19 research outputs found
Balanced electron-hole transport in spin-orbit semimetal SrIrO3 heterostructures
Relating the band structure of correlated semimetals to their transport
properties is a complex and often open issue. The partial occupation of
numerous electron and hole bands can result in properties that are seemingly in
contrast with one another, complicating the extraction of the transport
coefficients of different bands. The 5d oxide SrIrO3 hosts parabolic bands of
heavy holes and light electrons in gapped Dirac cones due to the interplay
between electron-electron interactions and spin-orbit coupling. We present a
multifold approach relying on different experimental techniques and theoretical
calculations to disentangle its complex electronic properties. By combining
magnetotransport and thermoelectric measurements in a field-effect geometry
with first-principles calculations, we quantitatively determine the transport
coefficients of different conduction channels. Despite their different
dispersion relationships, electrons and holes are found to have strikingly
similar transport coefficients, yielding a holelike response under field-effect
and thermoelectric measurements and a linear, electronlike Hall effect up to 33
T.Comment: 5 pages, 4 figure
Suppression of superconductivity and enhanced critical field anisotropy in thin flakes of FeSe
FeSe is a unique superconductor that can be manipulated to enhance its superconductivity using different routes, while ist monolayer form grown on different substrates reaches a record high temperature for a two-dimensional system. In order to understand the role played by the substrate and the reduced dimensionality on superconductivity, we examine the superconducting properties of exfoliated FeSe thin flakes by reducing the thickness from bulk down towards 9 nm. Magnetotransport measurements performed in magnetic fields up to 16 T and temperatures down to 2 K help to build up complete superconducting phase diagrams of different thickness flakes. While the thick flakes resemble the bulk behaviour, by reducing the thickness the superconductivity of FeSe flakes is suppressed. The observation of the vortex-antivortex unbinding transition in different flakes provide a direct signature of a dominant two-dimensional pairing channel. However, the upper critical field reflects the evolution of the multi-band nature of superconductivity in FeSe becoming highly two-dimensional and strongly anisotropic only in the thin limit. Our study provides detailed insights into the evolution of the superconducting properties of a multi-band superconductor FeSe in the thin limit in the absence of a dopant substrate
Significant change in the electronic behavior associated with structural distortions in the single crystalline SrAg4As2
We report a combined study of transport and thermodynamic measurements on the
layered pnictide material SrAg4As2. Upon cooling, a drop in electrical and Hall
resistivity, a jump in heat capacity and an increase in susceptibility and
magnetoresistance (MR) are observed around 110 K. These observations suggest
that non-magnetic phase transitions emerge at around 110 K, that are likely
associated with structural distortions. In sharp contrast with the
first-principles calculations based on the crystal structure at room
temperature, quantum oscillations reveal small Fermi pockets with light
effective masses, suggesting a significant change in the Fermi surface topology
caused by the low temperature structural distortion. No superconductivity
emerges in SrAgAs down to 2 K and under pressures up to 2.13 GPa;
instead, the low temperature structural distortion increases linearly with
temperature at a rate of ~13 K/GPa above 0.89 GPa
Unconventional localization of electrons inside of a nematic electronic phase
The magnetotransport behaviour inside the nematic phase of bulk FeSe reveals
unusual multiband effects that cannot be reconciled with a simple two-band
approximation proposed by surface-sensitive spectroscopic probes. In order to
understand the role played by the multiband electronic structure and the degree
of two-dimensionality we have investigated the electronic properties of
exfoliated flakes of FeSe by reducing their thickness. Based on
magnetotransport and Hall resistivity measurements, we assess the mobility
spectrum that suggests an unusual asymmetry between the mobilities of the
electrons and holes with the electron carriers becoming localized inside the
nematic phase. Quantum oscillations in magnetic fields up to 38 T indicate the
presence of a hole-like quasiparticle with a lighter effective mass and a
quantum scattering time three times shorter, as compared with bulk FeSe. The
observed localization of negative charge carriers by reducing dimensionality
can be driven by orbitally-dependent correlation effects, enhanced interband
spin-fluctuations or a Lifshitz-like transition which affect mainly the
electron bands. The electronic localization leads to a fragile two-dimensional
superconductivity in thin flakes of FeSe, in contrast to the two-dimensional
high-Tc induced with electron doping via dosing or using a suitable interface.Comment: 22 pages, 14 figure
Lifshitz transition enabling superconducting dome around the quantum critical point in TiSe
Superconductivity often emerges as a dome around a quantum critical point
(QCP) where long-range order is suppressed to zero temperature. So far, this
has been mostly studied in magnetically ordered materials. By contrast, the
interplay between charge order and superconductivity at a QCP is not fully
understood. Here, we present resistance measurements proving that a dome of
superconductivity surrounds the charge-density-wave (CDW) QCP in pristine
samples of 1-TiSe tuned with hydrostatic pressure. Furthermore, we use
quantum oscillation measurements to show that the superconductivity sets in at
a Lifshitz transition in the electronic band structure. We use density
functional theory to identify the Fermi pockets enabling superconductivity:
large electron and hole pockets connected by the CDW wave vector
which emerge upon partial suppression of the zero-pressure CDW gap. Hence, we
conclude that superconductivity is of interband type enabled by the presence of
hole and electron bands connected by the CDW vector. Earlier
calculations show that interband interactions are repulsive, which suggests
that unconventional s superconductivity is realised in TiSe -
similar to the iron pnictides. These results highlight the importance of
Lifshitz transitions in realising unconventional superconductivity and help
understand its interaction with CDW order in numerous materials.Comment: 21 pages, 5 figure
Quenching a Weyl-Kondo semimetal by magnetic field
With the advent of topology in electronic materials the number of predicted
quantum phases has literally exploded. Most of them, however, still await firm
experimental identification. In strongly correlated electron systems, scanning
their low-temperature phase diagrams by varying a nonthermal control parameter
has been instrumental in delineating phases defined by a Landau order
parameter. Here we show that this approach is versatile also for strongly
correlated topological phases. We use Hall effect measurements to probe how the
time reversal symmetry invariant Weyl-Kondo semimetal CeBiPd
transforms under magnetic-field tuning. We detect an intriguing two-stage
transition, which we associate with an annihilation of the Weyl nodes, making
the system more insulating, and a consecutive transition to a heavy fermion
metal phase. We expect our work to stimulate tuning studies in related systems,
thereby advancing the much needed identification of organizing principles for
strongly correlated electronic topology.Comment: 4 figures, 19 page
Truncated mass divergence in a Mott metal
The Mott metal–insulator transition represents one of the most fundamental phenomena in condensed matter physics. Yet, basic tenets of the canonical Brinkman-Rice picture of Mott localization remain to be tested experimentally by quantum oscillation measurements that directly probe the quasiparticle Fermi surface and effective mass. By extending this technique to high pressure, we have examined the metallic state on the threshold of Mott localization in clean, undoped crystals of NiS2. We find that i) on approaching Mott localization, the quasiparticle mass is strongly enhanced, whereas the Fermi surface remains essentially unchanged; ii) the quasiparticle mass closely follows the divergent form predicted theoretically, establishing charge carrier slowdown as the driver for the metal–insulator transition; iii) this mass divergence is truncated by the metal–insulator transition, placing the Mott critical point inside the insulating section of the phase diagram. The inaccessibility of the Mott critical point in NiS2 parallels findings at the threshold of ferromagnetism in clean metallic systems, in which criticality at low temperature is almost universally interrupted by first-order transitions or novel emergent phases such as incommensurate magnetic order or unconventional superconductivity