arXiv subjects

Imam Makhfudz

Publications and source records attributed to Imam Makhfudz.

At least 19 recordsLinked to original sources

arXiv2026

Topological Hall Response from Canted Antiferromagnetic Order in $d$-Electron Kagome Systems

In a two-dimensional kagome monolayer, a nontrivial intrinsic Berry curvature may arise in the $d$-electron system from the interaction with a non-collinear spin order induced by an underlying antiferromagnetic exchange. This opens the route for a quantum anomalous Hall effect in the multi-orbital system, even without an external magnetic field, explicit spin-orbit coupling or relativistic effects. For spin orders with an out-of-plane component, the scalar spin chirality is finite, and the integration of the Berry curvature over the Brillouin zone may yield integer Hall conductivities in units of $e^2/h$. For a Fermi level within a nontrivial gap, the canted configuration offers, at least in principle, the possibility of a maximal Chern number, $C=\pm 5$. Candidate materials are considered in this paper. In existing materials, the electron hopping is generally highly anisotropic, leading to a quantum anomalous Hall effect with smaller Chern numbers. A topological phase transition between Hall plateaus of opposite $C$ can be driven by flipping the out-of-plane component of the spin order, alluding to the potential of this system to applications in quantum information.

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arXiv2025

Controllable and Non-Dissipative Inertial Dynamics of Skyrmion in a Bosonic Platform

It has been understood in the past a decade or two that the dynamics of spin or magnetization in ultrafast regime necessarily involves inertial term that reflects the reluctance to follow abrupt or sudden change in the spin or magnetization orientation. The role of inertial spin dynamics in governing the motion of Skyrmion, a topological spin texture, is elucidated. Using nonequilibrium Green's function Keldysh formalism, an equation of motion is derived in terms of collective coordinates for a Skyrmion coupled via a ``minimal coupling'' to a bath of harmonic oscillators of frequency $ω$, modeling an optical phonon-like bosonic bath with its nearly-flat energy spectrum, and a coupling to the phonon energy density that dominates under resonance condition at the optical phonon frequency. A deterministic and non-dissipative dynamics equation of motion is obtained with an explicit mass term for the Skyrmion emerging due to the coupling, even within rigid Skyrmion picture. This results in a cyclotronic motion of Skyrmion, with a frequency that can go ultrafast, depending on that of the oscillator. Controlling the oscillator frequency can therefore guide the Skyrmion dynamics. Our theory bridges inertial dynamics and topology in magnetism and opens a pathway to ultrafast control of topological spin textures.

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arXiv2025

Quantum dot-based device for high-performance magnetic microscopy and spin filtering in the Kondo regime

We propose a nanoscale device consisting of a double quantum dot with a full exchange and pair hopping interaction. In this design, the current can only flow through the upper dot, but is sensitive to the spin state of the lower dot. The system is immersed in a highly inhomogeneous magnetic field, and only the bottom dot feels a substantial magnetic field, while the top dot experiences only a residual one. We show that our device exhibits very interesting magnetic field-dependent transport properties at low temperatures. The Kondo effect partially survives the presence of the magnetic field and allows to obtain conductances that differ by several orders of magnitude for the two spin types across the top dot. Interestingly, as a function of the magnetic field, our two-dot device changes from a spin singlet state to a spin triplet state, in which the amplitudes of the spin-dependent conductances are reversed. Our device is able to discriminate between positive and negative magnetic fields with a high sensitivity and is therefore particularly interesting for imaging the surface of anti-ferromagnetic (AF) insulating materials with alternated surface magnetic field, as well as for spin filtering applications.

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arXiv2024

Quantum Anomalous Hall Effect in $d$-Electron Kagome Systems: Chern Insulating States from Transverse Spin-Orbit Coupling

The possibility of quantum anomalous Hall effect (QAHE) in two-dimensional kagome systems with $d$-orbital electrons is studied within a multi-orbital tight-binding model. We concentrate on the case of isotropic Slater-Koster integrals which is realized in a recently discovered class of metal-organic frameworks TM$_3$C$_6$O$_6$ with transition metals (TM) in the beginning of the 3$d$ series. Furthermore, in the absence of exchange-type spin-orbit coupling, only isotropic Slater-Koster integrals give a perfect flatband in addition to the two dispersive bands hosting relativistic (Dirac) and quadratic band crossing points at high symmetry spots in the Brillouin zone. A quantized topological invariant requires a flux-creating spin-orbit coupling, giving Chern number (per spin sector) $C=1$ not only from the familiar Dirac points at the six corners of the Brillouin zone, but also from the quadratic band crossing point at the center $Γ$. In the case of isotropic Slater-Koster integrals the on-site spin-orbit coupling (SOC) is ineffective to create the QAHE and it is only the transfer or exchange-type SOC which can lead to a QAHE. Surprisingly, this QAHE comes from the nontrivial effective flux induced by the \textit{transverse} part of the spin-orbit coupling, exhibited by electrons in the $d$-orbital state with $m_l=0$ ($d_{z^2}$ orbital), in stark contrast to the more familiar form of QAHE due to the $d$-orbitals with $m_l \neq 0$, driven by the Ising part of spin-orbit coupling. The $C=1$ Chern plateau (per spin sector) due to Dirac point extends over a smaller region of Fermi energy than that due to quadratic band crossing. Our result hints at the promising potential of kagome $d$-electron systems as a platform for dissipationless electronics by virtue of its unique QAHE.

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arXiv2024

Interplay of Electron Trapping by Defect Midgap State and Quantum Confinement to Optimize Hot Carrier Effect in a Nanowire Structure

Hot carrier effect, a phenomenon where charge carriers generated by photon absorption remain energetic by not losing much energy, has been one of the leading strategies in increasing solar cell efficiency. Nanostructuring offers an effective approach to enhance hot carrier effect via the spatial confinement, as occurring in a nanowire structure. The recent experimental study by Esmaielpour et al. [ACS Applied Nano Materials 7, 2817 (2024)] reveals a fascinating non-monotonic dependence of the hot carrier effect in nanowire array on the diameter of the nanowire, contrary to what might be expected from quantum confinement alone. We show that this non-monotonic behavior can be explained by a simple model for electron energy loss that involves two principal mechanisms. First, electron-phonon scattering, that increases with nanowire diameter, leading to hot carrier effect that decreases with increasing diameter. Second, electron capture by a defect level within band gap, that is, a midgap state, that decreases with nanowire diameter, leading to hot carrier effect that increases with increasing diameter. The two mechanisms balance at a certain diameter corresponding to optimal hot carrier effect. Our result offers a guideline to optimize hot carrier effect in nanowire solar cells and ultimately their efficiency by adjusting the dimensions and micro-structural properties of nanowires.

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arXiv2024

Coexistence of symmetry-protected topological order and Neel order in the spin-1/2 ladder antiferromagnet C9H18N2CuBr4

Topological phases of matter are beyond the paradigm of Landau's symmetry breaking and have challenged our understanding of condensed matter systems. Here we report a new type of symmetry-protected topological phase of matter in the spin-1/2 coupled two-leg ladder antiferromagnet C9H18N2CuBr4, DLCB for short. In this two-sublattice antiferromagnet with a weak easy-axis anisotropy, we find no evidence of a conventional spin-flop transition in the magnetization with the magnetic field applied parallel to the easy axis at T=0.4 K, well below TN=2.0 K. Moreover, the temperature dependence of the gapped transverse excitations across TN indicates that they are not the conventional S=1 magnons associated with explicit symmetry breaking. Instead, the thermal renormalization of the gap energy shows a remarkable agreement with a calculation for the three-dimensional O(3) nonlinear sigma model. Accordingly, the spin gap in DLCB is not due to the spin anisotropy but to the separation between a spin singlet state and a triplet excited state. Since an antiferromagnetic spin-1/2 ladder systems can be mapped onto the spin-1 chain, the notion of the Haldane gap is proposed to explain the opening of the spin gap in DLCB. Therefore, the ground state of DLCB is best described as a quantum superposition of a Haldane phase and a Neel-ordered phase, which resembles the quantum state of a qubit in quantum computing. Our results indicate the presence of a symmetry-protected topological order coexisting with an antiferromagnetic order in this material.

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arXiv2023

Coherent Phonons-Driven Hot Carrier Effect in a Superlattice Solar Cell

Carrier thermalization in a superlattice solar cell made of polar semiconductors is studied theoretically by considering a minimal model where electron-phonon scattering is the principal channel of carrier energy loss. Importantly, the effect of an intrinsic quantum mechanical property; the phonon coherence, on carrier thermalization is investigated, within semiclassical picture in terms of phonon wave packet. It turns out that coherent longitudinal optical (LO) phonons weaken the effective electron-phonon coupling, thus supposedly lowering the carrier energy loss rate in solar cell. The resulting thermalization power is indeed significantly reduced by the coherent phonons, resulting in enhanced hot carrier effect, particularly for thin enough well layer where carrier confinement is also strong. A recent experiment on superlattice solar cell prototype is shown to manifest the coherent phonons-driven phenomenon. Our results demonstrate the practical implications of the fundamental quantum coherence property of phonons in semiconductors for improving superlattice solar cell performance, via hot carrier effect.

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arXiv2022

Enhancement of Hot Carrier Effects and Signatures of Confinement in Terms of Thermalization Power in Quantum Well Solar Cells

A theoretical model using electron-phonon scattering rate equations is developed for assessing carrier thermalization under steady-state conditions in two-dimensional systems. The model is applied to investigate the hot carrier effect in III-V hot-carrier solar cells with a quantum well absorber. The question underlying the proposed investigation is: what is the power required to maintain two populations of electron and hole carriers in a quasi-equilibrium state at fixed temperatures and quasi-Fermi level splitting? The obtained answer is that the thermalization power density is reduced in two-dimensional systems compared to their bulk counterpart, which demonstrates a confinement-induced enhancement of the hot carrier effect in quantum wells. This power overall increases with the well thickness, and it is moreover shown that the intra-subband contribution dominates at small thicknesses while the inter-subband contribution increases with thickness and dominates in the bulk limit. Finally, the effects of the thermodynamic state of phonons and screening are clarified. In particular, the two-dimensional thermalization power density exhibits a non-monotonic dependence on the thickness of the quantum well layer, when both out-of-equilibrium longitudinal optical phonons and screening effects are taken into account. Our theoretical and numerical results provide tracks to interpret intriguing experimental observations in quantum well physics. They will also offer guidelines to increase the yield of photovoltaic effect based on the hot carrier effect using quantum well heterostructures, a result critical to the research toward high-efficiency solar cell devices.

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arXiv2020

Nutation Wave as a Platform for Ultrafast Spin Dynamics in Ferromagnets

At short time scales the inertia term becomes relevant for the magnetization dynamics of ferromagnets and leads to nutation for the magnetization vector. For the case of spatially extended magnetic systems, for instance Heisenberg spin chains with isotropic spin-exchange interaction, this leads to the appearance of a novel collective excitation, the "nutation wave", whose properties are elucidated by analytic arguments and numerical studies. The one--particle excitations can be identified as relativistic massive particles. These particles, the "nutatons", acquire their mass via the Brout-Englert-Higgs mechanism, through the interaction of the wave with an emergent topological gauge field. This spin excitation would appear as a peak in the spectrum of the scattering structure factor in inelastic neutron scattering experiments. The high frequency and speed of the nutation wave can open new paths for realizing ultrafast spin dynamics.

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arXiv2018

Effective Field Theory of Chiral Spin Liquid between Ordered Phases in Kagomé Antiferromagnet

We propose in this work an effective field theory description of the chiral spin liquid state in Heisenberg spin system on kagomé lattice.To this end, we derive the low-energy effective theory of kagomé (isotropic) Heisenberg antiferromagnet around its ordered ground states found numerically and show that quantum fluctuations induced by further neighbor spin exchanges are equally strong as those from first neighbor.We use a chiral order parameter theory to argue for the occurrence of finite temperature chiral symmetry breaking transition into chiral ordered state in kagomé antiferromagnet with further neighbor spin exchange interactions.We compute the chiral symmetry breaking term in the effective ground state energy and show that chiral spin liquid necessarily occurs in the ground state of kagomé antiferromagnet with the first three nearest-neighbor spin exchange interactions.Finally, we consider the quantum criticality of the kagomé antiferromagnet and show that a Chern-Simons term emerges naturally at the transition between two ordered states that satisfy appropriate `matching condition' that we derive, providing explanation for why chiral spin liquid could occur at the transition between appropriate ordered states.This emergent Chern-Simons term is the low energy effective theory of the chiral spin liquid state, where the chirality is the immediate consequence of the breaking of discrete symmetries by this topological field theory.

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arXiv2018

On Anderson Localization and Chiral Anomaly in Disordered Time-Reversal Invariant Weyl Semimetals: Nonperturbative and Berry Phase Effects

Weyl semimetal, a three-dimensional electronic system with relativistic linear energy dispersion around gapless points carrying nontrivial Berry charge, is predicted to exhibit a wealth of unique response and transport properties.A crucial question is whether those properties are robust against disorder and whether Anderson localization occurs.In this work, the effects of nonperturbative topological (vortex loop) excitations and Berry phase in disordered time-reversal invariant 3d Weyl semimetal are studied.It is shown that the chiral symmetry is restored in the nonlinear sigma model describing the diffusons upon disorder average as any net topological term and its delocalization result do not take effect at sufficiently short length scales.Anderson localization occurs at sufficiently strong disorder and we predict that chirality and related phenomena disappear at such transition.Nevertheless, we uncover a mechanism that originates from Berry phase that impedes such localization effect.We show the occurrence of destructive interference between the vortex loops and between scattering paths due to the the vortex loops' Berry phase which resists the Anderson localization.We emphasize the applicability of our theory to the candidate Weyl materials where we point out the consistency of our theory with a recent experimental finding of the absent chiral anomaly in a noncentrosymmetric Weyl semimetal.

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arXiv2018

Quantum Spin Superfluid from Bose-Einstein Condensation of Spinons in Pyrochlore Spin Ice

Quantum spin ice in pyrochlore lattice exemplifies three dimensional frustrated spin systems.In existing studies, Bose-Einstein condensation of bosonic spinons gives rise to magnetically ordered ground state.A truly liquid quantum spin superfluid state that manifests a deconfined Higgs condensate of spinons is demonstrated.This state is shown to occur in the fully antiferromagnetic case of the non-Kramers quantum spin ice with a fluctuations-induced first-order quantum phase transition from the $U(1)$ spin liquid to the spin superfluid.The spin superfluid density jump is obtained analytically.

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arXiv2016

Signatures of Topological Phase Transition in 3d Topological Insulators from Dynamical Axion Response

Axion electrodynamics, first proposed in the context of particle physics, manifests itself in condensed matter physics in the topological field theory description of 3d topological insulators and gives rise to magnetoelectric effect, where applying magnetic (electric) field $\mathbf{B}(\mathbf{E})$ induces polarization (magnetization) $\mathbf{p}(\mathbf{m})$. We use linear response theory to study the associated topological current using the Fu-Kane-Mele model of 3d topological insulators in the presence of time-dependent uniform weak magnetic field. By computing the dynamical current susceptibility $χ^{\mathbf{j}_p\mathbf{j}_p}_{ij}(ω)$, we discover from its static limit an `order parameter' of the topological phase transition between weak topological (or ordinary) insulator and strong topological insulator, found to be continuous. The $χ^{\mathbf{j}_p\mathbf{j}_p}_{ij}(ω)$ shows a sign-changing singularity at a critical frequency with suppressed strength in the topological insulating state. Our results can be verified in current noise experiment on 3d TI candidate materials for the detection of such topological phase transition.

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arXiv2016

$d$-Density Wave (DDW) Scenario Description of the New Hidden Charge Order in Cuprates

In this paper, we show that the theory of high $T_c$ superconductivity based on a microscopic model with $d$-density wave (DDW) scenario in the pseudogap phase is able to reproduce some of the most important features of the recent experimentally discovered hidden charge order in several families of Cuprates. In particular, by computing and comparing energies of charge orders of different modulation directions derived from a full microscopic theory with $d$-density wave scenario, the axial charge order $ϕ_{X(Y)}$ with wavevector $\mathbf{Q}=(Q_0,0)((0,Q_0))$ is shown to be unambiguously energetically more favorable over the diagonal charge order $ϕ_{X\pm Y}$ with wavevector $\mathbf{Q}=(Q_0,\pm Q_0)$ at least in commensurate limit, to be expected also to hold even to more general incommensurate case, in agreement with experiment. The two types of axial charge order $ϕ_X$ and $ϕ_Y$ are degenerate by symmetry. We find that within the superconducting background, biaxial (checkerboard) charge order is energetically more favorable than uniaxial (stripe) charge order, and therefore checkerboard axial charge order should be the one observed in experiments, assuming a single domain of charge ordered state on each CuO$_2$ plane.

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arXiv2016

Majorana Zero Modes in Superconducting Proximity-coupled Magnetic Domain Wall

We propose a simple model consisting of a magnetic domain wall proximity-coupled to an $s$-wave superconductor for realization of Majorana zero-energy modes. A spin-dependent gauge transformation translates the rotating magnetic profile through the domain wall to effective spin-orbit and Zeeman terms. The Hamiltonian breaks time reversal and chiral symmetries, while preserving particle-hole symmetry, placing itself into topological D class characterized by the $\mathbb{Z}_{2}$ topological invariant for quasi one-dimensional system. The low-energy sector of the model maps to the one isomorphic with Kitaev Hamiltonian. The existence and localization of Majorana zero modes in the nontrivial phase are demonstrated explicitly and we obtain the topological phase diagram with extended regime of nontrivial phase and surprising occurrence of a re-entrance phase transition. Our calculation shows that the system can be easily tuned between trivial and topological ground states and can be implemented experimentally to realize non-Abelian statistics.

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arXiv2015

Protection against Spin Gap in 2-d Insulating Antiferromagnets with a Chern-Simons Term

We propose a novel mechanism for the protection against spin gapped states in doped antiferromagnets. It requires the presence of a Chern-Simons term that can be generated by a coupling between spin and an insulator. We first demonstrate that in the presence of this term the vortex loop excitations of the spin sector behave as anyons with fractional statistics. To generate such term, the fermions should have massive Dirac spectrum coupled to the emergent spin field of the spin sector. The Dirac spectrum can be realized by a planar spin configuration arising as the lowest-energy configuration of a square lattice antiferromagnet Hamiltonian involving a Dzyaloshinskii-Moriya interaction. The mass is provided by a combination of dimerization and staggered chemical potential. We finally show that for realistic parameters, anyonic vortex loop condensation will likely never occur and thus the spin gapped state is prevented. We also propose real magnetic materials for an experimental verification of our theory.

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arXiv2015

Theory of Quantum Phase Transition in Iron-based Superconductors with Half-Dirac Nodal Electron Fermi Surface

The quantum phase transition in iron-based superconductors with 'half-Dirac' node at the electron Fermi surface as a $T=0$ structural phase transition described in terms of nematic order is discussed. An effective low energy theory that describes half-Dirac nodal Fermions and their coupling to Ising nematic order that describes the phase transition is derived and analyzed using renormalization group (RG) study of the large-$N_f$ version of the theory. The inherent absence of Lorentz invariance of the theory leads to RG flow structure where the velocities $v_F$ and $v_Δ$ at the paired half-Dirac nodes ($1\overline{1}$ and $2\overline{2}$) in general flow differently under RG, implying that the nodal electron gap is deformed and the $C_4$ symmetry is broken, explaining the structural (orthogonal to orthorhombic) phase transition at the quantum critical point (QCP). The theory is found to have Gaussian fixed point $λ^*=0, (v_Δ/v_F)^*=0$ with stable flow lines toward it, suggesting a second order nematic phase transition. Interpreting the fermion-Ising nematic boson interaction as a decay process of nematic Ising order parameter scalar field fluctuations into half-Dirac nodal fermions, I find that the theory surprisingly behaves as systems with dynamical critical exponent $z = 1$, reflecting undamped quantum critical dynamics and emergent fully relativistic field theory arising from the non(fully)-relativistic field theory and is direct consequence of $(v_Δ/v_F)^*=0$ fixed point. The nematic critical fluctuations lead to remarkable change to the spectral function peak where at a critical point $λ_c$, directly related to nematic QCP, the central spectral peak collapses and splits into satellite spectral peaks around nodal point. The vanishing of the zero modes density of states leads to the undamped $z=1$ quantum critical dynamics.

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arXiv2015

Hole Properties On and Off Magnetization Plateaus in 2-d Antiferromagnets

The phenomenon of magnetization plateaus in antiferromagnets under magnetic field has always been an important topic in magnetism. We propose to probe the elusive physics of plateaus in 2-d by considering hole-doped antiferromagnet and studying the signatures of magnetization plateaus in terms of the properties of holes, coupled to an effective gauge field generated by the spin sector. The latter mediates interaction between the holes, found to be algebraically decaying long-ranged with both Coulombic and dipolar forms outside plateau and short-ranged (local) inside plateau. The resulting hole spectral weight is significantly broadened off-plateau, while it remains sharply-peaked on-plateau. We also extend the result obtained for 1-d system where finite hole doping gives rise to a shift in the magnetization value of the plateaus.

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