arXiv subjects

Dao-Xin Yao

Publications and source records attributed to Dao-Xin Yao.

At least 181 records · Page 10Linked to original sources

arXiv2016

Band Gap Engineering of Two-Dimensional Nitrogene

In our previous study, we have predicted the novel two-dimensional honeycomb monolayers of pnictogen. In particular, the structure and properties of the honeycomb monolayer of nitrogen, which we call nitrogene, are very unusual. In this paper, we make an in-depth investigation of its electronic structure. We find that the band structure of nitrogene can be engineered in several ways: controlling the stacking of monolayers, application of biaxial tensile strain, and application of perpendicular electric field. The band gap of nitrogene is found to decrease with the increasing number of layers. The perpendicular electric field can also reduce the band gap when it is larger than 0.18V/ Å, and the gap closes at 0.35V/ Å. A nearly linear dependence of the gap on the electric field is found during the process. Application of biaxial strain can decrease the band gap as well, and eventually closes the gap. After the gap-closing, we find six inequivalent Dirac points in the Brillouin zone under the strain between 17% and 28%, and the nitrogene monolayer becomes a Dirac semimetal. These findings suggest that the electronic structure of nitrogene can be modified by several techniques, which makes it a promising candidate for electronic devices.

cond-mat.mtrl-sci↗

arXiv2016

Strong ferromagnetic exchange interaction in the parent state of the superconductivity in BaFe$_2$S$_3$

Inelastic neutron scattering measurements have been performed to investigate the spin waves of the quasi-one-dimensional antiferromagnetic ladder compound BaFe$_2$S$_3$, where a superconducting transition was observed under pressure [H. Takahashi {\it et al.}, Nat. Mater. 14, 1008-1012 (2015); T. Yamauchi {\it et al.}, Phys. Rev. Lett. 115, 246402 (2015)]. By fitting the spherically averaged experimental data collected on a powder sample to a Heisenberg Hamiltonian, we find that the one-dimensional antiferromagnetic ladder exhibits a strong nearest neighbor ferromagnetic exchange interaction ($SJ_R=-71\pm4$ meV) along the rung direction, an antiferromagnetic $SJ_L=49\pm3$ meV along the leg direction and a ferromagnetic $SJ_2=-15\pm2$ meV along the diagonal direction. Our data demonstrate that the antiferromagnetic spin excitations are a common characteristic for the iron-based superconductors, while specific relative values for the exchange interactions do not appear to be unique for the parent states of the superconducting materials.

cond-mat.supr-con↗

arXiv2016

Spin waves and magnetic exchange interactions in the spin ladder compound RbFe$_2$Se$_3$

We report an inelastic neutron scattering study of the spin waves of the one-dimensional antiferromagnetic spin ladder compound RbFe$_2$Se$_3$. The results reveal that the products, $SJ$'s, of the spin $S$ and the magnetic exchange interactions $J$'s along the antiferromagnetic (leg) direction and the ferromagnetic (rung) direction are comparable with those for the stripe ordered phase of the parent compounds of the iron-based superconductors. The universality of the $SJ$'s implies nearly universal spin wave dynamics and the irrelevance of the fermiology for the existence of the stripe antiferromagnetic order among various Fe-based materials.

cond-mat.str-el↗

arXiv2016

Spin and quadrupolar orders in the spin-1 bilinear-biquadratic model for iron-based superconductors

Motivated by recent experimental and theoretical progresses of the magnetic properties of iron-based superconductors, we provide a comprehensive analysis of the spin-1 bilinear-biquadratic (BBQ) model on the square lattice. Using variational approach in the mean-field level, we identify the existence of various magnetic phases, including conventional spin dipolar orderings (ferro- or antiferromagnet), novel quadrupolar (spin nematic) orderings and mixed dipolar-quadrupolar orderings. In contrast to the usual Heisenberg model, the elementary excitations of the spin-1 BBQ model are described by the flavor-wave theory within the SU(3) representation. By fitting the experimental spin-wave dispersion, we determine the refined exchange couplings corresponding to the collinear antiferromagnetic iron pnictides. We also present the dynamic structure factors of both spin dipolar and quadrupolar components with connections to the future experiments.

cond-mat.str-el↗

arXiv2016

Chiral p-wave superconductivity in Sb(111) thin films close to Van Hove singularities

We theoretically investigate the development of unconventional superconductivity in the Sb(111) thin film when its Fermi level is tuned to near type-II Van Hove singularities (VHS), which locate at non-time-reversal invariant momenta. Via patch renormalization group analysis, we show that the leading instability is a chiral p + ip-wave superconducting order. The origin of such pairing relies on the hexagonal structure of the VHS and strong spin-orbit coupling, resulting in the anisotropy of the electron-electron scattering to provide an attractive channel. Our study hence suggests that superconducting Sb thin films originated from VHS physics may host Majorana zero modes in the magnetic vortices and provides another application perspective to such material.

cond-mat.str-el↗

arXiv2015

Two-dimensional octagon-structure monolayer of nitrogen group elements and the related nano-structures

In the purpose of expanding the family of two-dimensional materials, we predict the existence of two-dimensional octa-structure of nitrogen group elements that are composed of squares and octagons in first-principle method based on density functional theory (DFT). From our calculations, electronic structures of all monolayers show that they are semiconductors with indirect (N, P, Bi) and direct (As, Sb) band gaps (0.57-2.61eV). Nano-ribbons of three different unpassivated edges and their band structures are also investigated. Because of the reconstruction on the edges and dangling bonds, there exist ferromagnetic edge states in P, As, Sb nano-ribbons with different edges, and a Dirac point near π is found in the band structure of one specific N nano-ribbon. These structures may be useful in future applications, such as semiconductor devices, spintronics, hydrogen storage and quantum computation.

cond-mat.mtrl-sci↗

arXiv2015

Mott glass phase in a diluted bilayer Heisenberg quantum antiferromagnet

We use quantum Monte Carlo simulations to study a dimer-diluted $S=1/2$ Heisenberg model on a bilayer square lattice with intralayer interaction $J_{1}$ and interlayer interaction $J_{2}$. Below the classical percolation threshold $p_c$, the system has three phases reachable by tuning the interaction ratio $g=J_{2}/J_{1}$: a N$\acute{e}$el ordered phase, a gapless quantum glass phase, and a gapped quantum paramagnetic phase. We present the ground-state phase diagram in the plane of dilution $p$ and interaction ratio $g$. The quantum glass phase is certified to be of the gapless Mott glass type, having a uniform susceptibility vanishing at zero temperature $T$ and following a stretched exponential form at $T>0$; $χ_u \sim \exp(-b/T^α)$ with $α< 1$. At the phase transition point from N$\acute{e}$el ordered to Mott glass, we find that the critical exponents are different from those of the clean system described by the standard $O(3)$ universality class in 2+1 dimensions.

cond-mat.str-el↗

arXiv2015

Two-Dimensional Honeycomb Monolayer of Nitrogen Group Elements and the Related Nano-Structure: A First-Principle Study

Because of its novel physical properties, two-dimensional materials have attracted great attention. From first-principle calculations and vibration frequenceis analysis, we predict a new family of two-dimensional materials based on the idea of octet stability: honeycomb lattices of pnictogens (N, P, As, Sb, Bi). The buckled structures of materials come from the sp3 hybridization. These materials have indirect band gap ranging from 0.43eV to 3.7eV. From the analysis of projected density of states, we argue that the s and p orbitals together are sufficient to describe the electronic structure under tight-binding model, and the tight-binding parameters are obtained by fitting the band structures to first-principle results. Surprisingly large on-site spin-orbit coupling is found for all the pnictogen lattices except nitrogen. Investigation on the electronic structures of both zigzag and armchair nanoribbons reveals the possible existence of spin-polarized ferromagnetic edge states in some cases, which are rare in one-dimensional systems. These edge states and magnetism may exist under the condition of high vaccum and low temperature. This new family of materials would have promising applications in electronics, optics, sensors, and solar cells.

cond-mat.mtrl-sci↗

arXiv2015

Signatures of indirect K-edge resonant inelastic x-ray scattering on magnetic excitations in triangular lattice antiferromagnet

We compute the K-edge indirect resonant inelastic x-ray scattering (RIXS) spectrum of a triangular lattice antiferromagnet in its ordered coplanar 3- sublattice 120 degree magnetic state. By considering the first order self$-$energy corrections to the spin wave spectrum, magnon decay rate, bimagnon interactions within the ladder approximation Bethe-Salpeter scheme, and the effect of three-magnon contributions up to 1/S- order we find that the RIXS spectra is non-trivially affected. For a purely isotropic triangular lattice model, the peak splitting mechanism and the appearance of a multipeak RIXS structure is primarily dictated by the damping of magnon modes. At a scattering wavevector corresponding to the zone center Γpoint and at the roton point q=M, where the magnon decay rate is zero, a stable single peak forms. At the $Γ$ point, the contribution is purely trimagnon at the 1/S level and occurs approximately at the trimagnon energy of 6JS. The roton peak occurs at a lower energy of 4JS. The K-edge single peak RIXS spectra at the roton momentum can be utilized as an experimental signature to detect the presence of roton excitations. A unique feature of the triangular lattice K-edge RIXS spectra is the nonvanishing RIXS intensity at both the zone center $Γ$ point and the antiferromagnetic wavevector K point. This result is in sharp contrast to the vanishing K-edge RIXS intensity of the collinear magnetic phases on the square lattice. We find that including XXZ anisotropy leads to additional peak splitting, including at the roton scattering wavevector where the single peak destabilizes towards a two-peak structure. The observed splitting is consistent with our earlier theoretical prediction of the effects of spatial anisotropy on the RIXS spectra of a frustrated quantum magnet [Luo, Datta, and Yao, Phys. Rev. B 89, 165103 (2014)].

cond-mat.mtrl-sci↗

arXiv2015

Manipulating the Majorana Qubit with the Landau-Zener-Stückelberg Interference

Constructing a universal operation scheme for Majorana qubits remains a central issue for the topological quantum computation. We study the Landau-Zener-Stückelberg interference in a Majorana qubit and show that this interference can be used to achieve controllable operations. The Majorana qubit consists of an rf SQUID with a topological nanowire Josephson junction which hosts Majorana bound states. In the SQUID, a magnetic flux pulse can drive the quantum evolution of the Majorana qubit. The qubit experiences two Landau-Zener transitions when the amplitude of the pulse is tuned around the superconducting flux quanta $2e/\hbar$. The Landau-Zener-Stückelberg interference between the two transitions rotates the Majorana qubit, with the angle controlled by the time scale of the pulse. This rotation operation implements a high-speed single-qubit gate on the Majorana qubit, which is a necessary ingredient for the topological quantum computation.

cond-mat.supr-con↗

arXiv2015

Itinerancy enhanced quantum fluctuation of magnetic moments in iron-based superconductors

We investigate the influence of itinerant carriers on dynamics and fluctuation of local moments in Fe-based superconductors, via linear spin-wave analysis of a spin-fermion model containing both itinerant and local degrees of freedom. Surprisingly against the common lore, instead of enhancing the ($π$,0) order, itinerant carriers with well nested Fermi surfaces is found to induce significant amount of \textit{spatial} and temporal quantum fluctuation that leads to the observed small ordered moment. Interestingly, the underlying mechanism is shown to be intra-pocket nesting-associated long-range coupling, rather than the previously believed ferromagnetic double-exchange effect. This challenges the validity of ferromagnetically compensated first-neighbor coupling reported from short-range fitting to the experimental dispersion, which turns out to result instead from the ferro-orbital order that is also found instrumental in stabilizing the magnetic order.

cond-mat.str-el↗

arXiv2015

Proposal for a flux qubit in a dc SQUID with the $4π$ period Josephson effect

Constructing qubits which are suitable for quantum computation remains a notable challenge. Here, we propose a superconducting flux qubit in a dc SQUID structure, formed by a conventional insulator Josephson junction and a topological nanowire Josephson junction with Majorana bound states. The zero energy Majorana bound states transport $4π$ period Josephson currents in the nanowire junction. The interplay between this $4π$ period Josephson effect and the convectional $2π$ period Josephson effect in the insulator junction induces a double-well potential energy landscape in the SQUID. As a result, the two lowest energy levels of the SQUID are isolated from other levels. These two levels show contradicting circulating supercurrents, thus can be used as a flux qubit. We reveal that this flux qubit has the merits of stability to external noises, tolerance to the deviation of system parameters, and scalability to large numbers. Furthermore, we demonstrate how to couple this flux qubit with the Majorana qubit by tuning the junction parameters, and how to use this coupling to manipulate the Majorana qubit.

cond-mat.supr-con↗

arXiv2014

Fidelity Approach in Topological Superconductors with Disorders

We apply the fidelity approach to study the topological superconductivity in a spin-orbit coupled nanowire system. The wire is modeled as a one layer lattice chain with Zeeman energy and spin-orbit coupling, which is in proximity to a multi layer superconductor. In particular, we study the effects of disorders and find that the fidelity susceptibility show multiple peaks. It is revealed that the major peak indicates the topological quantum phase transition, while other peaks signal the pining of the Majorana bound states by disorders.

cond-mat.supr-con↗

arXiv2014

Criticality and Mott-glass phase in a disordered 2D quantum spin systems

We use quantum Monte Carlo simulations to study a disordered S=1/2 Heisenberg quantum spin model with three different nearest-neighbor interactions, J1<=J2<=J3, on the square lattice. We consider the regime in which J1 represents weak bonds, and J2 and J3 correspond to two kinds of stronger bonds (dimers) which are randomly distributed on columns forming coupled 2-leg ladders. When increasing the average intra-dimer coupling (J2+J3)/2, the system undergoes a Neel to quantum glass transition of the ground state and later a second transition into a quantum paramagnet. The quantum glass phase is of the gapless Mott glass type (i.e., in boson language it is incompressible at temperature T = 0), and we find that the temperature dependence of the uniform magnetic susceptibility follows the stretched exponential form x~exp(-b/T^alpha) with 0 < alpha < 1. At the Neel-glass transition we observe the standard O(3) critical exponents, which implies that the Harris criterion for the relevance of the disorder is violated in this system.

cond-mat.str-el↗

arXiv2014

Viewing Majorana Bound States by Rabi Oscillations

Rabi oscillation is a pure quantum phenomenon where the system jumps forth and back between two quantum levels under stimulation of a microwave, and a resonance occurs when the energy difference is matched by the photon energy. Rabi oscillations have been observed in various quantum systems so far, and especially are used to demonstrate the quantum coherence of quantum bits. In the present work, we explore Rabi oscillation in a system accommodating the elusive Majorana bound states (MBSs) under intensive search recently. The proposed setup is constructed by a quantum dot (QD) and a superconducting quantum interference device (SQUID), where a semiconductor nanowire with spin-orbital coupling in the topological state is introduced to form Josephson junction. When the coupling between QD and the Josephson junction is tuned by an ac gate voltage, Rabi oscillation takes place among quantum states formed by QD and MBSs, which makes it possible to reveal bizarre properties of MBSs by sensing the electron occupation on QD. Especially, one should be able to observe the fractional Josephson relation $\cos(πΦ/Φ_0)$ unique to MBSs with $Φ$ the magnetic flux applied in the SQUID and $Φ_0=hc/2e$ the flux quantum. The system has been investigated in terms of the analytic Floquet theorem and numerical simulations with fine agreement.

cond-mat.mes-hall↗

arXiv2014

Spectrum splitting of bimagnon excitations in a spatially frustrated Heisenberg antiferromagnet revealed by resonant inelastic x-ray scattering

We perform a comprehensive analysis of the bimagnon resonant inelastic x-ray scattering (RIXS) intensity spectra of the spatially frustrated Jx-Jy-J2 Heisenberg model on a square lattice in both the antiferromagnetic and the collinear antiferromagnetic phase. We study the model for strong frustration and significant spatial anisotropy to highlight the key signatures of RIXS spectrum splitting which may be experimentally discernible. Based on an interacting spin wave theory study within the ladder approximation Bethe-Salpeter scheme, we find the appearance of a robust two-peak structure over a wide range of the transferred momenta in both magnetically ordered phases. The unfrustrated model has a single-peak structure with a two-peak splitting originating due to spatial anisotropy and frustrated interactions. Our predicted two-peak structure from both magnetically ordered regime can be realized in iron pnictides.

cond-mat.str-el↗

arXiv2013

Effects of Long-Range Interactions on Magnetic Excitations and Phase Transition on a Magnetically Frustrated Square Lattice

We investigate the effects of long-range interaction on the magnetic excitations and the competition between magnetic phases on a frustrated square lattice. Applying the spin wave theory and assisted with symmetry analysis, we obtain analytical expression for spin wave spectrum of competing Neel and (pi, 0) stripe states of systems containing any-order long-range interactions. In the specific case of long-range interactions with power-law decay, we found surprisingly that staggered long-range interaction suppresses quantum fluctuation and enlarges the ordered moment, especially in the Neel state, and thus extends its phase boundary to the stripe state. Our findings only illustrate the rich possibilities of the roles of long-range interactions, and advocate future investigations in other magnetic systems with different structures of interactions.

cond-mat.str-el↗

arXiv2013

Fidelity in topological superconductors with end Majorana fermions

Fidelity and fidelity susceptibility are introduced to investigate the topological superconductors with end Majorana fermions. A general formalism is established to calculate the fidelity and fidelity susceptibility by solving Bogoliubov-de Gennes equations. It is shown that the fidelity susceptibility manifest itself as a peak at the topological quantum phase transition point for homogeneous Kitaev wire, thus serves as a valid indicator for the topological quantum phase transition which signals the appearance of Majorana fermions. The effect of disorders is investigated within this formalism. We consider three disordered systems and observe fidelity susceptibility peak in all of them. By analyzing the susceptibility peak, we notice that the local potential disorders and the hopping disorders can shift the phase transition point, while off-diagonal disorders have no obvious influence. Our results confirm that the existence of topological quantum phase transition is robust to these disorders, while the behavior of the phase transition might be influenced by disorders.

cond-mat.supr-con↗