246 research outputs found
Disrupted orbital order and the pseudo-gap in layered 1T-TaS
We present a state-of-the-art density functional theory (DFT) study which
models crucial features of the partially disordered orbital order stacking in
the prototypical layered transition metal dichalcogenide 1T-TaS2 . Our results
not only show that DFT models with realistic assumptions about the orbital
order perpendicular to the layers yield band structures which agree remarkably
well with experiments. They also demonstrate that DFT correctly predicts the
formation of an excitation pseudo-gap which is commonly attributed to
Mott-Hubbard type electron-electron correlations. These results highlight the
importance of interlayer interactions in layered transition metal
dichalcogenides and serve as an intriguing example of how disorder within an
electronic crystal can give rise to pseudo-gap features
Torque magnetometry study of magnetically ordered state and spin reorientation in the quasi-one-dimensional Heisenberg antiferromagnet CuSbO
We present an experimental study of macroscopic and microscopic magnetic
anisotropy of a spin tetramer system \cso using torque magnetometry and ESR
spectroscopy. Large rotation of macroscopic magnetic axes with temperature
observed from torque magnetometry agrees reasonably well with the rotation of
the tensor above ~K. Below 50~K, the
tensor is temperature independent, while macroscopic magnetic axes continue to
rotate. Additionally, the susceptibility anisotropy has a temperature
dependence which cannot be reconciled with the isotropic Heisenberg model of
interactions between spins. ESR linewidth analysis shows that anisotropic
exchange interaction must be present in \csos. These findings strongly support
the presence of anisotropic exchange interactions in the Hamiltonian of the
studied system. Below ~K, the system enters a long - range
antiferromagnetically ordered state with easy axis along the
direction. Small but significant rotation of magnetic axes is also observed in
the antiferromagnetically ordered state suggesting strong spin-lattice coupling
in this system.Comment: 10 pages, 10 figure
Interplay of electronic correlations and lattice instabilities in BaVS3
The quasi-one-dimensional metallic system BaVS3 with a metal-insulator
transition at T_MI=70 K shows large changes of the optical phonon spectrum, a
central peak, and an electronic Raman scattering continuum that evolve in a
three-step process. Motivated by the observation of a strongly fluctuating
precursor state at high temperatures and orbital ordering and a charge gap at
low temperatures we suggest a concerted action of the orbital, electronic, and
lattice subsystems dominated by electronic correlations.Comment: 4 pages, 4 figure
Mono- and Bilayer WS2 Light-Emitting Transistors
We have realized ambipolar ionic liquid gated field-effect transistors based
on WS2 mono- and bilayers, and investigated their opto-electronic response. A
thorough characterization of the transport properties demonstrates the high
quality of these devices for both electron and hole accumulation, which enables
the quantitative determination of the band gap ({\Delta}1L = 2.14 eV for
monolayers and {\Delta}2L = 1.82 eV for bilayers). It also enables the
operation of the transistors in the ambipolar injection regime with electrons
and holes injected simultaneously at the two opposite contacts of the devices
in which we observe light emission from the FET channel. A quantitative
analysis of the spectral properties of the emitted light, together with a
comparison with the band gap values obtained from transport, show the internal
consistency of our results and allow a quantitative estimate of the excitonic
binding energies to be made. Our results demonstrate the power of ionic liquid
gating in combination with nanoelectronic systems, as well as the compatibility
of this technique with optical measurements on semiconducting transition metal
dichalcogenides. These findings further open the way to the investigation of
the optical properties of these systems in a carrier density range much broader
than that explored until now.Comment: 22 pages, 6 figures, Nano Letters (2014
Scanning photocurrent microscopy reveals electron-hole asymmetry in ionic liquid-gated WS2 transistors
We perform scanning photocurrent microscopy on WS2 ionic liquid-gated field
effect transistors exhibiting high-quality ambipolar transport. By properly
biasing the gate electrode we can invert the sign of the photocurrent showing
that the minority photocarriers are either electrons or holes. Both in the
electron- and the hole-doping regimes the photocurrent decays exponentially as
a function of the distance between the illumination spot and the nearest
contact, in agreement with a two-terminal Schottky-barrier device model. This
allows us to compare the value and the doping dependence of the diffusion
length of the minority electrons and holes on a same sample. Interestingly, the
diffusion length of the minority carriers is several times larger in the hole
accumulation regime than in the electron accumulation regime, pointing out an
electron-hole asymmetry in WS2
Magnetic order and transitions in the spin-web compound Cu3TeO6
The spin-web compound Cu3TeO6, belongs to an intriguing group of materials
where magnetism is governed by 3d9 copper Cu2+ ions. This compound has been
sparsely experimentally studied and we here present the first investigation of
its local magnetic properties using muon-spin relaxation/rotation ({\mu}+SR).
Our results show a clear long-range 3D magnetic order below TN as indicated by
clear zero-field (ZF) muon-precessions. At TN = 61.7 K a very sharp transition
is observed in the weak transverse-field (wTF) as well as ZF data. Contrary to
suggestions by susceptibility measurements and inelastic neutron scattering, we
find no evidence for either static or dynamic (on the time-scale of {\mu}+SR)
spin-correlations above TN
Giant Kohn anomaly and the phase transition in charge density wave ZrTe_3
A strong Kohn anomaly in ZrTe_3 is identified in the mostly transverse
acoustic phonon branch along the modulation vector q_P with polarization along
the a* direction. This soft mode freezes to zero frequency at the transition
temperature T_P and the temperature dependence of the frequency is strongly
affected by fluctuation effects. Diffuse x-ray scattering of the incommensurate
superstructure shows a power law scaling of the intensity and the correlation
length that is compatible with an order parameter of dimension n = 2.Comment: 4 pages, 4 figures. accepted at Phys. Rev. Let
Strong interface-induced spin-orbit coupling in graphene on WS2
Interfacial interactions allow the electronic properties of graphene to be
modified, as recently demonstrated by the appearance of satellite Dirac cones
in the band structure of graphene on hexagonal boron nitride (hBN) substrates.
Ongoing research strives to explore interfacial interactions in a broader class
of materials in order to engineer targeted electronic properties. Here we show
that at an interface with a tungsten disulfide (WS2) substrate, the strength of
the spin-orbit interaction (SOI) in graphene is very strongly enhanced. The
induced SOI leads to a pronounced low-temperature weak anti-localization (WAL)
effect, from which we determine the spin-relaxation time. We find that
spin-relaxation time in graphene is two-to-three orders of magnitude smaller on
WS2 than on SiO2 or hBN, and that it is comparable to the intervalley
scattering time. To interpret our findings we have performed first-principle
electronic structure calculations, which both confirm that carriers in
graphene-on-WS2 experience a strong SOI and allow us to extract a
spin-dependent low-energy effective Hamiltonian. Our analysis further shows
that the use of WS2 substrates opens a possible new route to access topological
states of matter in graphene-based systems.Comment: Originally submitted version in compliance with editorial guidelines.
Final version with expanded discussion of the relation between theory and
experiments to be published in Nature Communication
Magnetic anisotropy of spin tetramer system SeCuO studied by torque magnetometry and ESR spectroscopy
We present an experimental study of macroscopic and microscopic magnetic
anisotropy of a spin tetramer system SeCuO using torque magnetometry and
ESR spectroscopy. Large rotation of macroscopic magnetic axes with temperature
observed from torque magnetometry agrees reasonably well with the rotation of
the tensor above ~K. Below 50~K, the
tensor is temperature independent, while macroscopic magnetic axes continue to
rotate. Additionally, the susceptibility anisotropy has a temperature
dependence which cannot be reconciled with the isotropic Heisenberg model of
interactions between spins. ESR linewidth analysis shows that anisotropic
exchange interaction must be present in SeCuO. These findings strongly
support the presence of anisotropic exchange interactions in the Hamiltonian of
the studied system. Below ~K, the system enters a long - range
antiferromagnetically ordered state with easy axis along the
direction. Small but significant rotation of magnetic axes is also observed in
the antiferromagnetically ordered state suggesting strong spin-lattice coupling
in this system.Comment: 14 pages, 13 figure
Ambipolar Nernst effect in NbSe
The first study of Nernst effect in NbSe reveals a large quasi-particle
contribution with a magnitude comparable and a sign opposite to the vortex
signal. Comparing the effect of the Charge Density Wave(CDW) transition on Hall
and Nernst coefficients, we argue that this large Nernst signal originates from
the thermally-induced counterflow of electrons and holes and indicates a
drastic change in the electron scattering rate in the CDW state. The results
provide new input for the debate on the origin of the anomalous Nernst signal
in high-T cuprates.Comment: 5 pages including 4 figure
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