arXiv subjects
Publications and source records attributed to Oleg P. Sushkov.
I analyze available $μ$SR data and show that it is inconsistent with the spin stripe but consistent with the coplanar spin spiral. The plane of the spiral coincides with the CuO$_2$-plane. The static expectation value of the spin is $S=0.37\times\frac{1}{2}$.
The electronic properties of solids are determined by the crystal structure and interactions between electrons, giving rise to a variety of collective phenomena including superconductivity, strange metals and correlated insulators. The mechanisms underpinning many of these collective phenomena remain unknown, driving interest in creating artificial crystals which replicate the system of interest while allowing precise control of key parameters. Here we demonstrate the formation of highly tunable artificial crystals by superimposing a periodic electrostatic potential on the 2D electron gas in an ultra-shallow (25 nm deep) GaAs quantum well. The 100 nm period artificial crystal is identified by the formation of a new bandstructure, different from the original cubic crystal and specific to the artificial triangular lattice: transport measurements show the Hall coefficient changing sign as the chemical potential sweeps through the artificial bands. Uniquely, the artificial bandstructure can be continuously tuned to form linear graphene-like and flat kagome-like bands in a single device. A strong insulating state is observed at half filling of the kagome flat band, which is not expected in the absence of strong interactions. This state, unique to the kagome lattice, is consistent with a loop-current Wigner insulator, which arises from long-range Coulomb interaction and delocalised electrons between neighbouring empty sites. The ability to continuously tune the bandstructure and access flat bands through electrical gating within a single device opens a new route to studying collective quantum states.
Exciton condensation in semiconductors and semimetals has long been predicted but remains elusive. In a semiconductor, condensation occurs when the exciton binding energy matches the band gap. This binding energy results from a balance between Coulomb attraction, which enhances it, and kinetic energy, which suppresses it. However, reducing kinetic energy typically increases screening, weakening Coulomb attraction. Empirically, in most candidate materials, the binding energy remains below the band gap, with few external parameters capable of altering this balance. Here, we propose an in-plane electric field as a control parameter. This field induces hybridisation between even- and odd-parity excitons, and the resulting level repulsion effectively enhances binding energy. We argue that this mechanism is generic to excitons in semiconductors and illustrate it with a model of biased bilayer graphene. Bilayer graphene is chosen since it has a tunable band gap, making it an excitonic condensate candidate and moreover, the Zener tunnelling rate contains, in addition to the usual exponential decay, a non-standard oscillating component -- thanks to details of the electron dispersion. Analogous to quantum oscillations, we propose that Fourier spectrum of the current-voltage data allows for a novel test of exciton condensation. Finally, we show that the is a large excitonic gap to critical temperature ratio -- a clear prediction for STM studies.
Employing a semiclassical method based on analytic continuation, we compute the electron-hole pair production rate in biased bilayer graphene subject to an in-plane electric field. This approach, originally due to Zwaan, bypasses the need for exact solutions at turning points, which are generally unavailable beyond linear or quadratic band structures. Applying this technique to biased bilayer graphene reveals non-standard features of the asymptotic wavefunctions, in particular the necessity of retaining decaying components even in classically allowed regions. By providing a fully analytic solution, this work complements and clarifies earlier results based on hybrid analytical-numerical treatments, and importantly establishes the absolute normalisation of the pair production rate -- and hence of the tunnelling current.
It is well known that a direct band gap may be opened in bilayer graphene via the application of a perpendicular electric field (bias). The bias and the chemical potential are controlled by electrostatic gating where the top and bottom gate voltages are tuned separately. The value of the band gap opened by the bias field is influenced by the self screening of the bilayer graphene. The Hartree contribution to the self screening is well known in literature, with Hartree screening significantly renormalizing the gap. In the present work we derive the Fock contribution to the self screening and demonstrate that it is equally important and in the low density regime even more important than the Hartree contribution. We calculate the Hartree-Fock screened band gap as a function of electron doping at zero temperature and also as a function of temperature at zero doping.
Two Dimensional (2D) Transition Metal Dichalcogenides (TMDs) possess a large direct band gap which has been experimentally observed to shrink with increasing charge carrier density (doping). The effect has been the subject of theoretical study in recent years using various approaches and approximations. In this work we develop the theory of bandgap renormalization based on Feynman diagrammatic technique in the imaginary frequency formalism. We consider dynamical screening from conduction band electrons using the random phase approximation (RPA), as well as screening from a metallic gate. While our theory is general for any 2D semiconductor, to be specific we consider MoS$_2$ and WSe$_2$ and compare with available experimental data. In both cases we calculate large band gap renormalization that reaches several hundred meV at relatively low carrier density. This is in good agreement with experimental data.
Two-dimensional hole gases (2DHGs) have strong intrinsic spin-orbit coupling and could be used to build spin filters by utilising transverse magnetic focusing (TMF). However, with an increase in the spin degree of freedom, holes demonstrate significantly different behaviour to electrons in TMF experiments, making it difficult to interpret the results of these experiments. In this paper, we numerically model TMF in a 2DHG within a GaAs/Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As heterostructure. Our band structure calculations show that the heavy $(\langle J_{z} \rangle = \pm\frac{3}{2})$ and light $(\langle J_{z} \rangle = \pm\frac{1}{2})$ hole states in the valence band mix at finite $k$, and the heavy hole subbands which are spin-split due to the Rashba effect are not spin-polarised. This lack of spin polarisation casts doubt on the viability of spin filtering using TMF in 2DHGs within conventional GaAs/Al$_{\mathrm{x}}$Ga$_{\mathrm{1-x}}$As heterostructures. We then calculate transport properties of the 2DHG with spin projection and offer a new perspective on interpreting and designing TMF experiments in 2DHGs.
The magnetic field dependence of photoabsorption provides direct insights into the band structure of semiconductors. It is perhaps surprising that there is a large discrepancy between electron, hole, and reduced mass reported in the recent literature. Motivated by this puzzle we reconsider excitonic magneto-absorption and find that the commonly employed perturbative approach, namely for computing the diamagnetic shift, is inadequate to account for the parameter ranges considered in existing data. In particular, we develop the theory for strong magnetic field and, upon analysis of the data, arrive at the set of exciton parameters different to what has been estimated perturbatively in the literature. Only s-wave excitons are visible in photoluminescence as the spectral weight of p-wave states is too small, this limits the amount of information that can be extracted about the underlying band structure. To overcome this, we propose to study p-wave states by mixing them with s-wave states by external in-plane electric field and show that a moderate DC electric field would provide sufficient mixing to brighten p-wave states. We calculate energies of the p-wave states including the effects of valley-orbital splitting and the orbital Zeeman shift, and show that this provides direct information on the electron-hole mass asymmetry.
We consider the hydrodynamic flow of an electron fluid in a channel formed in a two-dimensional electron gas (2DEG) with no-slip boundary conditions. To generate vorticity in the fluid the flow is influenced by an array of micromagnets on the top of 2DEG. We analyse the viscous boundary layer and demonstrate anti-Poiseuille behaviour in this region. Furthermore we predict a longitudinal voltage modulation, where a periodic magnetic field generates a voltage term periodic in the direction of transport.From the experimental point of view we propose a method for a boundary-independent measurement of the viscosity of different electron fluids. The results are applicable to graphene away from the charge neutrality point and to semiconductors.
In nearly free electron theory the imposition of a periodic electrostatic potential on free electrons creates the bandstructure of a material, determined by the crystal lattice spacing and geometry. Imposing an artificially designed potential to the electrons confined in a GaAs quantum well makes it possible to engineer synthetic two-dimensional band structures, with electronic properties different from those in the host semiconductor. Here we report the fabrication and study of a tuneable triangular artificial lattice on a GaAs/AlGaAs heterostructure where it is possible to transform from the original GaAs bandstructure and Fermi surface to a new bandstructure with multiple artificial Fermi surfaces simply by altering a gate bias. For weak electrostatic potential modulation magnetotransport measurements reveal quantum oscillations from the GaAs two-dimensional Fermi surface, and classical oscillations due to these electrons scattering from the artificial lattice. Increasing the strength of the modulation reveals new quantum oscillations due to the formation of multiple artificial Fermi surfaces, and ultimately to new classical oscillations of the electrons from the artificial Fermi surface scattering from the superlattice modulation. These results show that low disorder gate-tuneable lateral superlattices can be used to form artificial two dimensional crystals with designer electronic properties.
The hydrodynamic behavior of electron fluids in a certain range of temperatures and densities is well established in graphene and in 2D semiconductor heterostructures. The hydrodynamic regime is intrinsically based on electron-electron interactions, and therefore it provides a unique opportunity to study electron correlations. Unfortunately, in all existing measurements, the relative contribution of hydrodynamic effects to transport is rather small. Viscous hydrodynamic effects are masked by impurities, interaction with phonons, uncontrolled boundaries and ballistic effects. This essentially limits the accuracy of measurements of electron viscosity. Fundamentally, what causes viscous friction in the electron fluid is the property of the flow called vorticity. In this paper, we propose to use micromagnets to increase the vorticity by orders of magnitude. Experimental realization of this proposal will bring electron hydrodynamics to a qualitatively new precision level, as well as opening a new way to characterize and externally control the electron fluid.
Stimulated by recent suggestion of Cosmic Axion Spin Precession Experiment with Eu contained compound we develop a new method for accurate calculation of Schiff moments of even-odd deformed nuclei. The method is essentially based on experimental data on magnetic moments and E1,E3-amplitudes in the given even-odd nucleus and in adjacent even-even nuclei. Unfortunately such sets of data are not known yet for most of interesting nuclei. Fortunately the full set of data is available for $^{153}$Eu. Hence, we perform the calculation for $^{153}$Eu and find value of the Schiff moment. The value is about 30 times larger than a typical Schiff moment of a spherical heavy nucleus. The enhancement of the Schiff moment in $^{153}$Eu is related to the low energy octupole mode. On the other hand the value of Schiff moment we find is 30 times smaller than that obtained in the assumption of static octupole deformation.
Laboratory-scale precision experiments are a promising approach to searching for physics beyond the standard model. Non-centrosymmetric solids offer favorable statistical sensitivity for efforts that search for new fields, whose interactions violate the discrete parity and time-reversal symmetries. One example is the electric Cosmic Axion Spin Precession Experiment (CASPEr-e), which is sensitive to the defining interaction of the QCD axion dark matter with gluons in atomic nuclei. The effective electric field is the parameter that quantifies the sensitivity of such experiments to new physics. We describe the theoretical approach to calculating the effective electric field for non-centrosymmetric sites in ionic insulating solids. We consider the specific example of the EuCl$_3\cdot$6H$_2$O crystal, which is a particularly promising material. The optimistic estimate of the effective electric field for the $^{153}$Eu isotope in this crystal is 10 MV/cm. The calculation uncertainty is estimated to be two orders of magnitude, dominated by the evaluation of the Europium nuclear Schiff moment.
Recent experiments on kagome metals AV$_3$Sb$_5$ (A=K,Rb,Cs) [M. Kang et al., Nat. Phys. 18, 301-308 (2022)] identify twofold van Hove singularities (TvHS) with opposite concavity near the Fermi energy, generating two approximately hexagonal Fermi surfaces -- one electron-like and the other hole-like. Here we propose that a TvHS generates a novel time-reversal symmetry breaking excitonic order -- arising due to bound pairs of electrons and holes located at opposite concavity van Hove singularities. We introduce a minimal model for the TvHS and investigate interaction induced many-body instabilities via the perturbative renormalisation group technique and a free energy analysis. Specialising to parameters appropriate for the kagome metals AV$_3$Sb$_5$, we construct a phase diagram comprising chiral excitons, charge density wave and a region of coexistence. We propose this as an explanation of a diverse range of experimental observations in AV$_3$Sb$_5$. Notably, the chiral excitonic state gives rise to a quantum anomalous Hall conductance, providing an appealing interpretation of the observed anomalous Hall effect in kagome metals. Possible alternative realisations of the TvHS mechanism in bilayer materials are also discussed. We suggest that TvHS open up interesting possibilities for correlated phases, enriching the set of competing ground states to include excitonic order.
We consider suspened bilayer graphene under applied perpendicular electric bias field that is known to generate a single particle gap $2Δ$ and a related electric polarization ${\cal P}$. We argue that the bias also drives a quantum phase transition from band insulator to superfluid exciton condensate. The transition occurs when the exciton binding energy exceeds the band gap $2Δ$. We predict the critical bias (converted to band gap), $Δ_c\approx 60$ meV, below which the excitons condense. The critical temperature, $T_c(Δ)$, is maximum at $Δ\approx 25$ meV, $T_c^\text{max}\approx 115$ K, decreasing significantly at smaller $Δ$ due to thermal screening. Entering the condensate phase, the superfluid transition is accompanied by a cusp in the electric polarization ${\cal P}(Δ)$ at $Δ\toΔ_c$, which provides a striking testable signature. Additionally, we find that the condensate prefers to form a pair density wave.
Excitonic bound states are characterised by a binding energy $ε_b$ and a single-particle band gap $Δ_b$. This work provides a theoretical description for both strong ($ε_b\simΔ_b$) and weak ($ε_b\llΔ_b$) excitonic bound states, with particular application to biased bilayer graphene. Standard description of excitons is based on a wave function that is determined by a Schrödinger-like equation with screened attractive potential. The wave function approach is valid only in the weak binding regime $ε_b\llΔ_b$. The screening depends on frequency (dynamical screening) and this implies retardation. In the case of strong binding, $ε_b\simΔ_b$, a wave function description is not possible due to the retardation. Instead we appeal to the Bethe-Salpeter equation, written in terms of the electron-hole Green's function, to solve the problem. So far only the weak binding regime has been achieved experimentally. Our analysis demonstrates that the strong binding regime is also possible and we specify conditions in which it can be achieved for the prototypical example of biased bilayer graphene. The conditions concern the bias, the configuration of gates, and the substrate material. To verify the accuracy of our analysis we compare with available data for the weak binding regime. We anticipate applying the developed dynamical screening Bethe-Salpeter techniques to various 2D materials with strong binding.
Classical electromagnetism is linear. However, fields can polarize the vacuum Dirac sea, causing quantum nonlinear electromagnetic phenomena, e.g., scattering and splitting of photons, that occur only in very strong fields found in neutron stars or heavy ion colliders.We show that strong nonlinearity arises in Dirac materials at much lower fields $\sim 1\:\text{T}$, allowing us to explore the nonperturbative, extremely high field limit of quantum electrodynamics in solids. We explain recent experiments in a unified framework and predict a new class of nonlinear magneto-electric effects, including a magnetic enhancement of dielectric constant of insulators and a strong electric modulation of magnetization. We propose experiments and discuss the applications in novel materials.
Fluid dynamics is one of the cornerstones of modern physics and has recently found applications in the transport of electrons in solids. In most solids electron transport is dominated by extrinsic factors, such as sample geometry and scattering from impurities. However in the hydrodynamic regime Coulomb interactions transform the electron motion from independent particles to the collective motion of a viscous `electron fluid'. The fluid viscosity is an intrinsic property of the electron system, determined solely by the electron-electron interactions. Resolving the universal intrinsic viscosity is challenging, as it only affects the resistance through interactions with the sample boundaries, whose roughness is not only unknown but also varies from device to device. Here we eliminate all unknown parameters by fabricating samples with smooth sidewalls to achieve the perfect slip boundary condition, which has been elusive both in molecular fluids and electronic systems. We engineer the device geometry to create viscous dissipation and reveal the true intrinsic hydrodynamic properties of a 2D system. We observe a clear transition from ballistic to hydrodynamic electron motion, driven by both temperature and magnetic field. We directly measure the viscosity and electron-electron scattering lifetime (the Fermi quasiparticle lifetime) over a wide temperature range without fitting parameters, and show they have a strong dependence on electron density that cannot be explained by conventional theories based on the Random Phase Approximation.