[Show abstract][Hide abstract] ABSTRACT: Topological phases with insulating bulk and gapless surface or edge modes
have attracted much attention because of their fundamental physics implications
and potential applications in dissipationless electronics and spintronics. In
this review, we mainly focus on the recent progress in the engineering of
topologically nontrivial phases (such as $\mathbb{Z}_2$ topological insulators,
quantum anomalous Hall effects, quantum valley Hall effects \textit{etc.}) in
two-dimensional material systems, including quantum wells, atomic crystal
layers of elements from group III to group VII, and the transition metal
compounds.
[Show abstract][Hide abstract] ABSTRACT: Magneto-optical Kerr effect, normally found in magnetic materials with
nonzero magnetization such as ferromagnets and ferrimagnets, has been known for
more than a century. Here, using first-principles density functional theory, we
demonstrate large magneto-optical Kerr effect in high temperature noncollinear
antiferromagnets Mn$_{3}X$ ($X$ = Rh, Ir, or Pt), in contrast to usual wisdom.
The calculated Kerr rotation angles are large, being comparable to that of
transition metal magnets such as bcc Fe. The large Kerr rotation angles and
ellipticities are found to originate from the lifting of the band
double-degeneracy due to the absence of spatial symmetry in the Mn$_{3}X$
noncollinear antiferromagnets which together with the time-reversal symmetry
would preserve the Kramers theorem. Our results indicate that Mn$_{3}X$ would
provide a rare material platform for exploration of subtle magneto-optical
phenomena in noncollinear magnetic materials without net magnetization.
[Show abstract][Hide abstract] ABSTRACT: Within the wave-packet semiclassical approach, the Bloch electron energy is derived to second order in the magnetic field and classified into gauge-invariant terms with clear physical meaning, yielding a fresh understanding of the complex behavior of orbital magnetic susceptibility. The Berry curvature and quantum metric of the Bloch states give rise to a geometrical magnetic susceptibility, which can be dominant when bands are filled up to a small energy gap. There is also an energy polarization term, which can compete with the Peierls-Landau and Pauli magnetism on a Fermi surface. All these, and an additional Langevin susceptibility, can be calculated from each single band, leaving the Van Vleck susceptibility as the only term truly from interband coupling.
Physical Review B 06/2015; 91(21). DOI:10.1103/PhysRevB.91.214405 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Monolayer transition-metal dichalcogenides possess a pair of degenerate
helical valleys in the band structure that exhibit fascinating optical valley
polarization. Optical valley polarization, however, is limited by carrier
lifetimes of these materials. Lifting the valley degeneracy is therefore an
attractive route for achieving valley polarization. It is very challenging to
achieve appreciable valley degeneracy splitting with applied magnetic field. We
propose a strategy to create giant splitting of the valley degeneracy by
proximity-induced Zeeman effect. As a demonstration, our first principles
calculations of monolayer MoTe$_2$ on a EuO substrate show that valley
splitting over 300 meV can be generated. The proximity coupling also makes
interband transition energies valley dependent, enabling valley selection by
optical frequency tuning in addition to circular polarization. The valley
splitting in the heterostructure is also continuously tunable by rotating
substrate magnetization. The giant and tunable valley splitting adds a readily
accessible dimension to the valley-spin physics with rich and interesting
experimental consequences, and offers a practical avenue for exploring device
paradigms based on the intrinsic degrees of freedom of electrons.
Physical Review B 04/2015; 92(12). DOI:10.1103/PhysRevB.92.121403 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We derive a general scaling relation for the anomalous Hall effect in
ferromagnetic metals involving multiple competing scattering mechanisms,
described by a quadratic hypersurface in the space spanned by the partial
resistivities. We also present experimental findings, which show strong
deviation from previously found scaling forms when different scattering
mechanism compete in strength but can be nicely explained by our theory.
[Show abstract][Hide abstract] ABSTRACT: The two inequivalent valleys in graphene preclude the protection against
inter-valley scattering offered by an odd-number of Dirac cones characteristic
of Z2 topological insulator phases. Here we propose a way to engineer a chiral
single-valley metallic phase with quadratic crossover in a honeycomb lattice
through tailored \sqrt{3}N *\sqrt{3}N or 3N *3N superlattices. The possibility
of tuning valley-polarization via pseudo-Zeeman field and the emergence of
Dresselhaus-type valley-orbit coupling are proposed in adatom decorated
graphene superlattices. Such valley manipulation mechanisms and metallic phase
can also find applications in honeycomb photonic crystals.
Physical Review B 01/2015; 91(24). DOI:10.1103/PhysRevB.91.245415 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: The dynamics of the nonlinear generalized quantum delta-kicked rotator is investigated with different initial states, under quantum anti-resonance (AR) and quantum resonance (QR) conditions. With the help of an analytical stationary solution of nonlinear Schrodinger equation with periodic condition, the relationship between the different stationary solutions obtained by Carr et al. [Phys. Rev. A 62, 063610 (2000)] is presented. We find that for weak nonlinearity, the quantum beating does not depend on the initial states except the beating frequency for the AR case, whereas the different rates of suppression depend on the initial states for the QR case. It is interesting to note that the energy of the system evolving in a truly irregular manner for large nonlinearity is bounded within some limits for both AR and QR conditions.
Romanian Reports in Physics 01/2015; 67(1):207-221. · 1.52 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Within the wave-packet semiclassical approach, the Bloch electron energy is
derived to second order in the magnetic field and classified into
gauge-invariant terms with clear physical meaning, yielding a fresh
understanding of the complex behavior of orbital magnetism. The Berry curvature
and quantum metric of the Bloch states give rise to a geometrical magnetic
susceptibility, which can be dominant when bands are filled up to a small
energy gap. There is also an energy polarization term, which can compete with
the Peierls-Landau and Pauli magnetism on a Fermi surface. All these, and an
additional Langevin susceptibility, can be calculated from each single band,
leaving the Van Vleck susceptibility as the only term truly from interband
coupling.
[Show abstract][Hide abstract] ABSTRACT: A topological insulator is a novel state of quantum matter, characterized by symmetry-protected Dirac interfacial states within its bulk gap. Tremendous effort has been invested into the search for topological insulators. To date, the discovery of topological insulators has been largely limited to natural crystalline solids. Therefore, it is highly desirable to tailor-make various topological states of matter by design, starting with but a few accessible materials or elements. Here, we establish that valley-dependent dimerization of Dirac surface states can be exploited to induce topological quantum phase transitions, in a binary superlattice bearing symmetry-unrelated interfacial Dirac states. This mechanism leads to a rich phase diagram and allows for rational design of strong topological insulators, weak topological insulators, and topological crystalline insulators. Our ab initio simulations further demonstrate this mechanism in [111] and [110] superlattices of calcium and tin tellurides. While our results reveal a remarkable phase diagram for the binary superlattice, the mechanism is a general route to design various topological states.
[Show abstract][Hide abstract] ABSTRACT: Bilayer graphene is susceptible to a family of unusual broken symmetry states with spin and valley dependent layer polarization. We report on a microscopic study of the domain walls in these systems, demonstrating that they have interesting microscopic structures related to order-induced topological characters. We use our results to estimate Ginzburg-Landau model parameters and transition temperatures for the ordered states of bilayer graphene. Introduction.— Neutral bilayer graphene (BLG) [1, 2] and its ABC-stacked multilayer cousins [3–6], are attrac-tive platforms for unconventional two-dimensional elec-tron systems physics because they have flat band contact near their Fermi levels, and because order induces large momentum-space Berry curvatures [6] in their quasiparti-cle bands. Theoretical studies have identified a variety of potential broken symmetry states in neutral suspended BLG [6–29]. The band eigenstates in bilayer graphene are equal weight coherent sums of components local-ized in each layer, and have an interlayer phase that is strongly wavevector dependent. When lattice-scale cor-rections to bilayer graphene's massive Dirac model [1, 2] are neglected, the broken symmetry states predicted by mean-field theory have a charged quasiparticle energy gap [6, 7, 10–12] and spontaneous layer polarization within each of the four spin-valley flavors. Recent ex-periments [30–38] appear to rule out a competing family of nematic states [13–15], which do not have a quasipar-ticle gap and break rotational symmetry [39].
[Show abstract][Hide abstract] ABSTRACT: The valley dependent optical selection rules in recently discovered monolayer
group-VI transition metal dichalcogenides (TMDs) make possible optical control
of valley polarization, a crucial step towards valleytronic applications.
However, in presence of Landaul level(LL) quantization such selection rules are
taken over by selection rules between the LLs, which are not necessarily valley
contrasting. Using MoS$_{2}$ as an example we show that the spatial
inversion-symmetry breaking results in unusual valley dependent inter-LL
selection rules, which directly locks polarization to valley. We find a
systematic valley splitting for all Landau levels (LLs) in the quantum Hall
regime, whose magnitude is linearly proportional to the magnetic field and in
comparable with the LL spacing. Consequently, unique plateau structures are
found in the optical Hall conductivity, which can be measured by the
magneto-optical Faraday rotations.
Physical Review B 07/2014; 90:045427. DOI:10.1103/PhysRevB.90.045427 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We study the anomalous Nernst effect (ANE) and anomalous Hall effect (AHE) in
proximity-induced ferromagnetic palladium and platinum which is widely used in
spintronics, within the Berry phase formalism based on the relativistic band
structure calculations. We find that both the anomalous Hall ($\sigma_{xy}^A$)
and Nernst ($\alpha_{xy}^A$) conductivities can be related to the spin Hall
conductivity ($\sigma_{xy}^S$) and band exchange-splitting ($\Delta_{ex}$) by
relations $\sigma_{xy}^A =\Delta_{ex}\frac{e}{\hbar}\sigma_{xy}^S(E_F)'$ and
$\alpha_{xy}^A =
-\frac{\pi^2}{3}\frac{k_B^2T\Delta_{ex}}{\hbar}\sigma_{xy}^s(\mu)"$,
respectively. In particular, these relations would predict that the
$\sigma_{xy}^A$ in the magnetized Pt (Pd) would be positive (negative) since
the $\sigma_{xy}^S(E_F)'$ is positive (negative). Furthermore, both
$\sigma_{xy}^A$ and $\alpha_{xy}^A$ are approximately proportional to the
induced spin magnetic moment ($m_s$) because the $\Delta_{ex}$ is a linear
function of $m_s$. Using the reported $m_s$ in the magnetized Pt and Pd, we
predict that the intrinsic anomalous Nernst conductivity (ANC) in the magnetic
platinum and palladium would be gigantic, being up to ten times larger than,
e.g., iron, while the intrinsic anomalous Hall conductivity (AHC) would also be
significant.
Physical Review B 06/2014; 89(21). DOI:10.1103/PhysRevB.89.214406 · 3.74 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An intersection between one-dimensional chiral acts as a topological current splitter. We find that the splitting of a chiral zero-line mode obeys very simple, yet highly counterintuitive, partition laws which relate current paths to the geometry of the intersection. Our results have far reaching implications for device proposals based on chiral zero-line transport in the design of electron beam splitters and interferometers, and for understanding transport properties in systems where multiple topological domains lead to a statistical network of chiral channels. A massive chiral two-dimensional electron gas (C2DEG) has a valley Hall conductivity that has the same sign as its mass. The valley Hall effect leads to conducting edge states and also, when the mass param-eter varies spatially, to conducting states localized along mass zero-lines. 1–3 Provided that inter-valley scattering is weak, zero-line state properties are closely analogous 1–3
[Show abstract][Hide abstract] ABSTRACT: We derive the field correction to the Berry curvature of Bloch electrons, which can be traced back to a positional shift due to the interband mixing induced by external electromagnetic fields. The resulting semiclassical dynamics is accurate to second order in the fields, in the same form as before, provided that the wave packet energy is derived up to the same order. As applications, we discuss the orbital magnetoelectric polarizability and predict nonlinear anomalous Hall effects.
[Show abstract][Hide abstract] ABSTRACT: Spin pumping and spin-transfer torques are two widely studied reciprocal
phenomena in ferromagnets. However, pumping phenomena in homogeneous
antiferromagnets and their relations to current-induced torques have not been
explored. By calculating how electrons scatter off a normal
metal-antiferromagnetic interface, we derive pumped spin and staggered spin
currents in terms of the staggered field, the magnetization, and their rates of
change. For both compensated and uncompensated interfaces, spin pumping is
large and of a similar magnitude with a direction controlled by the microwave
polarization. The pumped currents are connected to current-induced torques via
Onsager reciprocity relations.
[Show abstract][Hide abstract] ABSTRACT: We derive the field correction to the Berry curvature of Bloch electrons,
which can be traced back to a positional shift due to the interband mixing
induced by external electromagnetic fields. The resulting semiclassical
dynamics is accurate to second order in the fields, in the same form as before,
provided that the wave packet energy is derived up to the same order. As
applications, we discuss the orbital magnetoelectric polarizability and predict
nonlinear anomalous Hall effects.
[Show abstract][Hide abstract] ABSTRACT: We propose realizing the quantum anomalous Hall effect by proximity coupling graphene to an antiferromagnetic insulator that provides both broken time-reversal symmetry and spin-orbit coupling. We illustrate our idea by performing ab initio calculations for graphene adsorbed on the (111) surface of BiFeO3. In this case, we find that the proximity-induced exchange field in graphene is about 70 meV, and that a topologically nontrivial band gap is opened by Rashba spin-orbit coupling. The size of the gap depends on the separation between the graphene and the thin film substrate, which can be tuned experimentally by applying external pressure.
[Show abstract][Hide abstract] ABSTRACT: Quantum transport measurements including the Altshuler-Aronov-Spivak (AAS)
and Aharonov-Bohm (AB) effects, universal conductance fluctuations (UCF), and
weak anti-localization (WAL) have been carried out on epitaxial Bi thin films
($10-70$ bilayers) on Si(111). The results show that while the film interior is
insulating all six surfaces of the Bi thin films are robustly metallic. We
propose that these properties are the manifestation of a novel phenomenon,
namely, a topologically trivial bulk system can become topologically
non-trivial when it is made into a thin film. We stress that what's observed
here is entirely different from the predicted 2D topological insulating state
in a single bilayer Bi where only the four side surfaces should possess
topologically protected gapless states.
[Show abstract][Hide abstract] ABSTRACT: Large bulk band gap is critical for the application of the quantum spin Hall
(QSH) insulator or two dimensional (2D) Z2 topological insulator (TI) in
spintronic device operating at room temperature (RT). Based on the
first-principles calculations, here we predict a group of 2D topological
insulators BiX/SbX (X = H, F, Cl, and Br) with extraordinarily large bulk gaps
from 0.32 to a record value of 1.08 eV. These giant-gaps are entirely due to
the result of strong spin-orbit interaction being related to the px and py
orbitals of the Bi/Sb atoms around the two valley K and K' of honeycomb
lattice, which is different significantly from the one consisted of pz orbital
just like in graphene and silicene. The topological characteristic of BiX/SbX
monolayers is confirmed by the calculated nontrivial Z2 index and an explicit
construction of the low energy effective Hamiltonian in these systems. We show
that the honeycomb structure of BiX remains stable even at a temperature of 600
K. These features make the giant-gap Tls BiX/SbX an ideal platform to realize
many exotic phenomena and fabricate new quantum devices operating at RT.
[Show abstract][Hide abstract] ABSTRACT: When a spin-polarized current flows through a ferromagnetic (FM) metal,
angular momentum is transferred to the magnetization via spin transfer torques.
In antiferromagnetic (AFM) materials, however, the corresponding problem is
unsolved. We derive microscopically the dynamics of an AFM system driven by
spin current generated by an attached FM polarizer, and find that the spin
current exerts a driving force on the local staggered order parameter. The
mechanism does not rely on the conservation of spin angular momentum, nor does
it depend on the induced FM moments on top the AFM background. Two examples are
studied: (i) A domain wall is accelerated to a terminal velocity by purely
adiabatic effect where the Walker's break-down is avoided. (ii) Spin injection
modifies the AFM resonance frequency, and spin current injection triggers spin
wave instability of local moments above a threshold.
Physical Review B 02/2014; 89(081105(R)). DOI:10.1103/PhysRevB.89.081105 · 3.74 Impact Factor