arXiv subjects

Ruqian Wu

Publications and source records attributed to Ruqian Wu.

At least 73 records · Page 4Linked to original sources

arXiv2017

Weyl node assisted conductivity switch in interfacial phase change memory with van der Waals interfaces

The interfacial phase-change memory (iPCM) GeTe/Sb2Te3, promising candidates for the next generation non-volatile random-access memories, exhibits fascinating topological properties. Depending on the atomic-layer-stacking sequence of the GeTe block, the iPCM can be either in the SET (Ge-Te-Ge-Te) or RESET (Te-Ge-Ge-Te) states, where the former exhibits ferroelectric polarization and electrical conductivity two orders of magnitude larger than that of the RESET state. But, its origin remains elusive. Here, we predict the emergence of a Weyl semimetal phase in the SET state induced by the ferroelectric polarization which breaks the crystal inversion symmetry. We show that the giant conductivity enhancement of the SET phase is due to the appearance of gapless Weyl nodes. The Ge-Te- or Sb-Te-terminated surfaces of Weyl semimetal iPCM produce surface states with completely distinctive topology, where the former consists solely of Fermi arcs while the latter consists of both closed Fermi surface and open Fermi arcs. The iPCM with van der Waals interfaces offers an ideal platform for exploiting the exotic Weyl properties for future memory device applications.

cond-mat.mtrl-sci↗

arXiv2016

New class of 3D topological insulator in double perovskite

We predict a new class of three-dimensional topological insulators (TIs) in which the spin-orbit coupling (SOC) can more effectively generate a large band gap at $Γ$ point. The band gap of conventional TI such as Bi$_2$Se$_3$ is mainly limited by two factors, the strength of SOC and, from electronic structure perspective, the band gap when SOC is absent. While the former is an atomic property, we find that the latter can be minimized in a generic rock-salt lattice model in which a stable crossing of bands {\it at} the Fermi level along with band character inversion occurs for a range of parameters in the absence of SOC. Thus, large-gap TI's or TI's comprised of lighter elements can be expected. In fact, we find by performing first-principle calculations that the model applies to a class of double perovskites A$_2$BiXO$_6$ (A = Ca, Sr, Ba; X = Br, I) and the band gap is predicted up to 0.55 eV. Besides, more detailed calculations considering realistic surface structure indicate that the Dirac cones are robust against the presence of dangling bond at the boundary with a specific termination.

cond-mat.mes-hall↗

arXiv2016

How to control Spin-Seebeck current in a metal-quantum dot-magnetic insulator junction

The control of the spin-Seebeck current is still a challenging task for the development of spin caloritronic devices. Here, we construct a spin-Seebeck device by inserting a strongly correlated quantum dot (QD) between the metal lead and magnetic insulator. Using the slave-particle approach and non-crossing approximation, we find that the spin-Seebeck effect increases significantly when the energy level of the QD locates near the Fermi level of the metal lead due to the enhancement of spin flipping and occurrences of quantum resonance. Since this can be easily realized by applying a gate voltage in experiments, the spin-Seebeck device proposed here can also work as a thermovoltaic transistor. Moreover, the optimal correlation strength and the energy level position of the QD are discussed to maximize the spin-Seebeck current as required for applications in controllable spin caloritronic devices.

cond-mat.str-el↗

arXiv2016

Tunable spin-orbit coupling and symmetry-protected edge states in graphene/WS$_2$

We demonstrate clear weak anti-localization (WAL) effect arising from induced Rashba spin-orbit coupling (SOC) in WS$_2$-covered single-layer and bilayer graphene devices. Contrary to the uncovered region of a shared single-layer graphene flake, WAL in WS$_2$-covered graphene occurs over a wide range of carrier densities on both electron and hole sides. At high carrier densities, we estimate the Rashba SOC relaxation rate to be $\sim 0.2 \rm{ps}^{-1}$ and show that it can be tuned by transverse electric fields. In addition to the Rashba SOC, we also predict the existence of a `valley-Zeeman' SOC from first-principles calculations. The interplay between these two SOC's can open a non-topological but interesting gap in graphene; in particular, zigzag boundaries host four sub-gap edge states protected by time-reversal and crystalline symmetries. The graphene/WS$_2$ system provides a possible platform for these novel edge states.

cond-mat.mes-hall↗

arXiv2016

Spin-Wave Fiber

Spin waves are collective excitations propagating in the magnetic medium with ordered magnetizations. Magnonics, utilizing the spin wave (magnon) as information carrier, is a promising candidate for low-dissipation computation and communication technologies. We discover that, due to the Dzyaloshinskii-Moriya interaction, the scattering behavior of spin wave at a magnetic domain wall follows a generalized Snell's law, where two magnetic domains work as two different mediums. Similar to optical total reflection that occurs at the water-air interfaces, spin waves may experience total reflection at magnetic domain walls when their incident angle larger than a critical value. We design a spin wave fiber using a magnetic domain structure with two domain walls, and demonstrate that such a spin wave fiber can transmit spin waves over long distance by total internal reflections, in analogy to an optical fiber. Our design of spin wave fiber opens up new possibilities in pure magnetic information processing.

cond-mat.mes-hall↗

arXiv2015

A spin wave diode

A diode, a device allowing unidirectional signal transmission, is a fundamental element of logic structures and lies in the heart of modern information systems. Spin wave or magnon, representing a collective quasi-particle excitation of the magnetic order in magnetic materials, is a promising candidate of information carrier for the next generation energy-saving technologies. Here we propose a scalable and reprogrammable pure spin wave logic hardware architecture using domain walls and surface anisotropy stripes as waveguides on a single magnetic wafer. We demonstrate theoretically the design principle of the simplest logic component, a spin wave diode, utilizing the chiral bound states in a magnetic domain wall with Dzyaloshiskii-Moriya interaction, and confirm its performance through micromagnetic simulations. Our findings open a new vista for realizing different types of pure spin wave logic components and finally achieving an energy-efficient and hardware-reprogrammable spin wave computer.

cond-mat.mtrl-sci↗

arXiv2015

The Trapping and Characterization of a Single Hydrogen Molecule in a Continuously Tunable Nanocavity

Using inelastic electron tunneling spectroscopy with the scanning tunneling microscope (STM-IETS) and density functional theory calculations (DFT), we investigated properties of a single H2 molecule trapped in nanocavities with controlled shape and separation between the STM tip and the Au (110) surface. The STM tip not only serves for the purpose of characterization, but also is directly involved in modification of chemical environment of molecule. The bond length of H2 expands in the atop cavity, with a tendency of dissociation when the gap closes, whereas it remains unchanged in the trough cavity. The availability of two substantially different cavities in the same setup allows understanding of H2 adsorption on noble metal surfaces and sets a path for manipulating a single chemical bond by design.

cond-mat.mtrl-sci↗

arXiv2015

Chern Half Metals: A New Class of Topological Materials to Realize the Quantum Anomalous Hall Effect

The search of new topological insulators that demonstrate the quantum anomalous Hall effect (QAHE) is a cutting-edge research topic in condensed matter physics and materials science. So far, the QAHE has been observed only in Cr-doped (Bi,Sb)2Te3 at extremely low temperature. Therefore, it is important to find new materials with large topological band gap and high thermal stability for the realization of the QAHE. Based on first-principles and tight-binding model calculations, we discovered a new class of topological phase$-$Chern half metal which manifests the QAHE in one spin channel while is metallic in the other spin channel$-$in Co or Rh deposited graphene. Meanwhile, these systems have large perpendicular magnetic anisotropy energies of 5.3 and 11.5 meV, necessary for the observation of the QAHE at reasonably high temperature.

cond-mat.mtrl-sci↗

arXiv2015

Magnetic anisotropy of FePt nanoparticles

We carry out a systematic theoretical investigation of Magneto Crystalline Anisotropy (MCA) of L10 FePt clusters with alternating Fe and Pt planes along the (001) direction. We calculate the structural relaxation and magnetic moment of each cluster by using ab initio spin-polarized density functional theory (DFT), and the MCA with both spin-polarized DFT (including spin-orbit coupling self-consistently) and the torque method. We find that the MCA of any composite structure of a given size is enhanced with respect to that of the same-sized pure Pt or pure Fe cluster as well as to that of any pair of Fe and Pt atoms in bulk L10 FePt. This enhancement results from the hybridization we observe between the 3d orbital of the Fe atoms and the 5d orbital of their Pt neighbors. This hybridization, however, affects the electronic properties of the component atoms in significantly different ways. While it somewhat increases the spin moment of the Fe atoms, it has little effect on their orbital moment; at the same time, it greatly increases both the spin and orbital moment of the Pt atoms. Given the fact that the spin-orbit coupling (SOC) constant of Pt is about 7 times greater than that of Fe, this Fe-induced jump in the orbital moment of the Pt atoms produces the increase in MCA of the composite structures over that of their pure counterparts. That any composite structure exhibits higher MCA than bulk L10 FePt results from the lower coordination of Pt atoms in the cluster, whether Fe or Pt predominates within it. We also find that bipyramidal clusters whose central layer is Pt have higher MCA than their same-sized counterparts whose central layer is Fe. This results from the fact that Pt atoms in such configurations are coordinated with more Fe atoms than in the latter. By thus participating in more instances of hybridization, they contribute higher orbital moments to the overall MCA of the unit.

cond-mat.mes-hall↗

arXiv2014

Giant magnetic anisotropy of transition-metal dimers on defected graphene

Continuous miniaturization of magnetic units in spintronics and quantum computing devices inspires efforts to search for magnetic nanostructures with large magnetic anisotropy energy (MAE). Typical nanostructures including molecular magnets, magnetic nanoclusters and magnetic nanowires have MAEs of a few meV so their blocking temperature is mostly lower than 50 K. In this work, we demonstrated the feasibility of achieving giant MAE in systems with transition metal dimers on defected and decorated graphene, based on density functional theory calculations. In particular, either a Pt-Ir dimer on a single vacancy or an Os-Ru dimer on a nitrogen-decorated divacancy possesses an MAE larger than 60 meV and high structural stability. Interestingly, their magnetic anisotropy can be conveniently manipulated by using external electric field. These features make them good candidates for the use in room temperature spintronics and quantum computing devices.

cond-mat.mtrl-sci↗

arXiv2014

Magnetism and Magnetic Anisotropy of Transition Metal-Phthalocyanine Molecules

Search for single-molecule magnets (SMMs) with high blocking temperature (TB) is urgent for practical applications in magnetic recording, molecular spintronics and quantum computing. Based on the First-principles calculations, magnetic anisotropy energies (MAE) of the transition metal-Phthalocyanine (TM-Pc) molecules are investigated and the mechanism that determines the MAE of TM-Pc molecules is established. In particular, colossal MAE > 100 meV can be obtained by adding an Os atom on RuPc and OsPc, so these molecules may offer ultrahigh thermal stability in devices.

cond-mat.mtrl-sci↗

arXiv2014

Control of the magnetism and magnetic anisotropy of a single-molecule magnet with an electric field

Through systematics density functional calculations, the mechanism of the substrate induced spin reorientation transition in FePc/O-Cu(110) was explained in terms of charge transfer and rearrangement of Fe-d orbitals. Moreover, we found giant magnetoelectric effects in this system, manifested by the sensitive dependences of its magnetic moment and magnetic anisotropy energy on external electric field. In particular, the direction of magnetization of FePc/O-Cu(110) is switchable between in-plane and perpendicular axes, simply by applying an external electric field of 0.5 eV/Å along the surface normal.

cond-mat.mtrl-sci↗

arXiv2013

Intrinsic Spin Seebeck Effect in Au/YIG

The acute magnetic proximity effects in Pt/YIG compromise the suitability of Pt as a spin current detector. We show that Au/YIG, with no anomalous Hall effect and a negligible magnetoresistance, allows the measurements of the intrinsic spin Seebeck effect with a magnitude much smaller than that in Pt/YIG. The experiment results are consistent with the spin-polarized density-functional calculations for Pt with a sizable and Au with a negligible magnetic moment near the interface with YIG.

cond-mat.mtrl-sci↗

arXiv2012

Giant topological insulator gap in graphene with 5d adatoms

Two-dimensional topological insulators (2D TIs) have been proposed as platforms for many intriguing applications, ranging from spintronics to topological quantum information processing. Realizing this potential will likely be facilitated by the discovery of new, easily manufactured materials in this class. With this goal in mind we introduce a new framework for engineering a 2D TI by hybridizing graphene with impurity bands arising from heavy adatoms possessing partially filled d-shells, in particular osmium and iridium. First principles calculations predict that the gaps generated by this means exceed 0.2 eV over a broad range of adatom coverage; moreover, tuning of the Fermi level is not required to enter the TI state. The mechanism at work is expected to be rather general and may open the door to designing new TI phases in many materials.

cond-mat.mes-hall↗

arXiv2011

Engineering a robust quantum spin Hall state in graphene via adatom deposition

The 2007 discovery of quantized conductance in HgTe quantum wells delivered the field of topological insulators (TIs) its first experimental confirmation. While many three-dimensional TIs have since been identified, HgTe remains the only known two-dimensional system in this class. Difficulty fabricating HgTe quantum wells has, moreover, hampered their widespread use. With the goal of breaking this logjam we provide a blueprint for stabilizing a robust TI state in a more readily available two-dimensional material---graphene. Using symmetry arguments, density functional theory, and tight-binding simulations, we predict that graphene endowed with certain heavy adatoms realizes a TI with substantial band gap. For indium and thallium, our most promising adatom candidates, a modest 6% coverage produces an estimated gap near 80K and 240K, respectively, which should be detectable in transport or spectroscopic measurements. Engineering such a robust topological phase in graphene could pave the way for a new generation of devices for spintronics, ultra-low-dissipation electronics and quantum information processing.

cond-mat.mes-hall↗

arXiv2010

Thioglycolic acid on the gold (111) surface and Raman vibrational spectra

The interaction of thioglycolic acid with the Au(111) surface is investigaged, and it is found that at the low coverage the molecule lies down on the substrate. If the mercaptan-hydrogen atom is eliminated, the resulting SCH_2COOH molecule is randomly oriented on the surface. If the carboxylic acid group in the HSCH_2COOH molecule is deprotonated instead, the HSCH_2COO^ molecule lies down on the surface. However, when the mercaptan-hydrogen atom in the HSCH_2COO^- molecule is removed, the resulting SCH_2COO^- molecule rises up to a certain level on the substrate. The calculated Raman vibrational spectra decipher which compounds and atomic displacements contribute to the corresponding frequencies. We thus propose a consistent mechanism for the deposition of thioglycolic acid on the Au(111) surface.

cond-mat.mtrl-sci↗

arXiv2007

Low temperature heat capacity of Fe_{1-x}Ga_{x} alloys with large magneostriction

The low temperature heat capacity C_{p} of Fe_{1-x}Ga_{x} alloys with large magnetostriction has been investigated. The data were analyzed in the standard way using electron ($γT$) and phonon ($βT^{3}$) contributions. The Debye temperature $Θ_{D}$ decreases approximately linearly with increasing Ga concentration, consistent with previous resonant ultrasound measurements and measured phonon dispersion curves. Calculations of $Θ_{D}$ from lattice dynamical models and from measured elastic constants C_{11}, C_{12} and C_{44} are in agreement with the measured data. The linear coefficient of electronic specific heat $γ$ remains relatively constant as the Ga concentration increases, despite the fact that the magnetoelastic coupling increases. Band structure calculations show that this is due to the compensation of majority and minority spin states at the Fermi level.

cond-mat.mtrl-sci↗

arXiv2007

Composition and reactivity on Pd/Au(111) surface: A first-principle study

Atomic configurations of two or three Pd substituents on the Au(111) surface are investigated using the first-principles pseudopotential plane wave approach. Pd atoms are found to form second neighborhoods on PdAu(111). The Pd-d band becomes narrow and well below the Fermi level, very different from those in a Pd film or bulk Pd. Nevertheless, surface Pd atoms are still active and serve as independent attractive centers towards adsorbates. The special ensembles are important for catalysis applications because of their ability to confine reactants in a small region.

cond-mat.mtrl-sci↗