Recent Publications
Quantum magnetic oscillations in the absence of closed electron trajectories
Z. E. Krix, O. A. Tkachenko, V. A. Tkachenko, D. Q. Wang, O. Klochan, A. R. Hamilton, O. P. Sushkov, arXiv:2404.04592
Quantum magnetic oscillations in crystals are typically understood in terms of Bohr-Sommerfeld quantisation, the frequency of oscillation is given by the area of a closed electron trajectory. However, since the 1970s, oscillations have been observed with frequencies that do not correspond to closed electron trajectories and this effect has remained not fully understood. Previous theory has focused on explaining the effect using various kinetic mechanisms, however, frequencies without a closed electron orbit have been observed in equilibrium and so a kinetic mechanism cannot be the entire story. In this work we develop a theory which explains these frequencies in equilibrium and can thus be used to understand measurements of both Shubnikov-de Haas and de Haas-van Alphen oscillations. We show, analytically, that these frequencies arise due to multi-electron correlations. We then extend our theory to explain a recent experiment on artificial crystals in GaAs two-dimensional electron gases, which revealed for the first time magnetic oscillations having frequencies that are half of those previously observed. We show that the half-frequencies arise in equilibrium from single-particle dynamics with account of impurities. Our analytic results are reinforced by exact numerics, which we also use clarify prior works on the kinetic regime.
Formation of artificial Fermi surfaces with a triangular superlattice on a conventional two dimensional electron gas
Daisy Q. Wang, Zeb Krix, Oleg P. Sushkov, Ian Farrer, David A. Ritchie, Alexander R. Hamilton, Oleh Klochan, arXiv:2403.06426
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.
Tuning the bandstructure of electrons in a two-dimensional artificial electrostatic crystal in GaAs quantum wells
Daisy Q. Wang, Zeb Krix, Olga A. Tkachenko, Vitaly A. Tkachenko, Chong Chen, Ian Farrer, David A. Ritchie, Oleg P. Sushkov, Alexander R. Hamilton, Oleh Klochan, arXiv:2402.12769
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. Cold atoms trapped in optical lattices provide great flexibility and tunability, but cannot replicate the long range Coulomb interactions and long range hopping that drive collective phenomena in real crystals. Solid state approaches support long range hopping and interactions, but previous attempts with laterally patterned semiconductor systems were not able to create tunable low disorder artificial crystals, while approaches based on Moire superlattices in twisted two-dimensional (2D) materials have limited tunability and control of lattice geometry. Here we demonstrate the formation of highly tunable artificial crystals by superimposing a periodic electrostatic potential on the 2D electron gas in an ultrashallow (25 nm deep) GaAs quantum well. (See more.)
Electron Magneto-Hydrodynamics in Graphene
Jack N. Engdahl, Aydın Cem Keser, Thomas Schmidt, Oleg P. Sushkov, arXiv:2312.04896
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 Hall effect, where a periodic magnetic field generates a voltage modulation in the direction of transport. From the experimental point of view we propose a method for a precise measurement of the properties of different electron fluids. The results are applicable to graphene away from the charge neutrality point and to semiconductors.
Probing Fermi surface shifts with spin resolved transverse magnetic focussing
M. J. Rendell, S. D. Liles, S. Bladwell, A. Srinivasan, O. Klochan, I. Farrer, D. A. Ritchie, O. P. Sushkov, A. R. Hamilton, arXiv:2310.04005
Transverse magnetic focussing is the solid state equivalent of a mass spectrometer. It is unique among 2D measurement techniques as it is able to measure a well defined section of the Fermi surface, making it possible to detect changes that would be averaged out over the whole Fermi surface. Here, we utilise this unique property to probe non-adiabatic spin dynamics and spin dependent scattering of holes. We combine spin-resolved magnetic focussing with an additional independent in-plane magnetic field and observe a change in focussing peak amplitude that is not symmetric with respect to the field direction (i.e. +B∥≠−B∥), and is extremely sensitive to the magnitude of the in-plane magnetic field. We show that the magnetic focussing signal is extremely sensitive to small changes in the Fermi velocity, which can be used to detect small shifts in the Fermi surface caused by an in-plane magnetic field. We also find that focussing can be used to detect the proximity between spin-split Fermi surfaces, which cause non-adiabatic spin dynamics.
Schiff moments of deformed nuclei
Oleg P. Sushkov, arXiv:2307.04299
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 153Eu. Hence, we perform the calculation for 153Eu 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 153Eu 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.
Effective electric field: quantifying the sensitivity of searches for new P,T-odd physics with EuCl3⋅6H2O
Alexander O. Sushkov, Oleg P. Sushkov, Alexander Yaresko, Phys. Rev. A 107, 062823 (2023) DOI: https://doi.org/10.1103/PhysRevA.107.062823
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 EuCl3⋅6H2O crystal, which is a particularly promising material. The optimistic estimate of the effective electric field for the 153Eu 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.
Formation of Artificial Fermi Surfaces with a Triangular Superlattice on a Conventional Two-Dimensional Electron Gas
Daisy Q. Wang, Zeb Krix, Oleg P. Sushkov, Ian Farrer, David A. Ritchie, Alexander R. Hamilton, and Oleh Klochan (2023) Nano Lett. 2023, 23, 5, 1705-1710. DOI: https://doi.org/10.1021/acs.nanolett.2c04358
Imposing an external periodic electrostatic potential to the electrons confined in a 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 tunable triangular artificial lattice on a GaAs/AlGaAs heterostructure where it is possible to transform from the original GaAs band structure and a circular Fermi surface to a new band structure with multiple artificial Fermi surfaces simply by altering a gate bias. For weak electrostatic modulation magnetotransport measurements reveal multiple quantum oscillations and commensurability oscillations due to the electron scattering from the artificial lattice. Increasing the strength of the modulation reveals new commensurability oscillations of the electrons from the artificial Fermi surface scattering from the triangular artificial lattice. These results show that low disorder gate-tunable lateral superlattices can be used to form artificial two-dimensional crystals with designer electronic properties.
Exciton condensation in biased bilayer graphene
Harley D. Scammell and Oleg P. Sushkov, Phys. Rev. Research 5, 043176 (2023)
We consider suspended bilayer graphene under applied perpendicular electric bias field that is known to generate a single particle gap 2∆ and a related electric polarization 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 2∆. We predict the critical bias (converted to band gap), ∆c ≈ 60 meV, below which the excitons condense. The critical temperature, Tc(∆), is maximum at ∆ ≈ 25 meV, T max c ≈ 115 K, decreasing significantly at smaller ∆ due to thermal screening. Entering the condensate phase, the superfluid transition is accompanied by a cusp in the electric polarization P(∆) at ∆ → ∆c, which provides a striking testable signature. Additionally, we find that the condensate prefers to form a pair density wave.
Observation of oscillating g-factor anisotropy arising from strong crystal lattice anisotropy in GaAs spin-3/2 hole quantum point contacts
Karina Hudson, Ashwin Srinivasan, Dmitry Miserev, Qingwen Wang, Oleh Klochan, Oleg Sushkov, Ian Farrer, David Ritchie, Alex Hamilton, arXiv:2211.00253
Many modern spin-based devices rely on the spin-orbit interaction, which is highly sensitive to the host semiconductor heterostructure and varies substantially depending on crystal direction, crystal asymmetry (Dresselhaus), and quantum confinement asymmetry (Rashba). One-dimensional quantum point contacts are a powerful tool to probe both energy and directional dependence of spin-orbit interaction through the effect on the hole g-factor. In this work we investigate the role of cubic crystal asymmetry in driving an oscillation in the in-plane hole g-factor anisotropy when the quantum point contact is rotated with respect to the crystal axes, and we are able to separate contributions to the Zeeman Hamiltonian arising from Rashba and cubic crystal asymmetry spin-orbit interactions. The in-plane g-factor is found to be extremely sensitive to the orientation of the quantum point contact, changing by a factor of 5 when rotated by 45∘. This exceptionally strong crystal lattice anisotropy of the in-plane Zeeman splitting cannot be explained within axially symmetric theoretical models. Theoretical modelling based on the combined Luttinger, Rashba and Dresselhaus Hamiltonians that we use here reveals new spin-orbit contributions to the in-plane hole g-factor and provides an excellent agreement with our experimental data.
Patterned bilayer graphene as a tunable strongly correlated system
Z. E. Krix, O. P. Sushkov, Phys. Rev. B 107, 165158 (2023)
Recent observations of superconductivity in Moire graphene have led to an intense interest in that system, with subsequent studies revealing a more complex phase diagram including correlated insulators and ferromagnetic phases. Here we propose an alternate system, electrostatically patterned bilayer graphene (PBG), in which a supermodulation is induced via metallic gates rather than the Moire effect. We show that, by varying either the gap or the modulation strength, bilayer graphene can be tuned into the strongly correlated regime. Further calculations show that this is not possible in monolayer graphene. We present a general technique for addressing Coulomb screening of the periodic potential and demonstrate that this system is experimentally feasible.
Dynamical screening and excitonic bound states in biased bilayer graphene
Harley D. Scammell, Oleg P. Sushkov, Phys. Rev. B 107, 085104 (2023) DOI: 10.1103/PhysRevB.107.085104
Excitonic bound states are characterised by a binding energy εb and a single-particle band gap Δb. This work provides a theoretical description for both strong (εb∼Δb) and weak (εb≪Δb) 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 εb≪Δb. The screening depends on frequency (dynamical screening) and this implies retardation. In the case of strong binding, εb∼Δb, 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.
Micromagnets dramatically enhance effects of viscous hydrodynamic flow in two-dimensional electron fluid
Jack N. Engdahl, Aydin Cem Keser, Oleg P. Sushkov, Phys. Rev. Research 4, 043175, (2022). DOI: https://doi.org/10.1103/PhysRevResearch.4.043175
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.
Chiral excitonic order from twofold van Hove singularities in kagome metals
Harley D. Scammell, Julian Ingham, Tommy Li, and Oleg P. Sushkov, Nature Communications 14, 605 (2023)
Recent experiments on kagome metals AV3Sb5 (A=K,Rb,Cs) 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 the TvHS are responsible for the novel charge and superconducting order observed in kagome metals. We introduce a model of a quasi-two dimensional system hosting a TvHS, and investigate the interaction induced many-body instabilities via the perturbative renormalisation group technique. The TvHS generates a topological d-wave excitonic condensate, arising due to bound pairs of electrons and holes located at opposite concavity vHS. Varying the chemical potential, detuning from the TvHS, and tuning the bare interaction strength, we construct a phase diagram which features the excitonic condensate alongside d-wave superconductivity and spin density wave order. The chiral excitonic state supports a Chern band giving 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.
Geometric control of universal hydrodynamic flow in a two dimensional electron fluid
Aydın Cem Keser, Daisy Q. Wang, Oleh Klochan, Derek Y. H. Ho, Olga A. Tkachenko, Vitaly A. Tkachenko, Dimitrie Culcer, Shaffique Adam, Ian Farrer, David A. Ritchie, Oleg P. Sushkov, Alexander R. Hamilton, Phys. Rev. X 11, 031030 (2021). DOI: https://doi.org/10.1103/PhysRevX.11.031030
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.
Nonlinear Quantum Electrodynamics in Dirac materials
Aydin C. Keser, Yuli Lyanda-Geller, Oleg P. Sushkov, Phys. Rev. Lett. 128, 066402 (2022).
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 ∼1T, allowing us to explore the extremely high field limit of quantum electrodynamics in solids. We explain recent experiments in a unified framework and predict nonlinear magneto-electric response, including a magnetic enhancement of dielectric constant and electrically induced magnetization. We propose experiments and discuss the applications on novel materials.
Origin of hour-glass magnetic dispersion in underdoped cuprate superconductors
Y. A. Kharkov, O. P. Sushkov, Phys. Rev. B 100, 224510 (2019).
In the present work we explain the hour-glass magnetic dispersion in underdoped cuprates. The dispersion arises due to an interplay of the Lifshitz-type magnetic criticality and superconductivity. We provide a unified picture of the evolution of magnetic excitations in various cuprate families, including “hour-glass” and “wine glass” dispersions and emergent static incommensurate order. Besides explaining existing data on magnetic dispersion we propose a neutron scattering experiment that can directly test the developed theory.
Nature of the spin liquid in underdoped cuprate superconductors
Y. A. Kharkov, O. P. Sushkov, Phys. Rev. B 98, 155118 (2018).
In the present work we address a long standing problem of the magnetic ground state and magnetic excitations in underdoped cuprates. Modelling cuprates by the extended t − J model we show that there is a hidden dimensionless parameter λ which drives magnetic criticality at low doping x. Hence we derive the zero temperature λ − x phase diagram of the model. It is argued that all underdoped cuprates are close to the quantum tricritical point x = 0, λ = 1. The three phases “meet” at the tricritical point: (i) N´eel antiferromagnet, (ii) spin spiral with antinodal direction of the spiral wave vector, (iii) algebraic spin liquid. We argue that underdoped cuprates belong either to the spin liquid phase or they are on the borderline between the spin liquid and the spin spiral. We calculate the energy position Ecross of the inelastic neutron scattering response maximum at q = (π, π) and compare our results with experiments. We also explain softening of magnons in the intermediate regime observed in inelastic neutron scattering.