arXiv subjects

Alireza Akbari

Publications and source records attributed to Alireza Akbari.

At least 55 records · Page 3Linked to original sources

arXiv2020

Gapped Dirac cones and spin texture in thin film topological insulator

The protected surface states of topological insulators (TIs) form gapless Dirac cones corresponding non-degenerate eigenstates with helical spin polarisation. The presence of a warping term deforms the isotropic cone of the most simple model into snowflake Fermi surfaces as in Bi2Se3 and Bi2Te3. Their features have been identified in STM quasiparticle interference (QPI) experiments on isolated surfaces. Here we investigate the QPI spectrum for the TI thin-film geometry with finite tunnelling between the surface states. This leads to a dramatic change of spectrum due to gapping and a change in spin texture that should leave distinct signatures in the QPI pattern. We consider both normal and magnetic exchange scattering from the surface impurities and obtain the scattering t-matrix in Born approximation as well as the general closed solution. We show the expected systematic variation of QPI snowflake intensity features by varying film thickness and study, in particular, the influence on backscattering processes. We predict the variation of the QPI spectrum for Bi2Se3 thin films using the observed gap dependence from ARPES results.

cond-mat.mtrl-sci↗

arXiv2020

Optical and magnetic excitations in the underscreened quasi-quartet Kondo lattice

The underscreened Kondo lattice consisting of a single twofold degenerate conduction band and a CEF split 4f-electron quasi-quartet has non-conventional quasiparticle dispersions obtained from the constrained mean-field theory. An additional genuinely heavy band is found in the main hybridization band gap of the upper and lower hybridzed bands whose heavy effective mass is controlled by the CEF splitting. Its presence should profoundly influence the dynamical optical and magnetic response functions. In the former, the onset of the optical conductivity is not the main hybridisation energy but the much lower Kondo energy scale which appears in the direct transitions to the additional heavy band. The dynamical magnetic response is also strongly modified by the in-gap heavy band which can lead to unconventional resonant excitations that may be interpreted as coherent CEF-Kondo lattice magnetic exciton bands. Their instability at low temperature signifies the onset of induced excitonic magnetism in the underscreened Kondo lattice.

cond-mat.str-el↗

arXiv2020

Phase-sensitive determination of nodal $\mathbf{d}$-wave order parameter in single-band and multiband superconductors

Determining the exact pairing symmetry of the superconducting order parameter in candidate unconventional superconductors remains an important challenge. Recently a new method, based on phase sensitive quasiparticle interference measurements, was developed to identify gap sign changes in isotropic multiband systems. Here we extend this approach to the single-band and multiband nodal $d$-wave superconducting cases relevant, respectively, for the cuprates and likely for the infinite-layer nickelate superconductors. Combining analytical and numerical calculations, we show that the antisymmetrized correction to the tunneling density of states due to nonmagnetic impurities in the Born and intermediate-scattering limits shows characteristic features for sign-changing and sign-preserving scattering wavevectors, as well as for the momentum-integrated quantity. Furthermore, using a realistic approach accounting for the Wannier orbitals, we model scanning tunneling microscopy data of $\text{Bi}_2\text{Sr}_2\text{CaCu}_2\text{O}_{8+δ}$, which should allow the comparison of our theory with experimental data.

cond-mat.supr-con↗

arXiv2020

Dynamics of Quantum Coherence and Quantum Fisher Information After Sudden Quench

The dynamics of relative entropy and $l_{1}$-norm of coherence, as well as, the Wigner-Yanase-skew and quantum Fisher information are studied for a time-dependent coupled XY spin chain in presence of a time-dependent transverse magnetic field. Independent of the initial state of the system and while the relative entropy of coherence, $l_{1}$-norm of coherence, and quantum Fisher information are incapable, surprisingly, the dynamic Wigner-Yanase-skew information can truly spotlight the equilibrium critical point. We also observe that when the system is quenched to the critical point, these quantities show suppressions and revivals. Moreover, the first suppression (revival) time scales linearly with the system size and its scaling ratio is unique for all quenches independent to the initial phase. This is the promised universality of the first suppression (revival) time.

quant-ph↗

arXiv2020

Strain-induced stripe phase in charge ordered single layer NbSe$_2$

Charge density waves are ubiquitous phenomena in metallic transition metal dichalcogenides. In NbSe$_2$, a triangular $3\times3$ structural modulation is coupled to a charge modulation. Recent experiments reported evidence for a triangular-stripe transition at the surface, due to strain or accidental doping and associated to a $4\times4$ modulation. We employ \textit{ab-initio} calculations to investigate the strain-induced structural instabilities in a pristine single layer and analyse the energy hierarchy of the structural and charge modulations. Our results support the observation of phase separation between triangular and stripe phases in 1H-NbSe$_2$, relating the stripe phase to compressive isotropic strain, favouring the $4\times4$ modulation. The observed wavelength of the charge modulation is also reproduced with good accuracy.

cond-mat.mtrl-sci↗

arXiv2019

Multipolar order and excitations in rare-earth boride Kondo systems

We review the cubic rare-earth boride series which displays diverse electronic states like localized 4f electron multiplets split by the crystal electric field (CEF), itinerant heavy-fermion quasiparticle bands of the Kondo lattice as well as gapped Kondo insulator or mixed-valent semiconductor states. Fairly exotic states may appear due to the 'hidden' order of multipoles carried by degenerate CEF multiplets, in addition to magnetic order present in many RB6 (R = rare earth) compounds. Most prominent are CeB6 and its La-diluted alloys which exhibit antiferro-quadrupolar and octupolar ordering enabled by the cubic G8 quartet state. The associated collective excitations are multipolar waves with a dispersion characteristic for the underlying order and accessible by inelastic neutron scattering. This localized multipolar-moment picture of R-borides is complemented by the itinerant Kondo-lattice approach. Collective spin exciton modes inside the hybridization gap around symmetry points of the Brillouin zone may appear. This has been observed in heavy-fermion metal CeB6 and in particular in the Kondo insulators YbB12 and SmB6. The latter is also the prime candidate for a strongly correlated topological insulator.

cond-mat.str-el↗

arXiv2019

Superconductivity of mixed parity and frequency in an anisotropic spin-orbit coupling

We illuminate the superconducting phases in [001]-grown-noncentrosymmetric quantum wells with an anisotropic spin-orbit coupling in the presence of on-site Hubbard interaction. Within the random phase approximation, we investigate the spin-fluctuation-mediated pairing in the presence of Rashba/Dresselhaus antisymmetric spin-orbit couplings. Although the existence of spatial inversion symmetry desires a dominant d-wave pairing for all filling levels, a broken inversion symmetry generates antisymmetric spin-orbit coupling and mixes the even- and odd-parity in the superconducting gap. We study the symmetry of the mixed-parity gap for various strengths of Hubbard interaction. Besides, we consider a superconductor-ferromagnet junction to survey the modifications of superconducting order parameters and observe an admixture of even- and odd-frequencies due to the ferromagnet exchange field.

cond-mat.supr-con↗

arXiv2019

Geometrically Frustrated Anisotropic Four-Leg Spin-$1/2$ Nanotube

We develop a real space quantum renormalization group (QRG) to explore a frustrated anisotropic four-leg spin-1/2 nanotube in the thermodynamic limit. We obtain the phase diagram, fixed points, critical points, the scaling of coupling constants and magnetization curves. Our investigation points out that in the case of strong leg coupling the diagonal frustrating interaction is marginal under QRG transformations and does not affect the universality class of the model. Remarkably, the renormalization equations express that the spin nanotube prepared in the strong leg coupling case goes to the strong plaquette coupling limit (weakly interacting plaquettes). Subsequently, in the limit of weakly interacting plaquettes, the model is mapped onto a 1D spin-1/2 XXZ chain in a longitudinal magnetic field under QRG transformation. Furthermore, the effective Hamiltonian of the spin nanotube inspires both first and second order phase transitions accompanied by the fractional magnetization plateaus. Our results show that the anisotropy changes the magnetization curve and the phase transition points, significantly. Finally, we report the numerical exact diagonalization results to compare the ground state phase diagram with our analytical visions.

cond-mat.str-el↗

arXiv2019

Disordered Kitaev chain with long-range pairing: Loschmidt echo revivals and dynamical phase transitions

We explore the dynamics of long-range Kitaev chain by varying pairing interaction exponent, $α$. It is well known that distinctive characteristics on the nonequilibrium dynamics of a closed quantum system are closely related to the equilibrium phase transitions. Specifically, the return probability of the system to its initial state (Loschmidt echo), in the finite size system, is expected to exhibit very nice periodicity after a sudden quench to a quantum critical point. Where the periodicity of the revivals scales inversely with the maximum of the group velocity. We show that, contrary to expectations, the periodicity of the return probability breaks for a sudden quench to the non-trivial quantum critical point. Further, We find that, the periodicity of return probability scales inversely with the group velocity at the gap closing point for a quench to the trivial critical point of truly long-range pairing case, $α< 1$. In addition, analyzing the effect of averaging quenched disorder shows that the revivals in the short range pairing cases are more robust against disorder than that of the long rang pairing case. We also study the effect of disorder on the non-analyticities of rate function of the return probability which introduced as a witness of the dynamical phase transition. We exhibit that, the non-analyticities in the rate function of return probability are washed out in the presence of strong disorders.

cond-mat.stat-mech↗

arXiv2018

Quasi-one Dimensional Nanostructures as Sign of Nematicity in Iron Pnictides and Chalcogenides

Impurity scattering is found to lead to quasi-one dimensional nanoscale modulation of the local density of states in the iron pnictides and chalcogenides. This `quasiparticle interference' feature is remarkably similar across a wide variety of pnictide and chalcogenide phases, suggesting a common origin. We show that a unified understanding of the experiments can be obtained by simply invoking a four-fold symmetry breaking $d_{xz}-d_{yz}$ orbital splitting, of a magnitude already suggested by the experiments. This can explain the one-dimensional characteristics in the local density of states observed in the orthorhombic nematic, tetragonal paramagnetic, as well as the spin-density wave and superconducting states in these materials.

cond-mat.supr-con↗

arXiv2018

Unveiling hidden charge density waves in single-layer NbSe$_2$ by impurities

We employ {\it{ab-initio}} calculations to investigate the charge density waves in single-layer NbSe$_2$, and we explore how they are affected by transition metal atoms. Our calculations reproduce the observed orthorhombic phase in single-layer NbSe$_2$ in the clean limit, establishing the energy order between three different distorted structures, two consisting of triangular Nb-Nb clusters and a third, energetically unfavoured, consisting of hexagonal Nb-Nb clusters. Such energy order, in agreement with known experimental work, is reversed by the adsorption of Co and Mn, which favour the formation of hexagonal Nb-Nb clusters; this CDW structure is indeed allowed from symmetry point of view but hidden in pure single layers because it is at a higher energy. The other adsorbates, K and Ga, still favour one of the triangular Nb-Nb cluster, while suppressing the other. We report how the energy difference between such distorted structure varies with these adsorbates. Furthermore, transition metals induce magnetism and favour the reduction of the symmetry of the charge density distribution.

cond-mat.mtrl-sci↗

arXiv2018

Heavy quasiparticle bands in the underscreened quasiquartet Kondo lattice

We study the quasiparticle spectrum in an underscreened Kondo-lattice (KL) model that involves a single spin degenerate conduction band and two crystalline-electric-field (CEF) split Kramers doublets coupled by both orbital-diagonal and non-diagonal exchange interactions. We find the three quasiparticle bands of the model using a constrained fermionic mean field approach. While two bands are similar to the one-orbital model a new genuinely heavy band inside the main hybridization gap appears in the quasiquartet model. Its dispersion is due to effective hybridization with conduction states but the bandwidth is controled by the size of the CEF splitting. Furthermore several new indirect and direct hybridztion gaps may be identified. By solving the selfconsistency equation we calculate the CEF- splitting and exchange dependence of effective Kondo low energy scale, hybridization gaps and band widths. We also derive the quasiparticle spectral densities and their partial orbital contributions. We suggest that the two-orbital KL model can exhibit mixed CEF/Kondo excitonic magnetism.

cond-mat.str-el↗

arXiv2018

Drumhead surface states and their signatures in quasiparticle scattering interference

We consider a two-orbital tight-binding model defined on a layered three-dimensional hexagonal lattice to investigate the properties of topological nodal lines and their associated drumhead surface states. We examine these surface states in centrosymmetric systems, where the bulk nodal lines are of Dirac type (i.e., four-fold degenerate), as well as in non-centrosymmetric systems with strong Rashba and/or Dresselhaus spin-orbit coupling, where the bulk nodal lines are of Weyl type (i.e., two-fold degenerate). We find that in non-centrosymmetric systems the nodal lines and their corresponding drumhead surface states are fully spin polarized due to spin-orbit coupling. We show that unique signatures of the topologically nontrivial drumhead surface states can be measured by means of quasiparticle scattering interference, which we compute for both Dirac and Weyl nodal line semimetals. At the end, we analyze the possible crystal structures with a symmetry that supports flat surface states which are effectively ringlike.

cond-mat.str-el↗

arXiv2018

Dynamical quantum correlations after sudden quenches

We employ the mean-field approach in the fermionic picture of the spin-1/2 XXZ chain to investigate the dynamics of bipartite quantum discord and concurrence under sudden quenching. In the case, when quenching is performed in the anisotropy from an initial value to the critical point, the quantum correlations show periodic cusps in time. Moreover, the first suppression (cusp) in quantum correlations can be explained in terms of the semi-classical picture of quasiparticle propagation. On the other hand, quenching to, as well as away from the criticality point shows that the longtime pairwise quantum discord gets enhanced from its initial state. Finally, we show that in the gapped region a quench in the transverse field displays survival of the next-nearest-neighbor quantum discord. Our results provide a further insight into the dynamical behavior of quantum correlations and their connections to quantum criticality.

quant-ph↗

arXiv2018

Edge currents as a probe of the strongly spin-polarized topological noncentrosymmetric superconductors

Recently the influence of antisymmetric spin-orbit coupling has been studied in novel topological superconductors such as half-Heuslers and artificial hetero-structures. We investigate the effect of Rashba and/or Dresselhaus spin-orbit couplings on the band structure and topological properties of a two-dimensional noncentrosymetric superconductor. For this goal, the topological helical edge modes are analyzed for different spin-orbit couplings as well as for several superconducting pairing symmetries. To explore the transport properties, we examine the response of the spin-polarized edge states to an exchange field in a superconductor-ferromagnet heterostructure. The broken chiral symmetry causes the uni-directional currents at opposite edges.

cond-mat.supr-con↗

arXiv2017

Spin-orbit coupling, minimal model and potential Cooper-pairing from repulsion in BiS$_2$-superconductors

We develop the realistic minimal electronic model for recently discovered BiS$_2$ superconductors including the spin-orbit coupling based on a first-principles band structure calculations. Due to strong spin-orbit coupling, characteristic for the Bi-based systems, the tight-binding low-energy model necessarily includes $p_x$, $p_y$, and $p_z$ orbitals. We analyze a potential Cooper-pairing instability from purely repulsive interaction for the moderate electronic correlations using the so-called leading angular harmonics approximation (LAHA). For small and intermediate doping concentrations we find the dominant instabilities to be $d_{x^2-y^2}$-wave, and $s_{\pm}$-wave symmetries, respectively. At the same time, in the absence of the sizable spin fluctuations the intra and interband Coulomb repulsion are of the same strength, which yields the strongly anisotropic behaviour of the superconducting gaps on the Fermi surface in agreement with recent ARPES findings. In addition, we find that the Fermi surface topology for BiS$_2$ layered systems at large electron doping can resembles the doped iron-based pnictide superconductors with electron and hole Fermi surfaces with sufficient nesting between them. This could provide further boost to increase $T_c$ in these systems.

cond-mat.supr-con↗

arXiv2017

Correlation of Fe-based Superconductivity and Electron-Phonon Coupling in an FeAs/Oxide Heterostructure

Interfacial phonons between iron-based superconductors (FeSCs) and perovskite substrates have received considerable attention due to the possibility of enhancing preexisting superconductivity. Using scanning tunneling spectroscopy, we studied the correlation between superconductivity and e-ph interaction with interfacial-phonons in an iron-based superconductor Sr$_2$VO$_3$FeAs ($T_c \approx$ 33 K) made of alternating FeSC and oxide layers. The quasiparticle interference measurement over regions with systematically different average superconducting gaps due to the e-ph coupling locally modulated by O vacancies in VO$_2$ layer, and supporting self-consistent momentum-dependent Eliashberg calculations provide a unique real-space evidence of the forward-scattering interfacial phonon contribution to the total superconducting pairing.

cond-mat.supr-con↗

arXiv2017

Mixed-pairing superconductivity in 5d Mott insulators with antisymmetric exchange

We investigate the potential existence of a superconducting phase in $5d$ Mott insulators with an eye to hole doped Sr$_2$IrO$_4$. Using a mean-field method, a mixed singlet-triplet superconductivity, $d + p$, is observed due to the antisymmetric exchange originating from a quasi-spin-orbit-coupling. Our calculation on ribbon geometry shows possible existence of the topologically protected edge states, because of nodal structure of the superconducting gap. These edge modes are spin polarized and emerge as zero-energy flat bands, supporting a symmetry protected Majorana states, verified by evaluation of winding number and $\mathbb{Z}_2$ topological invariant. At the end, a possible experimental approach for observation of these edge states and determination of the superconducting gap symmetry are discussed based on the quasi-particle interference (QPI) technique.

cond-mat.supr-con↗