arXiv subjects

P. Vasilopoulos

Publications and source records attributed to P. Vasilopoulos.

At least 19 recordsLinked to original sources

arXiv2025

Off-resonant light-induced topological phase transition and thermoelectric transport in semi-Dirac materials

We show that a semi-Dirac (SD) system with an inversion symmetry breaking mass exhibits a topological phase transition when irradiated with off-resonant light. Using Floquet theory, we derive the band structure, Chern numbers, phase diagram, and we show that as the light intensity is swept at fixed mass, the SD system undergoes normal-Chern-normal insulator transition. Along the phase boundaries we observe single semi-Dirac-cone (SSDC) semimetal states in which one SD cone is gapless and the other gapped. The nontrivial Berry curvature distribution $\Omega(\mathbf{k}) \neq -\Omega ( - \mathbf{k} )$ generates an orbital magnetization $M$ and anomalous Nernst ($\alpha_{xy}$) and thermal Hall ($\kappa_{xy}$) conductivities. We show that $M$ remains constant as the Fermi level $E_F$ scans the insulating gap, but it changes linearly with it in the Chern insulator (CI) phase, as expected. In the normal insulator phase, we find that $\alpha_{xy}$ exhibits a dip-peak profile which is reversed in the CI phase. We also find that switching the light's circular polarization from left to right induces a sign change in $M$, $\alpha_{xy}$, and $\kappa_{xy}$, regardless of the topological phase, thereby allowing us to reverse the direction of flow of the transverse charge and heat currents. Further, we evaluate the components of the charge ($\sigma_{aa}$), thermoelectric ($\alpha_{aa}$), and thermal ($\kappa_{aa}$) conductivity tensors ($a=x, y$) and examine the effect of light on them. With a linear dispersion along the $y$-direction, we find that $\alpha_{yy}$ and $\kappa_{yy}$ are significantly larger than $\alpha_{xx}$ and $\kappa_{xx}$, respectively, due to the much larger squared Dirac velocity $v_y^2$ compared to $v_x^2$.

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arXiv2024

Valley-dependent transport through graphene quantum dots due to proximity-induced, staggered spin-orbit couplings

We study a system composed of graphene decorated with an array of islands with C_3v symmetry that induce quantum dot (IQD) regions via proximity effects and give rise to several spin-orbit couplings (SOCs). We evaluate transport properties for an array of IQDs and analyze the conditions for realizing isolated valley conductances and valley-state localization. The resulting transmission shows a square-type behavior with wide gaps that can be tuned by adjusting the strength of the staggered intrinsic SOCs. Realistic proximity effects are characterized by weak SOC strengths, and the analysis of our results in this regime shows that the Rashba coupling is the important interaction controlling valley properties. As a consequence, a top gate voltage can be used to tune the valley polarization and switch the valley scattering for positive or negative incident energies. A proper choice of SOC strengths leads to higher localization of valley states around the linear array of IQDs. These systems can be implemented in heterostructures composed of graphene and semiconducting transition-metal dichalcogenides (TMDs) such as MoSe2, WSe2, MoS2, or WS2. In these setups, the magnitudes of induced SOCs depend on the twist angle, and due to broken valley degeneracy, valley-polarized currents at the edges can be generated in a controllable manner as well as localized valley states. Our findings suggest an alternative approach for producing valley-polarized currents and propose a corresponding mechanism for valley-dependent electron optics and optoelectronic devices.

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arXiv2023

Valley-dependent tunneling through electrostatically created quantum dots in heterostructures of graphene with hexagonal boron nitride

Kelvin probe force microscopy (KPFM) has been employed to probe charge carriers in a graphene/hexagonal boron nitride (hBN) heterostructure [Nano Lett, 21, 5013 (2021)]. We propose an approach for operating valley filtering based on the KPFM-induced potential $U_0$ instead of using external or induced pseudo-magnetic fields in strained graphene. Employing a tight-binding model, we investigate the parameters and rules leading to valley filtering in the presence of a graphene quantum dot (GQD) created by the KPFM tip. This model leads to a resolution of different transport channels in reciprocal space, where the electron transmission probability at each Dirac cone ($K_1$= -K and $K_2$ = +K) is evaluated separately. The results show that U0 and the Fermi energy $E_F$ control (or invert) the valley polarization, if electrons are allowed to flow through a given valley. The resulting valley filtering is allowed only if the signs of $E_F$ and $U_0$ are the same. If they are different, the valley filtering is destroyed and might occur only at some resonant states affected by $U_0$. Additionally, there are independent valley modes characterizing the conductance oscillations near the vicinity of the resonances, whose strength increases with $U_0$ and are similar to those occurring in resonant tunneling in quantum antidots and to the Fabry-Perot oscillations. Using KPFM, to probe the charge carriers, and graphene-based structures to control valley transport, provides an efficient way for attaining valley filtering without involving external or pseudo-magnetic fields as in previous proposals.

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arXiv2023

Influence of interface-induced valley-Zeeman and spin-orbit couplings\\ on transport in graphene-on-WSe$_{2}$ heterostructures

We investigate the electronic dispersion and transport properties of graphene/WSe$_{2}$ heterostructures in the presence of a proximity induced spin-orbit coupling (SOC) using a low-energy Hamiltonian, with different types of symmetry breaking terms, obtained from a four-band, first and second nearest-neighbour tight-binding (TB) one. The competition between different perturbation terms leads to inverted SOC bands. Further, we study the effect of symmetry breaking terms on ac and dc transport by evaluating the corresponding conductivities within linear response theory. The scattering-independent part of the valley-Hall conductivity, as a function of the Fermi energy $E_{F}$, is mostly negative in the ranges $-λ_{R}\leqslant E_{F}$ and $E_{F}\geqslantλ_{R}$ when the strength $λ_{R}$ of the Rashba SOC increases except for a very narrow region around $E_{F}=0$ in which it peaks sharply upward. The scattering-dependent diffusive conductivity increases linearly with electron density, is directly proportional to $λ_{R}$ in the low- and high-density regimes, but weakens for $λ_{R}=0$. We investigate the optical response in the presence of a SOC-tunable band gap for variable $E_{F}$. An interesting feature of this SOC tuning is that it can be used to switch on and off the Drude-type intraband response. Furthermore, the ac conductivity exhibits interband responses due to the Rashba SOC. We also show that the valley-Hall conductivity changes sign when $E_F$ is comparable to $λ_R$ and vanishes at higher values of $E_F$. It also exhibits a strong dependence on temperature and a considerable structure as a function of the frequency.

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arXiv2023

Valley-controlled transport in graphene/ WSe$_{2}$ heterostructures under an off-resonant polarized light

We investigate the electronic dispersion and transport properties of graphene/WSe$_{2}$ heterostructures in the presence of a proximity-induced spin-orbit coupling $λ_{v}$, sublattice potential $Δ$, and an off-resonant circularly polarized light of frequency $Ω$ that renormalizes $Δ$ to $\barΔ_{ηp} = Δ+ηp Δ_Ω $ with $η$ and $p$ the valley and polarization indices, respectively, and $ Δ_Ω $ the gap due to the off-resonant circularly polarized light. Using a low-energy Hamiltonian we find that the interplay between different perturbation terms leads to inverted spin-orbit coupled bands. At high $Ω$ we study the band structure and dc transport using the Floquet theory and linear response formalism, respectively. We find that the inverted band structure transfers into the direct band one when the off-resonant light is present. The valley-Hall conductivity behaves as an even function of the Fermi energy in the presence and absence of this light. At $Δ_Ω$ = $λ_{v}$ - $Δ$ a transition occurs from the valley-Hall phase to the anomalous Hall phase. In addition, the valley-Hall conductivity switches sign when the polarization of the off-resonant light changes. The valley polarization vanishes for $Δ_Ω$ = 0 but it is finite for $Δ_Ω$ $\neq$ 0 and reflects the lifting of the valley degeneracy of the energy levels, for $Δ_Ω \neq 0$, when the off-resonant light is present. The corresponding spin polarization, present for $Δ_Ω$ = 0, increases for $Δ_Ω$ $\neq$ 0. Further, pure $K$ or $K^{\prime}$ valley polarization is generated when $Δ_Ω$ changes sign. Also, the charge Hall conductivity is finite for $Δ_Ω\neq 0$ and changes sign when the handedness of the light polarization changes.

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arXiv2022

Tunable topological phases in monolayer Pt$_2$HgSe$_3$ with exchange fields

We investigate topological phases of monolayer jacutingaite (Pt$_2$HgSe$_3$) that arise when considering the competing effects of spin-orbit coupling (SOC), magnetic exchange interactions, and staggered sublattice potential V. The interplay between the staggered potential and exchange field offers the possibility of attaining different topological phases. By analyzing the Berry curvatures and computing the Chern numbers and Hall conductivities, we demonstrate that the system is time-reversal-symmetry-broken quantum spin Hall insulator when $m_b<λ_{so}$, where $m_b$ is the exchange field operating on the bottom Hg sublattice and $λ_{so}$ is the intrinsic SOC. For $m_b > λ_{so}$ and in the presence of Rashba SOC, we find that the band gap at valley $K(K^{\prime})$ is topologically trivial (non-trivial) with Chern number $C=1$ and valley Chern number $C_v=-1$, indicating that the system is valley-polarized quantum anomalous Hall insulator. We show that the topology of each valley is swapped (the Chern number becomes $C=-1$) by reversing the sign of the exchange field. The system transitions to a valley-polarized metal and quantum valley Hall phase as V increases. Along the phase boundaries, we observe a single-Dirac cone semimetal states. These findings shed more light on the possibility of realizing and controlling topological phases in spintronics and valleytronics devices.

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arXiv2020

Berry curvature, orbital magnetization, and Nernst effect in biased bilayer WSe$_2$

A valley-contrasting Berry curvature in bilayer transition metal dichalcogenides with spin-orbit coupling can generate valley magnetization when the inversion symmetry is broken, for example, by an electric field, regardless of time-reversal symmetry. A nontrivial Berry curvature can also lead to anomalous transport responses, such as the anomalous Hall effect and the anomalous Nernst effect. Applied to a bilayer WSe$_2$, an electric field can tune the Berry curvature and orbital magnetic moment, which has important consequences for the orbital magnetization and the anomalous Nernst responses. The orbital magnetization and its two contributions, one due to the magnetic moment and one due to the Berry curvature, are calculated and interpreted in terms of opposite circulating currents of the bands in the two layers. The valley anomalous Nernst coefficient and spin Nernst coefficient are also calculated. We find that a finite electric field leads to peaks and dips in the Nernst responses that have the signs of the Berry curvatures of the bands and are proportional to their magnitudes; it also enhances the valley Nernst responses. These experimentally verifiable findings may be promising for caloritronic applications.

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arXiv2019

Transport in armchair graphene nanoribbons and in ordinary waveguides

We study dc and ac transport along armchair graphene nanoribbons using the ${\bf k\cdot p}$ spectrum and eigenfunctions and general linear-response expressions for the conductivities. Then we contrast the results with those for transport along ordinary waveguides. In all cases we assess the influence of elastic scattering by impurities, describe it quantitatively with a Drude-type contribution to the current previously not reported, and evaluate the corresponding relaxation time for long- and short-range impurity potentials. We show that this contribution dominates the response at very low frequencies. In both cases the conductivities increase with the electron density and show cusps when new subbands start being occupied. As functions of the frequency the conductivities in armchair graphene nanoribbons exhibit a much richer peak structure than in ordinary waveguides: in the former intraband and interband transitions are allowed whereas in the latter only the intraband ones occur. This difference can be traced to that between the corresponding spectra and eigenfunctions.

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arXiv2018

Quantum magnetotransport in a bilayer MoS2: influence of a perpendicular electric field

We first derive the energy dispersion of bilayer MoS$_{2}$ in the presence of a perpendicular electric field $E_z$. We show that the band gap and layer splitting can be controlled by the field $E_z$. Away from the $k$ point, the intrinsic SOC splitting increases in the conduction band but is weakly affected in the valence band. We then analyze the band structure in the presence of a perpendicular magnetic field $B$ and the field $E_z$, including spin and valley Zeeman terms, and evaluate the Hall and longitudinal conductivities. We discuss the numerical results as functions of the fields $B$ and $E_z$ for finite temperatures. The field $B$ gives rise to a significant spin splitting in the conduction band, to a beating in the Shubnikov-de Haas (SdH) oscillations when it's weak, and to their splitting when it's strong. The Zeeman terms and $E_{z}$ suppress the beating and change the positions of the beating nodes of the SdH oscillations at low $B$ fields and enhance their splitting at high $B$ fields. Similar beating patterns are observed in the spin and valley polarizations at low $B$ fields. Interestingly, a $90\%$ spin polarization and a $100\%$ square-wave-shaped valley polarization are observed at high $B$ fields. The Hall-plateau sequence depends on $E_z$. These findings may be pertinent to future spintronic and valleytronic devices.

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arXiv2018

Magneto-optical properties of bilayer transition metal dichalcogenides

In transition metal dichalcogenides the spin-orbit interaction affects differently the conduction and valence band energies as functions of $k$ and the band gap is large. Consequently, when a perpendicular magnetic field $B$ is applied the conduction and valence band Landau levels are also different and this leads to a splitting of the interband optical absorption lines in both the absence and presence of an external electric field $E_{z}$. When $B$ and $E_{z}$ are present the peaks in the imaginary part of the Hall conductivity give two distinct contributions of opposite sign to the interband spectrum. The real part of the right- and left-handed interband conductivity, however, retains its two-peak structure but the peaks are shifted in energy and amplitude with respect to each other in contrast with graphene. The response of the intraband conductivity is significantly modified when the Fermi energy $E_{F}$ and the field $B$ are varied. Its optical spectral weight is found to increase with $E_{F}$ in contrast with the decrease observed in graphene. Further, the position and amplitude of the intraband response depends on the field $B$. The absorption peaks vary linearly with $B$ for all fields similar to bilayer graphene for low fields but in contrast with the high-field $\sqrt{B}$ dependence in it.

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arXiv2018

Linear response function in the presence of elastic scattering: plasmons in graphene and the two-dimensional electron gas

Within linear-response theory we derive a response function that thoroughly takes into account the influence of elastic scattering and is valid beyond the long-wavelength limit. We apply the theo-ry to plasmons in graphene and the two-dimensional electron gas (2DEG), in the random-phase ap-proximation, and for the former take into account intraband and interband excitations. The scatte-ring-modified dispersion relation shows that below a critical scattering strength $γ_c$, simply related to the plasmon frequency $ω$, no plasmons are allowed. The critical strengths $γ_c$ and the allowed $(ω, q)$ plasmon spectra for intraband and interband transitions in graphene are different. In both graphene and the 2DEG the strength $γ_c$ falls rapidly for small $ω$ but much more slowly for large $ω$.

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arXiv2016

Magneto-optical transport properties of monolayer WSe2

The recent experimental realization of a high quality WSe$% _{2} $ leads to the possibility of magneto-optical measurements and the manipulation of the spin and valley degrees of freedom. We study the influence of the very strong spin-orbit coupling and of the anisotropic lifting of the valley pseudospin degeneracy on its magnetotransport properties. The energy spectrum of WSe$_{2} $ is derived and discussed in the presence of a perpendicular magnetic field $B$. Correspondingly we evaluate the magneto-optical Hall conductivity and the optical longitudinal conductivity as functions of the frequency, magnetic field, and Fermi energy. They are strongly influenced by the field $B$ and the strong spin splitting. The former exhibits valley polarization and the latter beatings of oscillations. The magneto-optical responses can be tuned in two different regimes: the mictrowave-to-terahertz regime and the visible-frequency one. The absorption peaks involving the $n=0$ LL appear in between these two regimes and show a magnetic control of the spin and valley splittings. We also evaluate the power absorption spectrum.

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arXiv2016

Unconventional quantum Hall effect in Floquet topological insulators

We study an unconventional quantum Hall effect for the surface states of ultrathin Floquet topological insulators in a perpendicular magnetic field. The resulting band structure is modified by photon dressing and the topological property is governed by the low-energy dynamics of a single surface. An exchange of symmetric and antisymmetric surface states occurs by reversing the light's polarization. We find a novel quantum Hall state in which the zeroth Landau level undergoes a phase transition from a trivial insulator state, with Hall conductivity $σ_{yx}=0$ at zero Fermi energy, to a Hall insulator state with $σ_{yx}=e^2/2h$. These findings open new possibilities for experimentally realizing nontrivial quantum states and unusual quantum Hall plateaux at $(\pm1/2,\pm3/2,\pm5/2,...)e^2/h$.

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arXiv2015

Aharonov-Bohm oscillations in a mesoscopic ring with asymmetric arm-dependent injection

Electron transport through mesoscopic, one-dimensional rings with asymmetric injection into the arms of the ring is studied, in the presence of a Aharonov-Bohm flux, by means of an appropriate $\boldsymbol{S}$ matrix. This matrix is expressed in terms of two parameters one of which ($λ$) accounts phenomenologically for this asymmetric injection into the arms of the ring. In addition, the effect of a scatterer placed in one arm of the ring is considered. Explicit expressions are obtained for the transmission as a function of the incident electron energy, the magnetic field, the asymmetry parameter $λ$, and the strength of the scatterer. Results of the literature for symmetric rings are described by $λ=1$ and readily recovered. We relate our results to rings of finite width.

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arXiv2015

Spin- and valley-dependent transport through arrays of ferromagnetic silicene junctions

We study ballistic transport of Dirac fermions in silicene through arrays of barriers, of width $d$, in the presence of an exchange field $M$ and a tunable potential of height $U$ or depth $-U$. The spin- and valley-resolved conductances as functions of $U$ or $M$, exhibit resonances away from the Dirac point (DP) and close to it a pronounced dip that becomes a gap when a critical electric field $E_z$ is applied. This gap widens by increasing the number of barriers and can be used to realize electric field-controlled switching of the current. The spin $p_s$ and valley $p_v$ polarizations of the current near the DP increase with $E_z$ or $M$ and can reach 100\% for certain of their values. These field ranges widen significantly by increasing the number of barriers. Also, $p_s$ and $p_v$ oscillate nearly periodically with the separation between barriers or wells and can be inverted by reversing $M$.

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arXiv2015

Magneto-optical transport properties of monolayer phosphorene

The electronic properties of monolayer phosphorene are exotic due to its puckered structure and large intrinsic direct band gap. We derive and discuss its band structure in the presence of a perpendicular magnetic field. Further, we evaluate the magneto-optical Hall and longitudinal optical conductivities, as functions of temperature, magnetic field, and Fermi energy, and show that they are strongly influenced by the magnetic field. The imaginary part of the former and the real part of the latter exhibit regular {\it interband} oscillations as functions of the frequency $ω$ in the range $\hslashω\sim 1.5-2$ eV. Strong {\it intraband} responses in the latter and week ones in the former occur at much lower frequencies. The magneto-optical response can be tuned in the microwave-to-terahertz and visible frequency ranges in contrast with a conventional two-dimensional electron gas or graphene in which the response is limited to the terahertz regime. This ability to isolate carriers in an anisotropic structure may make phosphorene a promising candidate for new optical devices.

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arXiv2015

Electrically tunable magnetoplasmons in a monolayer of silicene or germanene

We theoretically study electrically tunable magnetoplasmons in a monolayer of silicene or germanene. We derive the dynamical response function and take into account the effects of strong spin-orbit coupling (SOC) and of an external electric filed $E_z$ perpendicular to the plane of the buckled silicene/germanene. Employing the random-phase approximation we analyze the magnetoplasmon spectrum. The dispersion relation has the same form as in a two-dimensional electron gas with the cyclotron and plasma frequencies modified due to the SOC and the field $E_z$. In the absence of SOC and $E_z$, our results agree well with recent experiments on graphene. The predicted effects could be tested by experiments similar to those on graphene and would be useful for future spintronics and optoelectronic devices.

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arXiv2015

Off-resonant polarized light-controlled thermoelectric transport in ultrathin topological insulators

We study thermoelectric transport in ultrathin topological insulators under the application of circularly polarized off-resonant light of frequency Ω and amplitude A. We derive analytical expressions for the band structure, orbital magnetization Morb, and the thermal (\k{appa}xy) and Nernst (αxy) conductivities. Reversing the light polarization from right to left leads to an exchange of the conduction and valence bands of the symmetric and antisymmetric surface states and to a sign change in Morb,αxy, and \k{appa}xy. Varying the sample thickness or A/Ω leads to a strong enhancement of Morb and αxy. These effects, accessible to experiments, open the possibility for selective, state-exchanged excitations under light and the conversion of heat to electric energy.

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