37 research outputs found
Dissipative Quantum Hall Effect in Graphene near the Dirac Point
We report on the unusual nature of nu=0 state in the integer quantum Hall
effect (QHE) in graphene and show that electron transport in this regime is
dominated by counter-propagating edge states. Such states, intrinsic to
massless Dirac quasiparticles, manifest themselves in a large longitudinal
resistivity rho_xx > h/e^2, in striking contrast to rho_xx behavior in the
standard QHE. The nu=0 state in graphene is also predicted to exhibit
pronounced fluctuations in rho_xy and rho_xx and a smeared zero Hall plateau in
sigma_xy, in agreement with experiment. The existence of gapless edge states
puts stringent constraints on possible theoretical models of the nu=0 state.Comment: 4 pgs, 4 fg
Temperature-driven transition from a semiconductor to a topological insulator
We report on a temperature-induced transition from a conventional
semiconductor to a two-dimensional topological insulator investigated by means
of magnetotransport experiments on HgTe/CdTe quantum well structures. At low
temperatures, we are in the regime of the quantum spin Hall effect and observe
an ambipolar quantized Hall resistance by tuning the Fermi energy through the
bulk band gap. At room temperature, we find electron and hole conduction that
can be described by a classical two-carrier model. Above the onset of quantized
magnetotransport at low temperature, we observe a pronounced linear
magnetoresistance that develops from a classical quadratic low-field
magnetoresistance if electrons and holes coexist. Temperature-dependent bulk
band structure calculations predict a transition from a conventional
semiconductor to a topological insulator in the regime where the linear
magnetoresistance occurs.Comment: 7 pages, 6 figure
Symmetry and correlation effects on band structure explain the anomalous transport properties of (111) LaAlO/SrTiO
The interface between the two insulating oxides SrTiO and LaAlO gives
rise to a two-dimensional electron system with intriguing transport phenomena,
including superconductivity, which are controllable by a gate. Previous
measurements on the (001) interface have shown that the superconducting
critical temperature, the Hall density, and the frequency of quantum
oscillations, vary nonmonotonically and in a correlated fashion with the gate
voltage. In this paper we experimentally demonstrate that the (111) interface
features a qualitatively distinct behavior, in which the frequency of
Shubnikov-de Haas oscillations changes monotonically, while the variation of
other properties is nonmonotonic albeit uncorrelated. We develop a theoretical
model, incorporating the different symmetries of these interfaces as well as
electronic-correlation-induced band competition. We show that the latter
dominates at (001), leading to similar nonmonotonicity in all observables,
while the former is more important at (111), giving rise to highly curved Fermi
contours, and accounting for all its anomalous transport measurements.Comment: 6+7 pages, 4+6 figures, Published Versio
Observation of pseudo two dimensional electron transport in the rock salt type topological semimetal LaBi
Topological insulators are characterized by an inverted band structure in the
bulk and metallic surface states on the surface. In LaBi, a semimetal with a
band inversion equivalent to a topological insulator, we observe surface state
like behavior in the magnetoresistance. The electrons responsible for this
pseudo two dimensional transport, however, originate from the bulk states
rather topological surface states, which is witnessed by the angle dependent
quantum oscillations of the magnetoresistance and ab initio calculations. As a
consequence, the magnetoresistance exhibits strong anisotropy with large
amplitude (~ 10^5 %).Comment: 21 pages, 4 figures and supplementary informatio
Spin excitations of magnetoelectric LiNiPO in multiple magnetic phases
Spin excitations of magnetoelectric LiNiPO are studied by infrared
absorption spectroscopy in the THz spectral range as a function of magnetic
field through various commensurate and incommensurate magnetically ordered
phases up to 33\,T. Six spin resonances and a strong two-magnon continuum are
observed in zero magnetic field. Our systematic polarization study reveals that
some of the excitations are usual magnetic-dipole active magnon modes, while
others are either electromagnons, electric-dipole active, or magnetoelectric,
both electric- and magnetic-dipole active spin excitations. Field-induced
shifts of the modes for all three orientations of the field along the
orthorhombic axes allow us to refine the values of the relevant exchange
couplings, single-ion anisotropies, and the Dzyaloshinskii-Moriya interaction
on the level of a four-sublattice mean-field spin model. This model also
reproduces the spectral shape of the two-magnon absorption continuum, found to
be electric-dipole active in the experiment
Magnetoelectric effect and magnetic phase diagram of a polar ferrimagnet CaBaFe4O7
The magnetic phase diagram of a polar ferrimagnet
CaBaFe4O7 with a magnetic easy axis has been investigated by measurements of magnetization, specific heat, and magnetoelectricity. A ferrimagnetic transition takes place at TC1=275 K within the orthorhombic phase followed by a second magnetic transition at TC2=211 K. Below
TC2, successive metamagnetic transitions occur for magnetic fields applied perpendicular to the easy axis, implying a sequential emergence of magnetic states which are neither collinear nor coplanar. The observation of the static magnetoelectric effect was limited to temperatures below 120 K due to the conducting nature of the crystals at higher temperatures. The magnitude of the ferroelectric polarization shows large changes between the different field-induced magnetic phases. The low-field state is characterized by a large linear magnetoelectric coefficient of αcc=39 ps/m, while a gigantic polarization change of ΔP=850μC/m2 is observed for μoH=14 T applied along the easy axis
Design of compensated ferrimagnetic Heusler alloys for giant tunable exchange bias
The discovery of materials with improved functionality can be accelerated by
rational material design. Heusler compounds with tunable magnetic sublattices
allow to implement this concept to achieve novel magnetic properties. Here, we
have designed a family of Heusler alloys with a compensated ferrimagnetic
state. In the vicinity of the compensation composition in Mn-Pt-Ga, a giant
exchange bias (EB) of more than 3 T and a similarly large coercivity are
established. The large exchange anisotropy originates from the exchange
interaction between the compensated host and ferrimagnetic clusters that arise
from intrinsic anti-site disorder. We demonstrate the applicability of our
design concept on a second material, Mn-Fe-Ga, with a magnetic transition above
room temperature, exemplifying the universality of the concept and the
feasibility of room-temperature applications. Our study points to a new
direction for novel magneto-electronic devices. At the same time it suggests a
new route for realizing rare-earth free exchange-biased hard magnets, where the
second quadrant magnetization can be stabilized by the exchange bias.Comment: Four figure