductivities and the specific heat. We discuss our results in light of recent measurements on the mixed valence insulator ${\mathrm{SmB}}_{6}$ and find them to be consistent with the experiments.

Optical conductivity of metal alloys with residual resistivities near or above the Mott-Ioffe-Regel limit

Most interesting examples of violations of the Mott-Ioffe-Regel (MIR) resistivity limit are found in materials with strong electronic correlations that are not well understood by theory. We demonstrate that first principles theory can predict the experimentally observed frequency dependence of the optical conductivity for a novel class of metals where the residual resistivity is near or above the MIR limit, which we define as a ``bad metal.'' The predicted optical conductivity of a NiCoCr alloy is in good agreement with experiment. It is demonstrated that the width of the Drude peak describing the low-frequency part of optical conductivity is comparable to the Fermi energy. The latter, together with a mean free path comparable to the interatomic distance, indicates the absence of well-defined quasiparticles. In contrast to traditional bad metals with strong electron-electron interactions, both the high resistivity and the large width of the Drude peak in these alloys result from strong scattering on disordered atomic potentials that can be understood using modern density functionals.

Ultrafast coherent nonlinear nanooptics and nanoimaging of graphene.

With its linear energy dispersion and large transition dipole matrix element, graphene is an attractive material for nonlinear optoelectronic applications. However, the mechanistic origin of its strong nonlinear response, the ultrafast coherent dynamics and the associated nanoscale phenomena have remained elusive due to a lack of suitable experimental techniques. Here, using adiabatic nanofocusing and imaging, we study the broadband four-wave mixing (FWM) response of graphene with nanometre and femtosecond spatio-temporal resolution. We detect a nonlinear signal enhancement at the edges and dependence on the number of layers from excitation areas as small as 104 carbon atoms. Femtosecond FWM nanoimaging and concomitant frequency-domain measurements reveal dephasing on T2 ≈ 6 ± 1 fs timescales, which we attribute to a strong electron-electron interaction. We also identify an unusual non-local FWM response on ~100-400 nm length scales, which we assign to a Doppler effect controlling the nonlinear interaction between the tip near-field momenta and the graphene electrons with high Fermi velocity. These results illustrate the distinct nonlinear nanooptical properties of graphene, expected also in related classes of two-dimensional materials, that could form the basis for improved nonlinear and ultrafast nanophotonic devices.

Fractional Conductance in Strongly Interacting 1D Systems.

We study one-dimensional clean systems with few channels and strong electron-electron interactions. We find that in several circumstances, even when time-reversal symmetry holds, they may lead to two-terminal fractional quantized conductance and fractional shot noise. The condition on the commensurability of the Fermi momenta of the different channels and the strength of the interactions resulting in such remarkable phenomena are explored using Abelian bosonization. Finite temperature and length effects are accounted for by a generalization of the Luther-Emery refermionization at specific values of the interaction strength, in the strongly interacting regime. We discuss the connection of our model to recent experiments in a confined two-dimensional electron gas, featuring possible fractional conductance plateaus, including situations with a zero magnetic field, when time-reversal symmetry is conserved. One of the most dominant observed fractions, with two-terminal conductance equal to 2/5(e^{2}/h), is found in several scenarios of our model. Finally, we discuss how at very small energy scales the conductance returns to an integer value and the role of disorder.

Ground-State Spin Blockade in a Single-Molecule Junction.

It is known that the quantum mechanical ground state of a nanoscale junction has a significant impact on its electrical transport properties. This becomes particularly important in transistors consisting of a single molecule. Because of strong electron-electron interactions and the possibility of accessing ground states with high spins, these systems are eligible hosts of a current-blockade phenomenon called a ground-state spin blockade. This effect arises from the inability of a charge carrier to account for the spin difference required to enter the junction, as that process would violate the spin selection rules. Here, we present a direct experimental demonstration of a ground-state spin blockade in a high-spin single-molecule transistor. The measured transport characteristics of this device exhibit a complete suppression of resonant transport due to a ground-state spin difference of 3/2 between subsequent charge states. Strikingly, the blockade can be reversibly lifted by driving the system through a magnetic ground-state transition in one charge state, using the tunability offered by both magnetic and electric fields.

From Chaos to Order in Mesoscopic Systems

An interplay between regular and chaotic dynamics of particles, confined in effective potentials of various shapes, is discussed. It is demonstrated that dynamical symmetries emerging from this interplay in classical and quantum systems are related to existence of the conserved quantities of the dynamics and integrability of the considered systems under certain conditions. Important role of these symmetries is illustrated on a broad class of mesoscopic systems that include octupole deformed nuclei and quantum dots in a magnetic field. We consider also the case of a strong electron-electron interaction in quantum dots. It leads us to the problem of self-organization of one-component charged particles, confined in a two-dimensional potential with a circular boundary. As an example, disk and circular parabolic potentials are considered, where the circular symmetry is responsible for the self-organization mechanism. It provides conditions for a steady formation of a centered hexagonal lattice that smoothly transforms to valence circular rings in the ground state configurations for particle number $$n > 180.$$

Strong electron-electron interactions of a Tomonaga-Luttinger liquid observed in InAs quantum wires

We report strong electron-electron interactions in quantum wires etched from an InAs quantum well, a material known to have strong spin-orbit interactions. We find that the current through the wires as a function of the bias voltage and temperature follows the universal scaling behavior of a Tomonaga--Luttinger liquid. Using a universal scaling formula, we extract the interaction parameter and find strong electron-electron interactions, increasing as the wires become more depleted. We establish theoretically that spin-orbit interactions cause only minor modifications of the interaction parameter in this regime, indicating that genuinely strong electron-electron interactions are indeed achieved in the device. Our results suggest that etched InAs wires provide a platform with both strong electron-electron and strong spin-orbit interactions.

Competition between exchange-driven dimerization and magnetism in diamond (111)

Strong electron-electron interaction in ultraflat edge states can be responsible for correlated phases of matter, such as magnetism, charge-density wave, or superconductivity. Here we consider the diamond (111) surface that, after Pandey reconstruction, presents zigzag carbon chains, generating a flat surface band. By performing full structural optimization with hybrid functionals and neglecting spin polarization, we find that a substantial dimerization ($0.090\phantom{\rule{0.28em}{0ex}}\AA{}/0.076\phantom{\rule{0.28em}{0ex}}\AA{}$ bond disproportionation in the PBE0/HSE06) occurs on the chains; a structural effect absent in calculations with functionals based on the local density/generalized gradient approximation. This dimerization is the primary mechanism for the opening of an insulating gap in the absence of spin polarization. The single-particle direct gap is $1.7\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ ($1.0\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$) in the PBE0 (HSE06), comparable with the experimental optical gap of $1.47\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$, and on the larger (smaller) side of the estimated experimental single-particle gap window of 1.57--1.87 eV, after inclusion of excitonic effects. However, by including spin polarization in the calculation, we find that the exchange interaction stabilizes a different ground state, undimerized, with no net magnetization and ferrimagnetic along the Pandey $\ensuremath{\pi}$ chains with magnetic moments as large as $0.2--0.3{\ensuremath{\mu}}_{B}$ in the PBE0. The direct single-particle band gap in the equal spin channel is approximately $2.2\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$ ($1.5\phantom{\rule{0.28em}{0ex}}\mathrm{eV}$) with the PBE0 (HSE06) functional. Our work is relevant for systems with flat bands in general and wherever the interplay between structural, electronic, and magnetic degrees of freedom is crucial, as in twisted bilayer graphene, IVB atoms on IVB(111) surfaces such as Pb/Si(111), or molecular crystals.

High-Magnetic-Field Tunneling Spectra of ABC-Stacked Trilayer Graphene on Graphite.

ABC-stacked trilayer graphene (TLG) was predicted to exhibit novel many-body phenomena due to the existence of almost dispersionless flat bands near the charge neutrality point. Here, using high-magnetic-field scanning tunneling microscopy, we present Landau Level (LL) spectroscopy measurements of high-quality ABC-stacked TLG on graphite. We observe an approximately linear magnetic-field scaling of valley splitting and spin splitting in the ABC-stacked TLG. Our experiment indicates that the spin splitting decreases dramatically with increasing the LL index. When the lowest LL is partially filled, we find an obvious enhancement of the spin splitting, attributing to strong many-body effects. Moreover, we observe linear energy scaling of the inverse lifetime of quasiparticles, providing an additional evidence for the strong electron-electron interactions in the ABC-stacked TLG. These results imply that interesting broken-symmetry states and novel electron correlated effects could emerge in the ABC-stacked TLG in the presence of high magnetic fields.

Quantized conductance of one-dimensional strongly correlated electrons in an oxide heterostructure

Oxide heterostructures are versatile platforms with which to research and create novel functional nanostructures. We successfully develop one-dimensional (1D) quantum-wire devices using quantum point contacts on MgZnO/ZnO heterostructures and observe ballistic electron transport with conductance quantised in units of 2e^{2}/h. Using DC-bias and in-plane field measurements, we find that the g-factor is enhanced to around 6.8, more than three times the value in bulk ZnO. We show that the effective mass m^{*} increases as the electron density decreases, resulting from the strong electron-electron interactions. In this strongly interacting 1D system we study features matching the 0.7 conductance anomalies up to the fifth subband. This paper demonstrates that high-mobility oxide heterostructures such as this can provide good alternatives to conventional III-V semiconductors in spintronics and quantum computing as they do not have their unavoidable dephasing from nuclear spins. This paves a way for the development of qubits benefiting from the low defects of an undoped heterostructure together with the long spin lifetimes achievable in silicon.

Nonrenewal statistics in quantum transport from the perspective of first-passage and waiting time distributions

The waiting time distribution has, in recent years, proven to be a useful statistical tool for characterising transport in nanoscale quantum transport. In particular, as opposed to moments of the distribution of transferred charge, which have historically been calculated in the long-time limit, waiting times are able to detect non-renewal behaviour in mesoscopic systems. They have failed, however, to correctly incorporate backtunneling events. Recently, a method has been developed that can describe unidirectional and bidirectional transport on an equal footing: the distribution of first-passage times. Rather than the time between successive electron tunnelings, the first-passage refers to the first time the number of extra electrons in the drain reaches $+1$. Here, we demonstrate the differences between first-passage time statistics and waiting time statistics in transport scenarios where the waiting time either cannot correctly reproduce the higher order current cumulants or cannot be calculated at all. To this end, we examine electron transport through a molecule coupled to two macroscopic metal electrodes. We model the molecule with strong electron-electron and electron-phonon interactions in three regimes: (i) sequential tunneling and cotunneling for a finite bias voltage through the Anderson model, (ii) sequential tunneling with no temperature gradient and a bias voltage through the Holstein model, and (iii) sequential tunneling at zero bias voltage and a temperature gradient through the Holstein model. We show that, for each transport scenario, backtunneling events play a significant role; consequently, the waiting time statistics do not correctly predict the renewal and non-renewal behaviour, whereas the first-passage time distribution does.

Effective Ising model for correlated systems with charge ordering

Collective electronic fluctuations in correlated materials give rise to various important phenomena, such as existence of the charge ordering, superconductivity, Mott insulating and magnetic phases, plasmon and magnon modes, and other interesting features of such systems. Unfortunately, description of these correlation effects requires significant efforts, since they almost entirely rely on strong local and nonlocal electron-electron interactions. Some collective phenomena, such as magnetism, can be sufficiently described by a simple Heisenberg-like models that are formulated in terms of bosonic variables. This fact suggests that other many-body excitations can also be described by simple bosonic models in spirit of the Heisenberg theory. Here we derive an effective bosonic action for charge degrees of freedom for the extended Hubbard model and define a physical regime where the obtained action reduces to a classical Hamiltonian of an effective Ising model.

Theoretical Aspects of the Cross Effect Enhancement of Nuclear Polarization under Static Dynamic Nuclear Polarization Conditions.

In this study, we perform quantum calculations of the spin dynamics of a small spin system that includes nine coupled electrons and one nucleus placed in an external magnetic field and exposed to microwave irradiation. This is an extension of a previous work in which we have demonstrated on a system of 11 coupled electron spins the dynamics of the electron polarizations composing the electron paramagnetic resonance (EPR) line during static dynamic nuclear polarization (DNP) experiments. There we have shown that the electron polarizations are determined by a spectral diffusion process, induced by the dipolar interaction and cross-relaxation. Additionally, we showed that a distinction had to be made between strong and weak dipolar-coupled systems relative to the inhomogeneity of the EPR line with only the first behaving according to the thermal mixing DNP (with two electron spin temperatures) description. The EPR spectra in the weak and strong dipolar interaction cases show different types of spectral features. In the extended spin system, we again make a distinction between weak and strong electron-electron interactions and show that the DNP spectra for the two cases are different in nature but that the DNP spectra can be derived in all cases from the EPR line shapes using the indirect cross effect.

Evaluation of NSAIDs antioxidant activity on lipid peroxidation in splenocyte membranes

Here we summarized the nanoscale layered transition metal chalcogenide superconductors, mainly based on the experimental results. In transition-metal chalcogenides the interplay between strong electron-electron and electron-phonon interactions produces a wide variety of instability ranging from charge density wave to superconducting state. The majority of bulk transition-metal dichalcogenides’ Tcs are normally between 2 and 4 K. At present, superconducting transition-metal chalcogenides generally show low transition temperature (Tc < 10 K). Fe based transition-metal chalcogenides have certain higher Tc (in the range of 10 ~ 50 K). As have been reported, nano-transition-metal chalcogenides may be one of the routes to improve the superconducting transition temperature. In this review, we would like to give a brief introduction of superconductor development and crystal structure of transition-metal chalcogenides. Furthermore, we will describe major synthesis and physical properties of nano-transition-metal chalcogenides. Finally, recent status and outlook of superconductor based on nano-transition-metal chalcogenides are discussed.

First Principles Investigation of FeCo Alloy: Electronic and Optical Properties Study

The ground state electronic structures and optical properties of FeCo alloy have been reported using plane wave ultrasoft pseudopotential based on spin polarized density functional theory through first principles study. The crystallographic structure of FeCo consists with body-centered cubic lattice that is described in space group Im-3m (229). The geometry is optimized with zero applied pressure and the optimized lattice constant is found to be 2.854A. The electronic energy bands represent the overlapped between valence and conductance electronic states and confirm zero forbidden gaps i.e. metallic nature of the FeCo alloy. The Fermi surfaces manifest the anisotropic features of electronic energy dispersion along the high symmetry directions (X-R-M-G-R) of the Brillouin zone. The total density of states arises from the contribution of the electronic states of Co and Fe atoms. The calculated spin magnetic moments of FeCo alloy is 1.26μB. The spin magnetic moments mainly come from the exchange interactions among electronic spins, which confirms the strong electron-electron interactions. Moreover, the optical properties are computed which also attest the metallic behavior of the material. The optical measurements indicate that FeCo alloy is an optically anisotropic material. The obtained loss spectrum reveals the plasmonic excitations that is important for many magneto-optical applications.

Distribution of waiting times between electron cotunneling events

In the resonant tunneling regime sequential processes dominate single electron transport through quantum dots or molecules that are weakly coupled to macroscopic electrodes. In the Coulomb blockade regime, however, cotunneling processes dominate. Cotunneling is an inherently quantum phenomenon and thus gives rise to interesting observations, such as an increase in the current shot noise. Since cotunneling processes are inherently fast compared to the sequential processes, it is of interest to examine the short time behaviour of systems where cotunneling plays a role, and whether these systems display nonrenewal statistics. We consider three questions in this paper. Given that an electron has tunneled from the source to the drain via a cotunneling or sequential process, what is the waiting time until another electron cotunnels from the source to the drain? What are the statistical properties of these waiting time intervals? How does cotunneling affect the statistical properties of a system with strong inelastic electron-electron interactions? In answering these questions, we extend the existing formalism for waiting time distributions in single electron transport to include cotunneling processes via an $n$-resolved Markovian master equation. We demonstrate that for a single resonant level the analytic waiting time distribution including cotunneling processes yields information on individual tunneling amplitudes. For both a SRL and an Anderson impurity deep in the Coulomb blockade there is a nonzero probability for two electrons to cotunnel to the drain with zero waiting time inbetween. Furthermore, we show that at high voltages cotunneling processes slightly modify the nonrenewal behaviour of an Anderson impurity with a strong inelastic electron-electron interaction.

Magnetic field effect on polaron recombination in conjugated polymers

By using the one-dimensional tight-binding model, we theoretically investigate the effect of magnetic field on the collision of the oppositely charged polarons with their spin antiparallel in cis-polyacetylene. We adopt the non-adiabatic approach to simulate the dynamical evolution of polaron spin. Under the effect of electric field with a moderate strength, the polarons initially localized at the chain-end of polyacetylene move towards each other. During the collision process, an obvious spin precession is obtained, and the precession period is found to be inversely proportional to the strength of magnetic field. While after including electron-electron interaction in form of Hubbard model, the period is no longer constant. With the decrease of distance between the oppositely charged polarons, the precession of their spin slows down. We also find that there is a critical value of electron-electron interaction strength, over which the spin precession of polaron disappears. In addition, we demonstrate that under the effect of magnetic field, the polarons could recombine or pass through each other to form the singlet exciton, rather than be dissociated after collision between them with the strong electron-electron interaction, and then the electroluminescence efficiency of organic optoelectronic devices is improved.

Precise simulation of strongly correlated quantum impurity systems

In recent years, nano-sized systems involving local charge or spin states have received wide interests because of their potential application in emerging fields such as quantum information and quantum computation. For instance, organometallic molecular complexes may serve as building blocks of quantum storage devices, because the spin-unpaired d or f electrons at transition metal centers may be employed to construct spin qubits. Moreover, if the system contains strong electron-electron interaction, the involving local quantum states are subject to prominent electron correlation effects (such as the Kondo effect). Theoretically, spatially confined nano-systems are often described by quantum impurity models. Thus the accurate prediction of the intrinsic properties of quantum impurity systems and the deep understanding on the response and evolution of local quantum states under external fields or in dissipative environment are fundamentally important for the design and fabrication of quantum devices. The accurate characterization of quantum coherence, correlation, and entanglement in quantum impurity systems remains a great challenge. Enormous efforts have been made to achieve this goal. A variety of theoretical methods have been developed, including the numerical renormalization group method, the quantum Monte Carlo method, and many others. However, all the existing methods are subject to certain limitations regarding accuracy or efficiency. Therefore, we choose to view this problem from a new perspective—the perspective of open quantum systems. Over the past decade, we have developed a formally exact quantum dissipation theory, the hierarchical equations of motion (HEOM) theory, for fermionic open systems. The HEOM theory captures the combined effects of system-environment dissipation, many-body interaction, and non-Markovian memory in a nonperturbative manner. It is capable of addressing static and dynamic responses of system observables in both equilibrium and nonequilibrium situations. We have implemented the HEOM method in our self-designed computer program HEOM-QUICK. We have also devised a series of advanced algorithms which substantially enhance the numerical efficiency of HEOM. The HEOM-QUICK program thus provides an accurate, efficient, and versatile theoretical tool for the investigation of strongly correlated quantum impurity systems. We have applied the HEOM method to study a variety of problems associated with quantum impurity systems. These include the intrinsic properties of quantum impurity models and lattice models, the transient current response of quantum dots to ac voltages, the thermopower and local heating effect in nonequilibrium quantum dots, and the tuning of local spin states in adsorbed molecular magnets. In particular, the HEOM method has been combined with density functional theory (DFT) method, and thus allows for first-principles-based simulation on the precise tuning of local spin states in adsorbed magnetic molecules. Numerical simulations have discovered or reproduced many important quantum phenomena that are essential for relevant experiments, including the Kondo effect, Mott metal-insulator transition, quantum memristive effect, etc. This paper gives a brief overview of our development of the HEOM method for fermionic open systems, as well as its applications to various strongly correlated quantum impurity systems.