A5057155376- Graphene research and applications
- Topological Materials and Phenomena
- Quantum and electron transport phenomena
- Advanced Condensed Matter Physics
- Multiferroics and related materials
- Magnetic properties of thin films
- Electronic and Structural Properties of Oxides
- Iron-based superconductors research
- 2D Materials and Applications
- Magnetic and transport properties of perovskites and related materials
- Carbon Nanotubes in Composites
- Quantum, superfluid, helium dynamics
- Magneto-Optical Properties and Applications
- Molecular Junctions and Nanostructures
- Diamond and Carbon-based Materials Research
- Mechanical and Optical Resonators
- Physics of Superconductivity and Magnetism
- Surface and Thin Film Phenomena
- Cold Atom Physics and Bose-Einstein Condensates
- Perovskite Materials and Applications
- Ferroelectric and Piezoelectric Materials
- Photorefractive and Nonlinear Optics
- Advanced Chemical Physics Studies
- Magnetic Properties of Alloys
- High-pressure geophysics and materials
University of California, Riverside
2017-2024
University of California System
2024
Lawrence Berkeley National Laboratory
2012-2019
University of California, Berkeley
2012-2019
Rutgers, The State University of New Jersey
2008-2011
We present a systematic Raman study of unconventionally-stacked double-layer graphene, and find that the spectrum strongly depends on relative rotation angle between layers. Rotation-dependent trends in position, width intensity graphene 2D G peaks are experimentally established accounted for theoretically. Our theoretical analysis reveals changes electronic band structure due to interlayer interaction, such as rotational-angle dependent Van Hove singularities, responsible observed spectra...
Abstract While the basic principles of conventional solar cells are well understood, little attention has gone towards maximizing efficiency photovoltaic devices based on shift currents. By analysing effective models, here we outline simple design for optimization currents frequencies near band gap. Our method allows us to express edge current in terms a few model parameters and show it depends explicitly wavefunctions addition standard structure. We use our approach identify two classes...
We extend the recently-developed theory of bulk orbital magnetization to finite electric fields, and use it calculate magnetoelectric response periodic insulators. Working in independent-particle framework, we find that finite-field can be written as a sum three gauge-invariant contributions, one which has no counterpart at zero field. The extra contribution is collinear with explicitly dependent on expression for suitable first-principles implementations, allowing coefficients by numerical...
Abundant transition metal borides are emerging as substitute electrochemical hydrogen evolution reaction (HER) catalysts for noble metals. Herein, an unusual canonic-like behavior of the c lattice parameter in AlB2 -type solid solution Cr1-x Mox B2 (x = 0, 0.25, 0.4, 0.5, 0.6, 0.75, 1) and its direct correlation to HER activity 0.5 M H2 SO4 reported. The increases with increasing x, reaching maximum at x 0.6 before decreasing again. At high current densities, Cr0.4 Mo0.6 outperforms Pt/C, it...
We present a Wannier-based method to calculate the Chern-Simons orbital magnetoelectric coupling in framework of first-principles density-functional theory. In view recent developments connection with strong ${\mathbb{Z}}_{2}$ topological insulators, we anticipate that contribution could, special cases, be as large or larger than total known magnetoelectrics, such Cr${}_{2}$O${}_{3}$. The results our calculations for ordinary magnetoelectrics Cr${}_{2}$O${}_{3}$, BiFeO${}_{3}$, and...
The linear magnetoelectric response of Cr2O3 at zero temperature is calculated from first principles by tracking the change in magnetization under a macroscopic electric field. Both spin and orbital contributions to induced are computed, each case decomposed into lattice electronic parts. We find that transverse dominated spin-lattice spin-electronic contributions, whose values consistent with static optical measurements. In longitudinal response, dominate over but net remains much smaller...
The anisotropic magnetoresistance (AMR) of ${\mathrm{Cr}}_{2}{\mathrm{Ge}}_{2}{\mathrm{Te}}_{6}$ (CGT), a layered ferromagnetic insulator, is investigated under an applied hydrostatic pressure up to 2 GPa. easy-axis direction the magnetization inferred from AMR saturation feature in presence and absence pressure. At zero pressure, easy axis along $c$ or perpendicular layer. Upon application > 1 GPa, uniaxial anisotropy switches easy-plane which drives equilibrium $ab$ plane at magnetic...
Conventional approaches for lattice dynamics based on static interatomic forces do not fully account the effects of time-reversal-symmetry breaking in magnetic systems. Recent to rectify this involve incorporating first-order change with atomic velocities under assumption adiabatic separation electronic and nuclear degrees freedom. In work, we develop a first-principles method calculate velocity-force coupling extended solids, show via example ferromagnetic CrI$_3$ that, due slow spins...
We extend the Berry-phase concept of polarization to insulators having a nonzero value Chern invariant. The generalization such requires special care because partial occupation chiral edge states. show how integrated bulk current arising from an adiabatic evolution can be related difference polarizations. also surface charge polarization, but only with knowledge wave vector at which occupancy state is discontinuous. Furthermore, we present numerical calculations on model Hamiltonian provide...
We analyze both experimentally (where possible) and theoretically from first-principles the dielectric tensor components crystal structure of five classes Pbnm perovskites. All these materials are believed to be stable on silicon therefore promising candidates for high-K dielectrics. also with various simple models, decompose lattice contribution into force constant matrix eigenmode contributions, explore a peculiar correlation between structural anisotropies in compounds give phonon...
We show that electron–phonon coupling can induce strong electron pairing in an FeSe monolayer on a SrTiO3 substrate (experimental indications for superconducting are between 65 and 109 K). The role of the increasing is two-fold. First, interaction TiO2 terminated face prevents from undergoing shear-type (orthorhombic, nematic) structural phase transition. Second, allows anti-ferromagnetic ground state which opens channels within prevented by symmetry non-magnetic phase. spectral function ()...
In insulators, Born effective charges describe the electrical polarization induced by displacement of individual atomic sublattices. Such a physical property is at first sight irrelevant for metals and doped semiconductors, where macroscopic ill defined. Here we show that, in clean conductors, going beyond adiabatic approximation results nonadiabatic that are well defined low-frequency limit. addition, find sublattice sum does not vanish as it insulating case, but instead proportional to...
We study theoretically the Raman spectrum of rotated double-layer graphene, consisting two graphene layers with respect to each other by an arbitrary angle $\ensuremath{\theta}$. find a relatively simple dependence $G$ peak intensity on On hand, $2D$ position, intensity, and width show much more complicated account for all these effects, including incoming photon energy, in good agreement experimental data. that it is sufficient include interaction between electronic degrees freedom...
Maximally localized Wannier functions are widely used in electronic structure theory for analyses of bonding, electric polarization, orbital magnetization, and interpolation. The state the art method their construction is based on Marzari Vanderbilt. One practical difficulties this guessing (initial projections) that approximate final functions. Here we present an approach optimized projection can construct maximally without a guess. We describe demonstrate several realistic examples.
Using first-principles density functional theory calculations, we discover an anomalously large bi-axial strain-induced octahedral rotation axis reorientation in orthorhombic perovskites with tendency towards rhombohedral symmetry. The transition between crystallographically equivalent (isosymmetric) structures different magnitudes originates from strong strain--octahedral coupling available to and the energetic hierarchy among competing tilt patterns. By elucidating these criteria, suggest...
We report a scanning tunneling microscopy and noncontact atomic force study of close-packed 2D islands tetrafluorotetracyanoquinodimethane (F4TCNQ) molecules at the surface graphene layer supported by boron nitride. While F4TCNQ are known to form cohesive 3D solids, intermolecular interactions that attractive for in repulsive 2D. Our experimental observation molecular behavior on is thus unexpected. This self-assembly can be explained novel solid formation mechanism occurs when charged...
Abstract Understanding how photoexcited electron dynamics depend on electron-electron (e-e) and electron-phonon (e-p) interaction strengths is important for many fields, e.g. ultrafast magnetism, photocatalysis, plasmonics, others. Here, we report simple expressions that capture the interplay of e-e e-p interactions distribution relaxation times. We observe a dependence universal to most metals also counterintuitive. While only reduce total energy stored by excited electrons, time leave...
We have performed an exhaustive representation-theory-based search for the simplest structures allowing isotropic magnetoelectric coupling. find 30 such structures, all sharing a common pattern of atomic displacements in direction magnetic moments. focus on one these canonical and that it is generically realized class fractionally substituted pyrochlore compounds with all-in-all-out order. Furthermore, we substantial Chern-Simons orbital component (\theta=0.1--0.2). While this also formally...
Among the phases of ${\text{SiO}}_{2}$ are $\ensuremath{\alpha}$ and $\ensuremath{\beta}$ cristobalites, which have a long somewhat controversial history proposed structural assignments phase-transition mechanisms. Recently, Zhang Scott found new indications that higher-temperature phase has space group $I\overline{4}2d$ and, by assuming group-subgroup relationship between phases, they argued lower-temperature should lower symmetry than widely accepted $P{4}_{1}{2}_{1}2$ group. With this...
We investigate the influence of SiO${}_{2}$, Au, Ag, Cu, and Pt substrates on Raman spectrum graphene. Experiments reveal particularly strong modifications to signal graphene platinum, compared that suspended The strongly depend relative orientation platinum lattices. These observations are theoretically investigated shown originate basically from hybridization electronic states in $d$ orbitals platinum. It is expected that, quite generally, between any material with near Fermi level will...
The unusual electronic properties of the quantum spin Hall or Chern insulator become manifest in form robust edge states when samples with boundaries are studied. In this work, we ask if and how topologically nontrivial structure these two-dimensional systems can be passed on to their zero-dimensional relatives, namely, fullerenes other closed-cage molecules. To address question, study Haldane's honeycomb lattice model polyhedral nanosurfaces. We find that for sufficiently large surfaces,...
We group materials into five symmetry classes and determine in which of these phonons carry angular momentum the Brillouin zone, away from a high-symmetry point, line, or plane. In some acquire via forces induced by relative displacements atoms out their equilibrium positions. However, for other materials, such as ferromagnetic iron, phonon arises velocities atoms. These effects are driven spin-orbit interaction.
We formulate the spin contribution to inverse Faraday effect of non-magnetic metals. deal with role inversion symmetry, which forces all electronic bands be at least twice degenerate every point in Brillouin zone. show both analytically and numerically that our formulation is invariant under unitary rotation within doubly set bands. In addition, we importance resonance-like features band structure for effect. Our first-principles computed component a simple metal such as Au reminiscent its...