arXiv subjects

O. P. Sushkov

Publications and source records attributed to O. P. Sushkov.

At least 19 recordsLinked to original sources

arXiv2025

Probing Fermi surface parity with spin resolved transverse magnetic focussing

Measurements of the Fermi surface are a fundamental technique for determining the electrical and magnetic properties of solids. In 2D systems, the area and diameter of the Fermi surface is typically measured using Shubnikov-de Haas oscillations and commensurability oscillations respectively. However, these techniques are unable to detect changes in the parity of the Fermi surface (i.e. when +k $\neq$ -k). Here, we show that transverse magnetic focussing can be used to detect such changes, because focussing only measures a well defined section of the Fermi surface and does not average over +k and -k. Furthermore, our results show that focussing is an order of magnitude more sensitive to changes in the Fermi surface than other 2D techniques, and could be used to investigate similar Fermi surface changes in other 2D systems.

cond-mat.mes-hall↗

arXiv2024

Quantum magnetic oscillations in the absence of closed electron trajectories

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.

cond-mat.mes-hall↗

arXiv2022

Patterned bilayer graphene as a tunable, strongly correlated system

Recent observations of superconductivity in Moire graphene have lead 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.

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arXiv2021

Electrical control of the $g$-tensor of a single hole in a silicon MOS quantum dot

Single holes confined in semiconductor quantum dots are a promising platform for spin qubit technology, due to the electrical tunability of the $g$-factor of holes. However, the underlying mechanisms that enable electric spin control remain unclear due to the complexity of hole spin states. Here, we study the underlying hole spin physics of the first hole in a silicon planar MOS quantum dot. We show that non-uniform electrode-induced strain produces nanometre-scale variations in the HH-LH splitting. Importantly, we find that this \RR{non-uniform strain causes} the HH-LH splitting to vary by up to 50\% across the active region of the quantum dot. We show that local electric fields can be used to displace the hole relative to the non-uniform strain profile, allowing a new mechanism for electric modulation of the hole g-tensor. Using this mechanism we demonstrate tuning of the hole $g$-factor by up to 500\%. In addition, we observe a \RR{potential} sweet spot where d$g_{(1\overline{1}0)}$/d$V$ = 0, offering a configuration to suppress spin decoherence caused by electrical noise. These results open a path towards a previously unexplored technology: engineering of \RR{non-uniform} strains to optimise spin-based devices.

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arXiv2021

Correlated physics in an artificial triangular anti-dot lattice

This work considers a two-dimensional artificial triangular anti-dot lattice (TAL); a semiconductor based artificial crystal hosting Dirac cones, flat bands and Fermi surface nesting. All such single particle features have dramatic implications for the emergent correlated phases. This work predominantly focuses on the existence of a robust flatband and enumerates the possible correlated phases that follow. We find that the flatband is generated, in the single-particle theory, when charges align themselves along a kagome lattice with the same period as the TAL. The correlated phases are studied using complementary techniques of expansions in strong and weak Coulomb interaction. Our microscopic modelling shows that for the purpose of generating strongly correlated phases, hole doped TALs have significant advantages over electron doped.

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arXiv2019

Prediction of the spin triplet two-electron quantum dots in Si: towards controlled quantum simulations of magnetic systems

Ground state of two-electron quantum dots in single-valley materials like GaAs is always a spin singlet regardless of what the potential and interactions are. This statement cannot be generalized to the multi-valley materials like $n$-doped Si. Here we calculate numerically the spectrum of a two-electron Si quantum dot and show that the dot with the lateral size of several nm can have the spin triplet ground state which is impossible in the single-valley materials. Predicted singlet-triplet level crossing in two-electron Si quantum dots can potentially establish the platform for quantum simulation of magnetic many body systems based on quantum dots. We suggest several examples of such systems that open a way to controlled quantum simulations within the condensed matter setting.

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arXiv2019

The origin of hour-glass magnetic dispersion in underdoped cuprate superconductors

In the present work we explain the hour-glass magnetic dispersion in underdoped cuprates. The dispersion arises due to the Lifshitz-type magnetic criticality. Superconductivity also plays a role, but the role is secondary. We list six major experimental observations related to the hour-glass and explain all of them. The theory provides a unified picture of the evolution of magnetic excitations in various cuprate families, including "hour-glass" and "wine-glass" dispersions and an emergent static incommensurate order. We propose the Lifshitz spin liquid "fingerprint" sum rule, and show that the latest data confirm the validity of the sum rule.

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arXiv2019

Gauge phonon dominated resistivity in twisted bilayer graphene near magic angle

Recent experiments on twisted bilayer graphene (tBG) close to magic angle show that a small relative rotation in a van der Waals heterostructure greatly alters its electronic properties. We consider various scattering mechanisms and show that the carrier transport in tBG is dominated by a combination of charged impurities and acoustic gauge phonons. Charged impurities still dominate at low temperature and densities because of the inability of Dirac fermions to screen long-range Coulomb potentials at charge neutrality; however, the gauge phonons dominate for most of the experimental regime because although they couple to current, they do not induce charge and are therefore unscreened by the large density of states close to magic angle. We show that the resistivity has a strong monotonically decreasing carrier density dependence at low temperature due to charged impurity scattering, and weak density dependence at high temperature due to gauge phonons. Away from charge neutrality, the resistivity increases with temperature, while it does the opposite close to the Dirac point. A non-monotonic temperature dependence observed only at low temperature and carrier density is a signature of our theory that can be tested in experimentally available samples.

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arXiv2018

Tuning the topological insulator states of artificial graphene

We develop a robust, non-perturbative approach to study the band structure of artificial graphene. Artificial graphene, as considered here, is generated by imposing a superlattice structure on top of a two dimensional hole gas in a semiconductor heterostructure, where the hole gas naturally possesses large spin-orbit coupling. Via tuning of the system parameters we demonstrate how best to exploit the spin-orbit coupling to generate time reversal symmetry-protected topological insulator phases. Our major conclusion is the identification of a second set of topological Dirac bands in the band structure (with spin Chern number $C=3$), which were not reliably obtainable in previous perturbative approaches to artificial graphene. Importantly, the second Dirac bands host more desirable features than the previously studied first set of Dirac bands (with $C=1$). Moreover, we find that upon tuning of the system parameters, we can drive the system to the highly desirable regime of the topological flat band. We discuss the possibilities this opens up for exotic, strongly correlated phases.

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arXiv2018

Nature of the spin liquid in underdoped cuprate superconductors

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) Néel 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 $E_{cross}$ of the inelastic neutron scattering response maximum at ${\bm q}=(π,π)$ and compare our results with experiments. We also explain softening of magnons in the intermediate regime observed in inelastic neutron scattering.

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arXiv2017

Unifying static and dynamic properties in 3D quantum antiferromagnets

Quantum Monte Carlo simulations offer an unbiased means to study the static and dynamic properties of quantum critical systems, while quantum field theory provides direct analytical results. We study three dimensional, critical quantum antiferromagnets by performing a combined analysis using both quantum field theory calculations and quantum Monte Carlo data. Explicitly, we analyze the order parameter (staggered magnetization), Néel temperature, quasiparticle gaps, and the susceptibilities in the scalar and vector channels. We connect the two approaches by deriving descriptions of the quantum Monte Carlo observables in terms of the quasiparticle excitations of the field theory. The remarkable agreement not only unifies the description of the static and dynamic properties of the system, but also constitutes a thorough test of perturbative O(3) quantum field theory and opens new avenues for the analytical guidance of detailed numerical studies.

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arXiv2017

Mechanisms for strong anisotropy of in-plane g-factors in hole based quantum point contacts

In-plane hole g-factors measured in quantum point contacts based on p-type heterostructures strongly depend on the orientation of the magnetic field with respect to the electric current. This effect, first reported a decade ago and confirmed in a number of publications, has remained an open problem. In this work, we present systematic experimental studies to disentangle different mechanisms contributing to the effect and develop the theory which describes it successfully. We show that there is a new mechanism for the anisotropy related to the existence of an additional $B_+k_-^4σ_+$ effective Zeeman interaction for holes, which is kinematically different from the standard single Zeeman term $B_-k_-^2σ_+$ considered until now.

cond-mat.mes-hall↗

arXiv2017

Prediction of ultra-narrow Higgs resonance in magnon Bose-condensates

Higgs resonance modes in condensed matter systems are generally broad; meaning large decay widths or short relaxation times. This common feature has obscured and limited their observation to a select few systems. Contrary to this, the present work predicts that Higgs resonances in magnetic field induced, three-dimensional magnon Bose-condensates have vanishingly small decay widths. Specifically for parameters relating to TlCuCl$_3$, we find an energy ($Δ_H$) to width ($Γ_H$) ratio $Δ_H/Γ_H\sim500$, making this the narrowest predicted Higgs mode in a condensed matter system, some two orders of magnitude `narrower' than the sharpest condensed matter Higgs observed so far.

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arXiv2017

Bound states of skyrmions and merons near the Lifshitz point

We study topological defects in anisotropic ferromagnets with competing interactions near the Lifshitz point. We show that skyrmions and bi-merons are stable in a large part of the phase diagram. We calculate skyrmion-skyrmion and meron-meron interactions and show that skyrmions attract each other and form ring-shaped bound states in a zero magnetic field. At the Lifshitz point merons carrying a fractional topological charge become deconfined. These results imply that unusual topological excitations may exist in weakly frustrated magnets with conventional crystal lattices.

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arXiv2017

Detection and control of spin-orbit interactions in a GaAs hole quantum point contact

We investigate the relationship between the Zeeman interaction and the inversion asymmetry induced spin orbit interactions (Rashba and Dresselhaus SOIs) in GaAs hole quantum point contacts. The presence of a strong SOI results in crossing and anti-crossing of adjacent spin-split hole subbands in a magnetic field. We demonstrate theoretically and experimentally that the anti-crossing energy gap depends on the interplay between the SOI terms and the highly anisotropic hole g tensor, and that this interplay can be tuned by selecting the crystal axis along which the current and magnetic field are aligned. Our results constitute independent detection and control of the Dresselhaus and Rashba SOIs in hole systems, which could be of importance for spintronics and quantum information applications.

cond-mat.mes-hall↗

arXiv2017

Electrical control of the sign of the g-factor in a GaAs hole quantum point contact

Zeeman splitting of 1D hole subbands is investigated in quantum point contacts (QPCs) fabricated on a (311) oriented GaAs-AlGaAs heterostructure. Transport measurements can determine the magnitude of the g-factor, but cannot usually determine the sign. Here we use a combination of tilted fields and a unique off-diagonal element in the hole g-tensor to directly detect the sign of g*. We are able to tune not only the magnitude, but also the sign of the g-factor by electrical means, which is of interest for spintronics applications. Furthermore, we show theoretically that the resulting behavior of g* can be explained by the momentum dependence of the spin-orbit interaction.

cond-mat.mes-hall↗

arXiv2017

Multiple universalities in order-disorder magnetic phase transitions

Phase transitions in isotropic quantum antiferromagnets are associated with the condensation of bosonic triplet excitations. In three dimensional quantum antiferromagnets, such as TlCuCl$_3$, condensation can be either pressure or magnetic field induced. The corresponding magnetic order obeys universal scaling with thermal critical exponent $ϕ$. Employing a relativistic quantum field theory, the present work predicts the emergence of multiple (three) universalities under combined pressure and field tuning. Changes of universality are signalled by changes of the critical exponent $ϕ$. Explicitly, we predict the existence of two new exponents $ϕ=1$ and $1/2$ as well as recovering the known exponent $ϕ=3/2$. We also predict logarithmic corrections to the power law scaling.

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arXiv2016

Non-equilibrium quantum mechanics: A 'hot quantum soup' of paramagnons

Motivated by recent measurements of the lifetime (decay width) of paramagnons in quantum antiferromagnet TlCuCl3, we investigate paramagnon decay in a heat bath and formulate an appropriate quantum theory. Our formulation can be split into two regimes: (i) a non-perturbative, 'hot quantum soup' regime where paramagnon width is comparable to its energy; (ii) usual perturbative regime where paramagnon width is significantly lower than its energy. Close to the Neel tempera- ture the paramagnon width becomes comparable to its energy and falls into the hot quantum soup regime. To describe this regime we develop a new finite frequency, finite temperature technique for a nonlinear quantum field theory; the 'golden rule of quantum kinetics'. The formulation is generic and applicable to any three dimensional quantum antiferromagnet in the vicinity of a quantum critical point. Specifically we apply our results to TlCuCl3 and find agreement with experimental data. Additionally, we show that logarithmic running of the coupling constant in the upper critical dimension changes the commonly accepted picture of the quantum disordered and quantum critical regimes.

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