arXiv subjects

Hui-Tian Wang

Publications and source records attributed to Hui-Tian Wang.

At least 55 records · Page 3Linked to original sources

arXiv2018

Condensed matter realization of fermion quasiparticles in Minkowski space

"What is the difference between space and time?" is an ancient question that remains a matter of intense debate. In Newtonian mechanics time is absolute, while in Einstein's theory of relativity time and space combine into Minkowski spacetime. Here, we firstly propose Minkowski fermions in 2+1 dimensional Minkowski spacetime which have two space-like and one time-like momentum axes. These quasiparticles can be further classified as Klein-Gordon fermions and Dirac-Minkowski fermions according to the linearly and quadratically dispersing excitations. Realization of Dirac-Minkowski quasiparticles requires systems with particular topological nodal-line band degeneracies, such as hyperbolic nodal lines or coplanar band crossings. With the help of first-principles calculations we find that novel massless Dirac-Minkowski fermions are realized in a metastable bulk boron allotrope, Pnnm-B16.

cond-mat.mtrl-sci↗

arXiv2018

A novel superhard tungsten nitride predicted by machine-learning accelerated crystal structure searching

Transition metal nitrides have been suggested to have both high hardness and good thermal stability with large potential application value, but so far stable superhard transition metal nitrides have not been synthesized. Here, with our newly developed machine-learning accelerated crystal structure searching method, we designed a superhard tungsten nitride, h-WN6, which can be synthesized at pressure around 65 GPa and quenchable to ambient pressure. This h-WN6 is constructed with single-bonded N6 rings and presents ionic-like features, which can be formulated as W2.4+N62.4-. It has a band gap of 1.6 eV at 0 GPa and exhibits an abnormal gap broadening behavior under pressure. Excitingly, this h-WN6 is found to be the hardest among transition metal nitrides known so far (Vickers hardness around 57 GPa) and also has a very high melting temperature (around 1900 K). These predictions support the designing rules and may stimulate future experiments to synthesize superhard material.

cond-mat.mtrl-sci↗

arXiv2017

Superhard and Superconducting B6C

Crystal structure searching and ab initio calculations have been used here to explore low-energy structures of boron carbides under high pressure. Under pressures of 85-110 GPa, a metastable B6C with R-3m symmetry is found to be energetically more stable than the mixture of previous B4C and elemental boron. This B6C is a rhombohedral structure and contains mooncake-like B24 clusters with stuffing of C2 pairs. The mechanical and dynamical stabilities of this structure at ambient pressure are confirmed by its elastic constants and phonon dispersions. The bulk modulus and shear modulus of this newly predicted B6C at ambient pressure reach high values of 291 GPa and 272 GPa, respectively. The Vickers hardness is calculated to be around 48 GPa, and its melting temperature at ambient pressure is estimated to be around 2500 K, indicating that this metastable B6C is a potential superhard material with very good thermal stability. Interestingly, this superhard B6C structure is also found to be superconducting and its superconducting critical temperature (Tc) is evaluated to be around 12.5 K at ambient pressure by electron-phonon coupling.

cond-mat.mtrl-sci↗

arXiv2017

Complete orbital angular momentum Bell-state measurement and superdense coding

Quantum protocols require access to large-scale entangled quantum states, due to the requirement of channel capacity. As a promising candidate, the high-dimensional orbital angular momentum (OAM) entangled states have been implemented, but only one of four OAM Bell states in each individual subspace can be distinguished. Here we demonstrate the first realization of complete OAM Bell-state measurement (OAM-BSM) in an individual subspace, by seeking the suitable unitary matrix performable using only linear optics and breaking the degeneracy of four OAM Bell states in ancillary polarization dimension. We further realize the superdense coding via our complete OAMBSM with the average success probability of ~82% and the channel capacity of ~1.1(4) bits. This work opens the window for increasing the channel capacity and extending the applications of OAM quantum states in quantum information in future.

quant-ph↗

arXiv2017

Two-dimensional boron on Pb (110) surface

We simulate boron on Pb(110) surface by using ab initio evolutionary methodology. Interestingly, the two-dimensional (2D) Dirac Pmmn boron can be formed because of good lattice matching. Unexpectedly, by increasing the thickness of 2D boron, a three-bonded graphene-like structure (P2_1/c boron) was revealed to possess double anisotropic Dirac cones. It is 20 meV/atom lower in energy than the Pmmn structure, indicating the most stable 2D boron with particular Dirac cones. The puckered structure of P2_1/c boron results in the peculiar Dirac cones, as well as substantial mechanical anisotropy. The calculated Young's modulus is 320 GPa.nm along zigzag direction, which is comparable with graphene.

cond-mat.mtrl-sci↗

arXiv2017

A stable compound of helium and sodium at high pressure

Helium is generally understood to be chemically inert and this is due to its extremely stable closed-shell electronic configuration, zero electron affinity and an unsurpassed ionization potential. It is not known to form thermodynamically stable compounds, except a few inclusion compounds. Here, using the ab initio evolutionary algorithm USPEX and subsequent high-pressure synthesis in a diamond anvil cell, we report the discovery of a thermodynamically stable compound of helium and sodium, Na2He, which has a fluorite-type structure and is stable at pressures >113 GPa. We show that the presence of He atoms causes strong electron localization and makes this material insulating. This phase is an electride, with electron pairs localized in interstices, forming eight-centre two-electron bonds within empty Na8 cubes. We also predict the existence of Na2HeO with a similar structure at pressures above 15 GPa.

cond-mat.mtrl-sci↗

arXiv2015

Two-Dimensional Magnetic Boron

We predict a two-dimensional (2D) antiferromagnetic (AFM) boron (designated as M-boron) by using ab initio evolutionary methodology. M-boron is entirely composed of B20 clusters in a hexagonal arrangement. Most strikingly, the highest valence band of M-boron is isolated, strongly localized, and quite flat, which induces spin polarization on each cap of the B20 cluster. This flat band originates from the unpaired electrons of the capping atoms, and is responsible for magnetism. M-boron is thermodynamically metastable and is the first cluster-based 2D magnetic material in the elemental boron system.

cond-mat.mtrl-sci↗

arXiv2015

Arbitrary orbital angular momentum of photons

Orbital angular momentum (OAM) of photons, as a new fundamental degree of freedom, has excited a great diversity of interest, because of a variety of emerging applications. Arbitrarily tunable OAM has gained much attention, but its creation remains still a tremendous challenge. We demonstrate the realization of well-controlled arbitrary OAM in both theory and experiment. We present the concept of general OAM, which extends the OAM carried by the scalar vortex field to the OAM carried by the azimuthally varying polarized vector field. The arbitrary OAM has the same characteristics as the well-defined integer OAM: intrinsic OAM, uniform local OAM and intensity ring, and propagation stability. The arbitrary OAM has unique natures: it is allowed to be flexibly tailored and the radius of the focusing ring can have various choices for a desired OAM, which are of great significance to the benefit of surprising applications of the arbitrary OAM.

physics.optics↗

arXiv2015

Entanglement of Vector-Polarization States of Photons

Photons may have homogeneous polarization and may carry quantized orbital angular momentum (OAM). Photon entanglement has been realized in various degrees of freedom such as polarization and OAM. Using a pair of orthogonally polarized states carrying opposite-handedness quantized OAMs could create "quantized" vector polarization states with space-variant polarization structures. It is thus possible to extend the polarization degree of freedom from two dimensional space to indefinite dimensional discrete Hilbert space. We present a class of vector-polarization entangled Bell states, which use the spatial modes of the vector fields with space-variant polarization structure. We propose a scheme of creating the vector-polarization entangled Bell states using a Sagnac interferometer. We also design an analyzer for identifying the vector-polarization entangled Bell states. Such a class of entanglement is important for quantum information science and technology, and fundamental issues of quantum theory, due to its advantage of the increase in information capacity.

physics.optics↗

arXiv2015

Near-complete polarization Bell-state analysis based on symmetry-broken scheme with linear optics

Bell-state analysis is a considerable challenge and an essential requirement for reliable implementation of quantum communication proposals. An open question is the one for the maximal fraction of successful Bell measurements. It has been pointed out that no scheme using only linear elements can implement a Bell state analyzer. Some effort has paid attention to the complete polarization-entangled Bell-state analysis using linear optics, with the aid of auxiliary means. Here we present a symmetry-broken scheme with linear optics only, without any aid of other auxiliary means, for discriminating polarization-entangled Bell states. Although our scheme is unable of realizing complete Bell-state measurement for less photon-pairs situation, it can deterministically identify four Bell states with success probabilities beyond 99.2% provided that photon-pairs are not less than 8. Our scheme as a significant breakthrough is simpler and feasible with respect to the current technology for the near-complete Bell-state analysis. Symmetry breaking is indispensable in our scheme as in other physical systems.

quant-ph↗

arXiv2015

Trajectory-Based Unveiling of Angular Momentum of Photons

The Heisenberg uncertainty principle suggests that it is impossible to determine the trajectory of a quantum particle in the same way as a classical particle. However, we may still yield insight into novel behavior of photons based on the average photon trajectories (APTs). Here we explore the APTs of photons carrying spin angular momentum (SAM) and/or orbital angular momentum (OAM) under the paraxial condition. We define the helicity and differential helicity for unveiling the three-dimensional spiral structures of the APTs of photons. We clarify the novel behaviors of the APTs caused by the SAM and OAM as well as the SAM-OAM coupling. The APT concept is very helpful for profoundly understanding the motion of trapped particles and for elucidating other physical systems. Due to the presence of the helical path caused by the SAM and/or the OAM, the actual traveling distance of the photons might be much longer than the geometric distance.

physics.optics↗

arXiv2014

Generalized Poincaré Sphere

We present a generalized Poincaré sphere (G sphere) and generalized Stokes parameters (G parameters), as a geometric representation, which unifies the descriptors of a variety of vector fields. Unlike the standard Poincaré sphere, the radial dimension in the G sphere is not used to describe the partially polarized field. The G sphere is constructed by extending the basic Jones vector basis to the general vector basis with the continuously changeable ellipticity (spin angular momentum, SAM) and the higher dimensional orbital angular momentum (OAM). The north and south poles of different spherical shell in the G sphere represent the pair of different orthogonal vector basis with different ellipticity (SAM) and the opposite OAM. The higher-order Poincaré spheres are just the two special spherical shells of the G sphere. We present a quite flexible scheme, which can generate all the vector fields described in the G sphere. The salient properties of this representation are able to treat the complicated geometric phase of arbitrary vector fields in the future.

physics.optics↗

arXiv2014

Unexpected Reconstruction of the alpha-Boron (111) Surface

We report on a novel reconstruction of the alpha-boron (111) surface, discovered using an ab initio evolution structure search, and reveal that it has an unexpected neat structure and much lower surface energy than the recently proposed (111)-I_R,(a) surface. For this reconstruction, every single interstitial boron atom forms bridges with the unique polar-covalent bonds between neighboring B_12 icosahedra, which perfectly meet the electron counting rule and are responsible for the reconstruction-induced metal-semiconductor transition. The peculiar charge transfer between the interstitial atoms and the icosahedra plays an important role in stabilizing the surface.

cond-mat.mtrl-sci↗

arXiv2014

Semimetallic Two-Dimensional Boron Allotrope with Massless Dirac Fermions

It has been widely accepted that planar boron structures, composed of triangular and hexagonal motifs are the most stable two dimensional (2D) phases and likely precursors for boron nanostructures. Here we predict, based on ab initio evolutionary structure search, novel 2D boron structure with non-zero thickness, which is considerably, by 50 meV/atom lower in energy than the recently proposed alpha-sheet structure and its analogues. In particular, this phase is identified for the first time to have a distorted Dirac cone, after graphene and silicene the third elemental material with massless Dirac fermions. The buckling and coupling between the two sublattices not only enhance the energetic stability, but also are the key factors for the emergence of the distorted Dirac cone.

cond-mat.mtrl-sci↗

arXiv2011

Universal phase transitions of B1 structured stoichiometric transition-metal carbides

The high-pressure phase transitions of B1-structured stoichiometric transition metal carbides (TMCs, TM=Ti, Zr, Hf, V, Nb, and Ta) were systematically investigated using ab initio calculations. These carbides underwent universal phase transitions along two novel phase-transition routes, namely, B1\rightarrowdistorted TlI (TlI')\rightarrowTlI and/or B1\rightarrowdistorted TiB (TiB')\rightarrowTiB, when subjected to pressures. The two routes can coexist possibly because of the tiny enthalpy differences between the new phases under corresponding pressures. Four new phases result from atomic slips of the B1-structured parent phases under pressure. After completely releasing the pressure, taking TiC as a representative of TMCs, only its new TlI'-type phase is mechanically and dynamically stable, and may be recovered.

cond-mat.mtrl-sci↗

arXiv2010

Ab initio study of the formation of transparent carbon under pressure

A body-centered tetragonal carbon (bct-Carbon) allotrope has been predicted to be a transparent carbon polymorph obtained under pressure. The structural transition pathways from graphite to diamond, M-Carbon, and bct-Carbon are simulated and the lowest activation barrier is found for the graphite-bct transition. Furthermore, bct-Carbon has higher shear strength than diamond due to its perpendicular graphene-like structure. Our results provide a possible explanation for the formation of a transparent carbon allotrope via the cold compression of graphite. We also verify that this allotrope is hard enough to crack diamond.

cond-mat.mtrl-sci↗

arXiv2010

Unusual Compression Behavior of Columbite TiO2 via First-Principles Calculations

The physical mechanisms behind the reduction of the bulk modulus of a high-pressure cubic TiO2 phase are confirmed by first-principles calculations. An unusual and abrupt change occurs in the dependence of energy on pressure at 43 GPa, indicating a pressure-induced phase transition from columbite TiO2 to a newly-identified modified fluorite TiO2 with a Pca21 symmetry. Oxygen atom displacement in Pca21 TiO2 unexpectedly reduces the bulk modulus by 34% relative to fluorite TiO2. This discovering provides a direct evidence for understanding the compressive properties of such groups of homologous materials

cond-mat.mtrl-sci↗

arXiv2009

Creation of a new vector field and focusing engineering

Recently many methods have been proposed to create the vector fields, due to the academic interest and a variety of attractive applications such as for particle acceleration, optical trapping, particle manipulation, and fluorescence imaging. For the most of the created vector fields, the spatial distribution in states of polarization (SoPs) is dependent of azimuthal angle only. It is very interesting and crucial that if we can introduce the radial controlling freedom, which undoubtedly opens a new way to provide the flexibility for creating the desired vector fields and for fulfilling the requirement on a variety of applications. Here we present a new idea to create a new kind of vector filed with the radial-variant SoPs. This idea also permits to create flexibly vector fields with arbitrarily complex distribution of SoPs, based on a combination of radial and azimuthal dependency. This realization in both principle and experiment is paramount to be able to implement the focusing engineering for applications in a variety of realms. Specially arranging the SoPs of vector fields, purposefully and carefully, is anticipated to lead to new effects and phenomena that can expand the functionality and enhance the capability of optical systems, such as the optical trapping.

physics.optics↗