Interplay Of Coulomb Interactions Research Articles

Chiral Transport of Hot Carriers in Graphene in the Quantum Hall Regime.

Photocurrent (PC) measurements can reveal the relaxation dynamics of photoexcited hot carriers beyond the linear response of conventional transport experiments, a regime important for carrier multiplication. Here, we study the relaxation of carriers in graphene in the quantum Hall regime by accurately measuring the PC signal and modeling the data using optical Bloch equations. Our results lead to a unified understanding of the relaxation processes in graphene over different magnetic field strength regimes, which is governed by the interplay of Coulomb interactions and interactions with acoustic and optical phonons. Our data provide clear indications of a sizable carrier multiplication. Moreover, the oscillation pattern and the saturation behavior of PC are manifestations of not only the chiral transport properties of carriers in the quantum Hall regime but also the chirality change at the Dirac point, a characteristic feature of a relativistic quantum Hall effect.

Erratum: Interplay of Coulomb interactions and disorder in three-dimensional quadratic band crossings without time-reversal symmetry and with unequal masses for conduction and valence bands [Phys. Rev. B 97 , 125121 (2018)

Received 14 December 2021DOI:https://doi.org/10.1103/PhysRevB.105.039901©2022 American Physical SocietyPhysics Subject Headings (PhySH)Physical SystemsDisordered systemsStrongly correlated systemsCondensed Matter, Materials & Applied Physics

Screening the Coulomb interaction and thermalization of Anderson insulators

Long range interactions are relevant for a wide range of phenomena in physics where they often present a challenge to theory. In condensed matter, the interplay of Coulomb interaction and disorder remains largely an unsolved problem. In two dimensional films the long-range part of the Coulomb interaction may be screened by a nearby metallic overlay. This technique is employed in this work to present experimental evidence for its effectiveness in limiting the spatial range of the Coulomb interaction. We use this approach to study the effects of the long-range Coulomb interaction on the out-of-equilibrium dynamics of electron-glasses using amorphous indium-oxide films. The results demonstrate that electronic relaxation times, extending over thousands of seconds, do not hinge on the long-range Coulomb interaction nor on the presence of a real gap in the density of states. Rather, they emphasize the dominant role played by disorder in controlling the slow thermalization processes of Anderson insulators taken far from equilibrium.

Finite-frequency noise of interacting single-electron emitters: Spectroscopy with higher noise harmonics

We derive the symmetrized current-noise spectrum of a quantum dot, which is weakly tunnel-coupled to an electron reservoir and driven by a slow time-dependent gate voltage. This setup can be operated as an on-demand emitter of single electrons into a mesoscopic conductor. By extending a real-time diagrammatic technique which is perturbative in the tunnel coupling, we obtain the time-resolved finite-frequency noise as well as its decomposition into noise harmonics in the presence of both strong Coulomb interaction and slow time-dependent driving. We investigate the noise over a large range of frequencies and point out where the interplay of Coulomb interaction and driving leads to unique signatures in finite-frequency noise spectra, in particular in the first harmonic. Besides that, we employ the first noise harmonic as a spectroscopic tool to access individual fluctuation processes. We discuss how the inverse noise frequency sets a time scale for fluctuations, which competes with time scales of the quantum-dot relaxation dynamics as well as the driving.

Theoretical Model Study of Interplay of Coulomb Interaction and Electron-Phonon Interaction in the Thermal Properties of Monolayer Graphene

We propose here a tight-binding (TB) model Hamiltonian for monolayer graphene-on-substrate describing the nearest-neighbor-hopping, on-site Coulomb interaction on the sub-lattices and the electron-phonon interaction under the high-frequency limit of phonon vibration. Applying Lang-Firsov canonical transformation, the electron and phonon systems are decoupled in the atomic Hamiltonian, such that the effective Coulomb interaction and effective nearest-neighbor-hopping integral respectively appear as $\tilde {U}=U-2t_{1}\lambda $ and $\tilde {t}_{1}=t_{1}e^{\frac {-t_{1}\lambda } {\omega _{0}}}$ , where U, t1, λ and ω0 are respectively Coulomb energy, nearest-neighbor-hopping integral, electron-phonon (e-ph) coupling and phonon frequency. The effective Coulomb interaction in the Hamiltonian is considered within mean-field approximation. The Hamiltonian is solved by Zubarev’s Green’s function technique. The temperature-dependent electronic entropy and specific heat are calculated from the free energy of graphene system and are computed numerically. The temperature-dependent electronic specific heat exhibits a charge gap peak at room temperature arising due to the effect of Coulomb interaction and electron-phonon interaction. The evolution of these peaks in specific heat is investigated by varying the model parameters of the system.

Interplay of Coulomb interactions and disorder in three-dimensional quadratic band crossings without time-reversal symmetry and with unequal masses for conduction and valence bands

Coulomb interactions famously drive three dimensional quadratic band crossing semimetals into a non-Fermi liquid phase of matter. In a previous work, Phys. Rev. B 95, 205106 (2017), the effect of disorder on this non-Fermi liquid phase was investigated, assuming that the bandstructure was isotropic, assuming that the conduction and valence bands had the same band mass, and assuming that the disorder preserved exact time-reversal symmetry and statistical isotropy. It was shown that the non-Fermi liquid fixed point is unstable to disorder, and that a runaway flow to strong disorder occurs. In this work, we extend that analysis by relaxing the assumption of time-reversal symmetry and allowing the electron and hole masses to differ (but continuing to assume isotropy of the low energy bandstructure). We first incorporate time-reversal symmetry breaking disorder, and demonstrate that there do not appear any new fixed points. Moreover, while the system continues to flow to strong disorder, time-reversal-symmetry-breaking disorder grows asymptotically more slowly than time-reversal-symmetry-preserving disorder, which we therefore expect should dominate the strong-coupling phase. We then allow for unequal electron and hole masses. We show that whereas asymmetry in the two masses is irrelevant in the clean system, it is relevant in the presence of disorder, such that the 'effective masses' of the conduction and valence bands should become sharply distinct in the low-energy limit. We calculate the RG flow equations for the disordered interacting system with unequal band masses, and demonstrate that the problem exhibits a runaway flow to strong disorder. Along the runaway flow, time-reversal-symmetry-preserving disorder grows asymptotically more rapidly than both time-reversal-symmetry-breaking disorder and the Coulomb interaction.

Quantum phase transition and unusual critical behavior in multi-Weyl semimetals

The low-energy behaviors of gapless double- and triple-Weyl fermions caused by the interplay of long-range Coulomb interaction and quenched disorder are studied by performing a renormalization group analysis. It is found that an arbitrarily weak disorder drives the double-Weyl semimetal to undergo a quantum phase transition into a compressible diffusive metal, independent of the disorder type and the Coulomb interaction strength. In contrast, the nature of the ground state of triple-Weyl fermion system relies sensitively on the specific disorder type in the noninteracting limit: The system is turned into a compressible diffusive metal state by an arbitrarily weak random scalar potential or $z$ component of random vector potential but exhibits stable critical behavior when there is only $x$ or $y$ component of random vector potential. In case the triple-Weyl fermions couple to random scalar potential, the system becomes a diffusive metal in the weak interaction regime but remains a semimetal if Coulomb interaction is sufficiently strong. Interplay of Coulomb interaction and $x$, or $y$, component of random vector potential leads to a stable infrared fixed point that is likely to be characterized by critical behavior. When Coulomb interaction coexists with the $z$ component of random vector potential, the system flows to the interaction-dominated strong coupling regime, which might drive a Mott insulating transition. It is thus clear that double- and triple-Weyl fermions exhibit distinct low-energy behavior in response to interaction and disorder. The physical explanation of such distinction is discussed in detail. The role played by long-range Coulomb impurity in triple-Weyl semimetal is also considered.

Influence of Coulomb interaction on the anisotropic Dirac cone in graphene

Anisotropic Dirac cones can appear in a number of correlated electron systems, such as cuprate superconductors and deformed graphene. We study the influence of long-range Coulomb interaction on the physical properties of an anisotropic graphene by using the renormalization group method and 1/N expansion, where N is the flavor of Dirac fermions. Our explicit calculations reveal that the anisotropic fermion velocities flow monotonously to an isotropic fixed point in the lowest energy limit in clean graphene. We then incorporate three sorts of disorders, including random chemical potential, random gauge potential, and random mass, and show that the interplay of Coulomb interaction and disorders can lead to rich and unusual behaviors. In the presence of strong Coulomb interaction and a random chemical potential, the fermion velocities are driven to vanish at low energies and the system turns out to be an exotic anisotropic insulator. In the presence of Coulomb interaction and other two types of disorders, the system flows to an isotropic low-energy fixed point more rapidly than the clean case, and exhibits non-Fermi liquid behaviors. We also investigate the nonperturbative effects of Coulomb interaction, focusing on how the dynamical gap is affected by the velocity anisotropy. It is found that the dynamical gap is enhanced (suppressed) as the fermion velocities decrease (increase), but is suppressed as the velocity anisotropy increases.

Interplay of Coulomb interaction and spin-orbit effects in multilevel quantum dots

We study electron transport through a multilevel quantum dot with Rashba spin-orbit interaction in the presence of local Coulomb repulsion. We focus on the parameter regime in which the level spacing is larger than the level broadening. Motivated by recent experiments, we compute the level splitting induced by the spin-orbit interaction at finite Zeeman fields $B$, which provides a measure of the renormalized spin-orbit energy. This level splitting is responsible for the suppression of the Kondo ridges at finite $B$ characteristic for the multilevel structure. In addition, the dependence of renormalized $g$ factors on the relative orientation of the applied $B$ field and the spin-orbit direction following two different protocols used in experiments is investigated.

Study of self-heating effects in SOI and conventional MOSFETs with electro-thermal particle-based device simulator

In this paper we present a study of self-heating effects in nanoscale SOI (Silicon-On-Insulator) devices and conventional MOSFETs using an in-house electro-thermal particle-based device simulator. We first describe the key features of the electro-thermal Monte Carlo device simulator (the two-dimensional (2D) and the three-dimensional version (3D) of the tool) and then we present a series of representative simulation results that clearly illustrate the importance of self-heating in larger nanoscale devices made in SOI technology. Our simulation results for planar SOI devices (using 2D version of the tool) show that in the smallest devices considered, heat dissipation occurs in the contacts, not in the active channel region of the device. This is because of two factors: pronounced velocity overshoot effect and the smaller thermal resistance of the buried oxide layer. We propose methods in which heat can be effectively removed from the device by using silicon on diamond and silicon on AlN technologies. To simulate self heating in nanowire transistors, the 2D simulator was extended to three spatial dimensions. We study the interplay of Coulomb interactions due to the presence of a random trap at the source end of the channel in nanowire transistors, the influence of a positive and a negative trap on the magnitude of the on-current and the role of the potential barrier at the source end of the channel. Finally, we examine the importance of self-heating effects in conventional MOSFETs used for low-power applications. We find that the average temperature increase obtained with our simulator of about 10 K is almost identical to the value that has to be used in low-power circuit simulations.

Theory of transport through noncollinear single-electron spin-valve transistors

We study the electronic transport through a noncollinear single-electron spin-valve transistor. It consists of a small metallic island weakly coupled to two ferromagnetic leads with noncollinear magnetization directions. The electric current is influenced by Coulomb charging and by spin accumulation. Furthermore, the interplay of Coulomb interaction and tunnel coupling to spin-polarized leads yields a many-body exchange field in which the accumulated island spin precesses. We analyze the effects of this exchange field in both the linear and nonlinear transport regime. In particular, we find that the exchange field can give rise to a high sensitivity of the island's spin orientation on the gate voltage.

Probing the exchange field of a quantum-dot spin valve by a superconducting lead

Electrons in a quantum-dot spin valve, consisting of a single-level quantum dot coupled to two ferromagnetic leads with magnetizations pointing in arbitrary directions, experience an exchange field that is induced on the dot by the interplay of Coulomb interaction and quantum fluctuations. We show that a third, superconducting lead with large superconducting gap attached to the dot probes this exchange field very sensitively. In particular, we find striking signatures of the exchange field in the symmetric component of the supercurrent with respect to the bias voltage applied between the ferromagnets already for small values of the ferromagnets' spin polarization.

Spin-induced charge correlations in transport through interacting quantum dots with ferromagnetic leads

We study the full counting statistics of electronic transport through a single-level quantum dot weakly coupled to two leads, with either one or both of them being ferromagnetic. The interplay of Coulomb interaction and finite spin polarization implies spin-correlation induced charge correlations that give rise to super-Poissonian transport behavior and positive cross correlations of the currents of the two spin species. In the case of two ferromagnetic leads, we analyze the nontrivial dependence of the cumulants on the angle between the polarization directions of the leads. We find diverging second and higher cumulants for spin polarizations approaching unity.

Large system asymptotics of persistent currents in mesoscopic quantum rings

We consider a one-dimensional mesoscopic quantum ring filled with spinless electrons and threaded by a magnetic flux, which carries a persistent current at zero temperature. The interplay of Coulomb interactions and a single on-site impurity yields a nontrivial dependence of the persistent current on the size of the ring. We determine numerically the asymptotic power law for systems up to $32\text{ }000$ sites for various impurity strengths and compare with predictions from Bethe ansatz solutions combined with bosonization. The numerical results are obtained using an improved functional renormalization-group (fRG) method. We apply the density-matrix renormalization-group (DMRG) and exact diagonalization methods to benchmark the fRG calculations. We use DMRG to study the persistent current at low electron concentrations in order to extend the validity of our results to quasicontinuous systems. We briefly comment on the quality of calculated fRG ground-state energies by comparison with exact DMRG data.

Magnetic Brightening of Carbon Nanotube Photoluminescence through Symmetry Breaking

We report that symmetry breaking by a magnetic field can drastically increase the photoluminescence quantum yield of single-walled carbon nanotubes, by as much as a factor of 6, at low temperatures. To explain this we have developed a theoretical model based on field-dependent exciton band structure and the interplay of Coulomb interactions and the Aharonov-Bohm effect. This conclusively explains our data as the first experimental observation of dark excitons 5-10 meV below the bright excitons.

Photon absorption and collective excitations in spintronic superlattices

We calculate the electron density response function of electron-hole multilayer systems in the presence of spin-orbit interaction modeled by Rashba terms. It is found that the spin-orbit coupling leads to a redistribution of the absorption spectral weight between the particle-hole excitation and the plasmon excitation. The interplay of Coulomb interactions and spin-orbit interaction resulted in a suppression or enhancement of the optical absorption due to plasmons. Our result indicates that the spin-orbit coupling can serve as a tuning mechanism for optical absorption in spintronics systems.

Incoherent charge transport through molecular wires: interplay of Coulomb interaction and wire population

The influence of Coulomb interaction on the electron transport through molecular wires is studied in the regime of incoherent tunneling. In the limit of strong Coulomb repulsion, the current for spinless electrons is determined. It is shown that the voltage profile along the wire crucially influences the dependence of the current on the wire length. Upon inclusion of the spin degree of freedom one finds a blocking effect which depends both on the interaction strength and on the population of the wire. For finite Coulomb interaction, the temperature dependence of the blocking is studied and it is shown that several regimes with different blocking strength may exist.

Electronic stabilization of amorphous and quasicrystalline metals: importance of quantum correlations

Numerous experimental indications suggest that the Hume–Rothery mechanism plays an important role in stabilizing quasicrystalline and amorphous phases. However, the exponential damping of the conventional Friedel oscillations at the relevant, elevated temperatures T poses a severe challenge to the HR stabilization. In order to resolve this problem it is shown using a Feynman diagram technique that quantum correlations in the electron sea, arising from the interplay of Coulomb interaction and impurity scattering, can strongly enhance the Friedel oscillations in these systems even at elevated temperature. The resulting corrections to the Friedel potential are in agreement with available experimental results on amorphous HR alloys. It is proposed to include the enhancement of the Friedel amplitude derived in the present work into pseudopotentials through the local field factor.

Theoretical Aspects of Non-Debye Relaxation in Ionic Conductors

AbstractTracer diffusion as well as collective transport properties of ionic conductors are investigated within a lattice-gas model of charged particles. Results obtained from Monte Carlo simulations support the conclusion that the interplay of Coulomb-interactions among diffusing ions and structural disorder in the material provides a general mechanism for non-Debye relaxational effects commonly observed by experiment.

The Mott Anderson metal insulator transition in n orbital models

The interplay of Coulomb interactions and disorder is formulated on the basis of local gauge invariant models withn orbitals per site. Universality classes of correlation-influenced metal insulator transitions are examined from the conducting region of an approximately semi-elliptical band. The microscopic 1/n expansion is brought into direct correspondence with the 1/(k F l) expansion of Altshuler et al. Based on1. the assumption that theO(1/n)-expansion can be exponentiated and2. on the renormalization group β-function for finite length scaling, critical exponents are derived in leading order ofd-2 (d is dimensionality). The density of states-at-E F as the order parameter vanishes at the critical pointE c like\(\left| {E_F - E_c } \right|^{\beta _{MA} } \) with\(\beta _{MA} = \frac{1}{{4(d - 2)}}\) for the interacting real matrix (orthogonal) ensemble and\(\beta _{MA} = \frac{1}{{2(d - 2)}}\) for the unitary ensemble. If the bare Coulomb interactionU b (q)∝q 1-d is replaced by a general long range interactionV b (q)∝q −x withx>0 ford=2, βMA depends on the “interaction range exponent”x like βMA(x) = xβMA(1). Also in leading order ofd−2, the conductivity exponent ist=1 for both models with or without time reversal invariance respectively. This implies a correlation-induced crossover fromt=1/2 for unitary Anderson localization (broken time reversal invariance) tot=1 at the Mott Anderson transition.