Abstract
The quantum Hall effect (QHE) is traditionally considered to be a purely two-dimensional (2D) phenomenon. Recently, however, a three-dimensional (3D) version of the QHE was reported in the ...Dirac semimetal ZrTe
5
. It was proposed to arise from a magnetic-field-driven Fermi surface instability, transforming the original 3D electron system into a stack of 2D sheets. Here, we report thermodynamic, spectroscopic, thermoelectric and charge transport measurements on such ZrTe
5
samples. The measured properties: magnetization, ultrasound propagation, scanning tunneling spectroscopy, and Raman spectroscopy, show no signatures of a Fermi surface instability, consistent with in-field single crystal X-ray diffraction. Instead, a direct comparison of the experimental data with linear response calculations based on an effective 3D Dirac Hamiltonian suggests that the quasi-quantization of the observed Hall response emerges from the interplay of the intrinsic properties of the ZrTe
5
electronic structure and its Dirac-type semi-metallic character.
The minimal ingredients to explain the essential physics of layered copper-oxide (cuprates) materials remains heavily debated. Effective low-energy single-band models of the copper-oxygen orbitals ...are widely used because there exists no strong experimental evidence supporting multi-band structures. Here, we report angle-resolved photoelectron spectroscopy experiments on La-based cuprates that provide direct observation of a two-band structure. This electronic structure, qualitatively consistent with density functional theory, is parametrised by a two-orbital (Formula: see text and Formula: see text) tight-binding model. We quantify the orbital hybridisation which provides an explanation for the Fermi surface topology and the proximity of the van-Hove singularity to the Fermi level. Our analysis leads to a unification of electronic hopping parameters for single-layer cuprates and we conclude that hybridisation, restraining d-wave pairing, is an important optimisation element for superconductivity.
The analysis of functional connectivity at rest (rFC) enables us to know how brain regions within and between networks interact. In this study, we used resting-state functional magnetic resonance ...imaging and a creativity test of divergent thinking (DT) to investigate the relationship between creativity measured by DT and rFC. We took the medial prefrontal cortex (mPFC) to be the seed region and investigated correlations across subjects between the score of the DT test and the strength of rFC between the mPFC and other brain regions. Our results showed that the strength of rFC with the mPFC significantly and positively correlated with creativity as measured by the DT test in the posterior cingulate cortex (PCC). These results showed that higher creativity measured by DT is associated with rFC between the mPFC and the PCC, the key nodes of the default mode network (DMN). Increased rFC between these regions is completely opposite from that is generally expected from the association between higher creativity and reduced deactivation in DMN during an externally directed attention-demanding task shown in our previous study but is similar to the pattern seen in relatives of schizophrenia. These findings are comparable to the previously reported psychological associations between schizotypy and creativity.
Abstract
A paradigmatic case of multi-band Mott physics including spin-orbit and Hund’s coupling is realized in Ca
2
RuO
4
. Progress in understanding the nature of this Mott insulating phase has ...been impeded by the lack of knowledge about the low-energy electronic structure. Here we provide—using angle-resolved photoemission electron spectroscopy—the band structure of the paramagnetic insulating phase of Ca
2
RuO
4
and show how it features several distinct energy scales. Comparison to a simple analysis of atomic multiplets provides a quantitative estimate of the Hund’s coupling
J
=0.4 eV. Furthermore, the experimental spectra are in good agreement with electronic structure calculations performed with Dynamical Mean-Field Theory. The crystal field stabilization of the
d
xy
orbital due to
c
-axis contraction is shown to be essential to explain the insulating phase. These results underscore the importance of multi-band physics, Coulomb interaction and Hund’s coupling that together generate the Mott insulating state of Ca
2
RuO
4
.
Abstract Quantum fluctuations in low-dimensional systems and near quantum phase transitions have significant influences on material properties. Yet, it is difficult to experimentally gauge the ...strength and importance of quantum fluctuations. Here we provide a resonant inelastic x-ray scattering study of magnon excitations in Mott insulating cuprates. From the thin film of SrCuO 2 , single- and bi-magnon dispersions are derived. Using an effective Heisenberg Hamiltonian generated from the Hubbard model, we show that the single-magnon dispersion is only described satisfactorily when including significant quantum corrections stemming from magnon-magnon interactions. Comparative results on La 2 CuO 4 indicate that quantum fluctuations are much stronger in SrCuO 2 suggesting closer proximity to a magnetic quantum critical point. Monte Carlo calculations reveal that other magnetic orders may compete with the antiferromagnetic Néel order as the ground state. Our results indicate that SrCuO 2 —due to strong quantum fluctuations—is a unique starting point for the exploration of novel magnetic ground states.
Relativistic massless Dirac fermions can be probed with high-energy physics experiments, but appear also as low-energy quasi-particle excitations in electronic band structures. In condensed matter ...systems, their massless nature can be protected by crystal symmetries. Classification of such symmetry-protected relativistic band degeneracies has been fruitful, although many of the predicted quasi-particles still await their experimental discovery. Here we reveal, using angle-resolved photoemission spectroscopy, the existence of two-dimensional type-II Dirac fermions in the high-temperature superconductor La
Sr
CuO
. The Dirac point, constituting the crossing of Formula: see text and Formula: see text bands, is found approximately one electronvolt below the Fermi level (E
) and is protected by mirror symmetry. If spin-orbit coupling is considered, the Dirac point degeneracy is lifted and the bands acquire a topologically non-trivial character. In certain nickelate systems, band structure calculations suggest that the same type-II Dirac fermions can be realised near E
.
We present a soft x-ray angle-resolved photoemission spectroscopy study of overdoped high-temperature superconductors. In-plane and out-of-plane components of the Fermi surface are mapped by varying ...the photoemission angle and the incident photon energy. No k_{z} dispersion is observed along the nodal direction, whereas a significant antinodal k_{z} dispersion is identified for La-based cuprates. Based on a tight-binding parametrization, we discuss the implications for the density of states near the van Hove singularity. Our results suggest that the large electronic specific heat found in overdoped La_{2-x}Sr_{x}CuO_{4} cannot be assigned to the van Hove singularity alone. We therefore propose quantum criticality induced by a collapsing pseudogap phase as a plausible explanation for observed enhancement of electronic specific heat.
Abstract
The magnetic ground states in highly ordered double perovskites LaSr
1−
x
Ca
x
NiReO
6
(
x
= 0.0, 0.5, 1.0) are studied in view of the Goodenough-Kanamori rules of superexchange ...interactions in this paper. In LaSrNiReO
6
, Ni and Re sublattices are found to exhibit curious magnetic states separately, but no long range magnetic ordering is achieved. The magnetic transition at ~255 K is identified with the independent Re sublattice magnetic ordering. Interestingly, the sublattice interactions are tuned by modifying the Ni-O-Re bond angles through Ca doping. Upon Ca doping, the Ni and Re sublattices start to display a ferrimagnetically ordered state at low temperature. The neutron powder diffraction data reveals long range ferrimagnetic ordering of the Ni and Re magnetic sublattices along the crystallographic
b-
axis. The transition temperature of the ferrimagnetic phase increases monotonically with increasing Ca concentration.
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