arXiv subjects

Seyed Akbar Jafari

Publications and source records attributed to Seyed Akbar Jafari.

8 recordsLinked to original sources

arXiv2026

Non-perturbative theory of valley splitting in Si qubits from variational wave function: periodic effects of shear strain and asymptotic freedom in the wiggle-well potential

Valley splitting sets the energy scale at which spin and valley degrees of freedom hybridize in silicon quantum-well qubits, but its sensitivity to interface structure makes it difficult to predict. Using the valleyor basis, we formulate a two-band effective-mass model for intervalley coupling in a finite quantum well and develop an analytical framework for treating non-perturbative effects of shear strain and wiggle-well potentials. For a $z$-independent coupling $V_sτ_2$, corresponding to uniform shear strain, we construct a variational state from the exact plane-wave valleyors and a hard-wall envelope. This yields closed-form expressions for the two lowest states and their splitting, including the characteristic oscillatory $|\sin(k_{\rm min}L)|$ dependence. The variational result agrees with exact diagonalization to within $\sim1\%$ for realistic couplings, and provides a simple prescription for tuning the shear strain away from the nodes to enhance the valley splitting. We further show that finite barrier heights primarily renormalize the effective width of the quantum well while preserving the oscillatory dependence. For a wiggle-well coupling $V_w\cos(k_wz)τ_1$, the valleyor representation reveals a crossover near the $k_w=2k_1$ resonance from wiggle-dominated splitting to orbital quantization. Remarkably, at high wiggle-well amplitudes, the empty-box energy scale predominantly dictates the near-resonant valley splitting, while the wiggle-well potential enters only as a sub-leading correction. This behavior can be viewed as asymptotic freedom for spin qubits. Away from resonance, the splitting exhibits a broadly peaked resonance with diffraction-like side-lobes as a function of detuning.

cond-mat.mes-hall↗

arXiv2026

Valley-Enabled Intrinsic Dresselhaus Spin-Orbit Coupling in Silicon

We develop a symmetry-based theory of spin-orbit-valley coupling in silicon that reveals an intrinsic source of Dresselhaus spin-orbit coupling independent of interfaces or external electric fields. Treating the valley degree of freedom as a symmetry-carrying quantum degree of freedom, we show that the Dresselhaus interaction is necessarily valley off-diagonal and that a bulk contribution proportional to the valley Pauli matrix $τ_1$ is symmetry allowed. Tight-binding calculations yield a bulk coupling more than an order of magnitude larger than typical interface-induced spin-orbit coupling; achieving the same energy scale through the interface-induced mechanism would require electric fields roughly 50 times larger than typical fields. We further derive the symmetry-allowed spin-valley couplings generated by magnetic-field gradients and show how they account for the valley-dependent Zeeman splitting observed in micromagnet experiments. A slight tilt of the background magnetic field out of the plane produces an additional isotropic contribution linear in $B_z$, providing an experimentally accessible signature of the corresponding coupling constant. Finally, we predict a spin-independent micromagnet-induced valley splitting in the $τ_3$ channel, which is distinct from the $τ_{1,2}$ channels generated by alloy disorder and therefore remains robust against disorder-induced cancellation. These results establish valley symmetry as a fundamental ingredient in the spin-orbit physics of silicon and provide new mechanisms for controlling and probing spin and valley degrees of freedom in silicon quantum devices.

cond-mat.mtrl-sci↗

arXiv2025

Smart Holes: Analogue black holes with the right temperature and entropy

In analogue gravity studies, the goal is to replicate black hole phenomena, such as Hawking radiation, within controlled laboratory settings. In the realm of condensed matter systems, this may happen in 2D tilted Dirac cone materials based on honeycomb lattice. In particular, we compute the entropy of this system, and find it has the same form as black hole Bekenstein-Hawking entropy, if an analogue horizon forms. Hence, these systems can be potential analogues of quantum black holes. We show that this entropy is primarily concentrated in the region where the tilt parameter is close to one, which corresponds to the location of the analogue black hole horizon. Additionally, when nonlinear effects are taken into account, the entropy is peaked in a small pocket of the Fermi sea that forms behind the analogue event horizon, which we call the \textit{Fermi puddle}. We further refer to this new type of analogue black hole as a {\it smart hole}, since, in contrast to dumb holes, it can simulate both the correct temperature {\it and} entropy of general relativistic black holes. These results provide an opportunity to illuminate various quantum facets of black hole physics in a laboratory setting.

hep-th↗

arXiv2025

Two-dimensional M2X2 family with emerging semiconducting, semimetallic, and magnetic properties

The exploration for novel two-dimensional (2D) materials with diverse electronic characteristics has attracted growing interest in recent years. Using density functional theory (DFT) calculations, we have predicted a new family of 2D transition-metal (TM) based compounds under the nomenclature M_2X_2 (where M represents TMs, and X denotes chalcogen elements like S, Se, and Te). Our investigation delves into the examination of the formation energies, dynamical/thermal stabilities, mechanical properties, electronic structures, and magnetic properties of various systems within this family. Through our computational analyses, we have discovered a total of 35 thermodynamically and dynamically stable M_2X_2 monolayer materials that exhibit remarkable diversity in terms of their electronic and magnetic properties. Our findings will pave the way for the experimental realization of various M_2X_2 structures in the near future. In particular, among the predicted compounds, M_2X_2(M=Zn, Cd; X=S, Se, Te) are a direct band-gap semiconductor with band gaps between 0.9 to 2.6 eV (1.3 to 3.7 eV) by DFT+PBE (hybrid functional HSE) calculations. M_2X_2(M=Ti, Zr, Hf, Tc, Re) are zero-gap semiconductor (semimetals) in standard DFT+PBE calculation. Inclusion of spin-orbit coupling leads to a gap opening of 0.1 eV. Notably, our analysis has also unveiled the magnetic nature of certain materials, such as Mn_2X_2(X=S, Se), Fe_2X_2(X=Se, Te), and Ti_2Te_2. The prediction of semiconducting (magnetic) M_2X_2 materials not only offers valuable insights into the underlying electronic properties (magnetism) of 2D systems but also positions these materials as promising candidates for the development of advanced electronic (spintronic) devices.

cond-mat.mtrl-sci↗

arXiv2025

Solid-State Gravitational Redshift: Transport Signatures of Massless Dirac Fermions in Tilted Dirac Cone Heterostructures

We investigate the propagation of electron waves in a two-dimensional tilted Dirac cone heterostructure where tilt depends on the coordinate $z$ along the junction. The resulting Dirac equation in an emergent curved spacetime for the spinor $ψ(z)$ can be efficiently solved using 4th-order Runge-Kutta numerical method by a transformation to a "suitable" spinor $φ$ where the resulting Dirac cone looks locally upright. The spatial texture of the tilt induces oscillatory behaviors in key physical quantities such as the norm $|φ(z)|^2$ of the wave function, polar and azimuthal angles $Θ(z)$ and $Φ(z)$ of the pseudospin, and the integrated transmission $τ$ where oscillation wave-lengths get shorter (longer) in stronger (weaker) tilt regions. Such an oscillatory behavior reminiscent of gravitational red-shift is an indicator of an underlying spacetime metric that can be probed in tunneling experiments. We derive analytical approximations for the position-dependent wave numbers $Δk_z(z)$ that explain the red-shift patterns and corroborate them with numerical simulations. For a tilt "bump" spread over length scale $\ell$, upon increasing $\ell$, the amplitude of red-shifted oscillations reduces whereas the number of peaks increases. The scale invariance of Dirac equation allows to probe these aspects of $\ell$-dependence by a voltage sweep in transmission experiments. Smooth variations of the tilt reduce impedance mismatch of the electron waves, thereby giving rise to very high transmission rate. This concept can be used in combination with a sigmoid-shaped tilt texture for red-shift or blue-shift engineering of the transmitted waves, depending on whether the sigmoid is downswing or upswing.

cond-mat.mes-hall↗

arXiv2025

Realization of tilted Dirac-like microwave cone in superconducting circuit lattices

Dirac-like band crossings are paradigms in condensed matter systems to emulate high-energy physics phenomena. They are associated with two aspects: gap and tilting. The ability to design sign-changing gap gives rise to band topology, whereas the tilting of band crossings which is a gateway for large gravity-like effects remains uncharted. In this work, we introduce an experimental platform to realize tilted Dirac-like microwave cone in large-scale superconducting circuit lattices. The direction and magnitude of the tilt can be controlled by engineering the axially preferred second neighbor coupling. We demonstrate three lattices with 731-site LC resonator featuring tilt values of up to 59% of relative difference in the opposite-direction group velocities. This is obtained by reconstructing the density of states (DOS) of measured microwave resonance frequencies. Harnessing the tilt of Dirac-like band crossings lays the foundation for weaving the fabric of an emergent solid-state spacetime.

cond-mat.mes-hall↗

arXiv2024

Semiclassical transport in two-dimensional Dirac materials with spatially variable tilt

We use Boltzmann theory to study the semi-classical dynamics of electrons in a two-dimensional (2D) tilted Dirac material in which the tilt varies in space. The spatial variation of the tilt parameter induces a non-trivial spacetime geometry on the background of which the electrons roam about. As the first manifestation of graivto-electric phenomena, we find a geometric planar Hall effect according to which a current flows in a direction transverse to the chemical potential gradient and is proportional to $g^{xy}$ component of the emergent spacetime structure. The longitudinal conductivity contains information about the gravitational red-shift factors. Furthermore, in the absence of externally applied electric field there can be "free-fall" or zero-bias currents that can be used as detectors of terahertz radiation.

cond-mat.mes-hall↗

arXiv2024

Material and size dependent corrections to conductance quantization in anomalous Hall effect from anomaly inflow

In quantum anomalous Hall (QAH) systems, the Hall conductance is quantized and the corresponding effective topological theory of the system is the Chern-Simons theory. The conductance quantum is given by the universal constant $e^2/h$ -- the inverse von Klitzing constant -- that is independent of the bulk gap, as well as the size of the system. This picture relies on the assumption that the edge modes are sharply localized at the edge, i.e. they have zero width. We show that considering the physical case where the edge modes have finite localization length $b$, the effective action would not be topological in bulk direction anymore. Due to non-zero $b$ the conductance quantum will be corrected as $(1-\varepsilon)e^2/h$ where $\varepsilon$ encompasses the non-universal (i.e. material/sample dependent) part that is determined by the dimensionless ratios $\frac{gb}{\hbar v_F}$ and $\frac{b}{L}$ where $g,v_F,L$ are the bulk gap, Fermi velocity and sample length. To compute the non-universal correction $\varepsilon$ we use anomaly inflow framework according to which the bulk action produces the correct amount of anomaly inflow that would cancel the anomaly of the chiral edge modes. These corrections place limits on the precision of measurable quantization in units of the inverse von Klitzing constant for QAH systems with smaller sizes and/or smaller bulk gaps. Our result suggests that the failure of precision measurements to reproduce the exact conductance quantum $e^2/h$ is not an annoying sample quality issue, but it contains the quantitative physics of anomaly inflow that can be inferred by the systematic study of such corrections.

cond-mat.mes-hall↗