32 research outputs found
Superconductivity in few-layer iron chalcogenide and transition metal dichalcogenide devices:(Alternative Format Thesis)
Probing magnetic exchange interactions with helium
Funding: CT and PW acknowledge support from EPSRC (EP/R031924/1) and CMY and LSF from EP/S005005/1. C.H. acknowledges support by the Austrian Science Fund (FWF) Project No. P32144-N36 and the VSC-4 of the Vienna University of Technology. The work was partially supported by the Deutsche Forschungsgemeinschaft (DFG) through Transregional Research Collaboration TRR 80 (Augsburg, Munich, and Stuttgart).Controlling and sensing spin-polarization of electrons forms the basis of spintronics. Here, we report a study of the effect of helium on the spin-polarization of the tunneling current and magnetic contrast in spin-polarized Scanning Tunneling Microscopy. We show that the magnetic contrast in SP-STM images recorded in the presence of helium depends sensitively on the tunneling conditions. From tunneling spectra and their variation across the atomic lattice we establish that the helium can be reversibly ejected from the tunneling junction by the tunneling electrons. The energy of the tunneling electrons required to eject the helium depends on the relative spin-polarization of the tip and sample, making the microscope sensitive to the magnetic exchange interactions. We show that the time-averaged spin polarization of the tunneling current is suppressed in the presence of helium and thereby demonstrate voltage control of the spin polarization of the tunneling current across the tip-sample junction.PostprintPeer reviewe
Superconducting Quantum Interference in Twisted van der Waals Heterostructures
Modern Superconducting QUantum Interference Devices (SQUIDs) are commonly
fabricated from either Al or Nb electrodes, with an in-situ oxidation process
to create a weak link between them. However, common problems of such planar
nano- and micro-SQUIDs are hysteretic current-voltage curves, and a shallow
flux modulation depth. Here, we demonstrate the formation of both Josephson
junctions and SQUIDs using a dry transfer technique to stack and
deterministically misalign flakes of NbSe; allowing one to overcome these
issues. The Josephson dynamics of the resulting twisted NbSe-NbSe
junctions are found to be sensitive to the misalignment angle of the
crystallographic axes. A single lithographic process was then implemented to
shape the Josephson junction into a SQUID geometry with typical loop areas of
25 and weak links 600 nm wide. These devices
display large stable current and voltage modulation depths of up to 75 and 1.4 mV respectively.Comment: 12 pages, 6 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
Optical response of the bulk stabilized mosaic phase in Se doped TaSSe
The layered van der Waals material, TaS features a meta-stable mosaic
phase on the verge of a nearly commensurate to commensurate charge density wave
transition. This meta-stable or 'hidden' phase can be reached by laser pumping
the low temperature, commensurate charge density wave phase. Here we report the
stabilization of a bulk, equilibrium mosaic phase in 1T-TaSSe
single crystals observed with transport and optical spectroscopy experiments.
We identify a bulk pseudogap in the mosaic phase of approximately 200 meV at
the lowest temperatures, while the CCDW phase can be obtained by heating and
instead has a full optical gap of about 100 meV. Surprisingly, a spectral
weight analysis shows that Se doping gives rise to an increased charge density
despite the fact that this is formally an isovalent substitution. This finding
is consistent with the recent observation that the mosaic phase is stabilized
as equilibrium phase through the appearance of charged defects.Comment: 7 pages, 3 figure
Unconventional localization of electrons inside of a nematic electronic phase
The magnetotransport behavior 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.</p
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
Correlation between crystal purity and the charge density wave in 1T-VSe2
We examine the charge density wave (CDW) properties of 1T-VSe2 crystals grown by chemical vapor transport (CVT) under varying conditions. Specifically, we find that upon lowering the growth temperature (Tg < 630\u25e6C), there is a significant increase in both the CDW transition temperature and the residual resistance ratio (RRR) obtained from electrical transport measurements. Using x-ray photoelectron spectroscopy, we correlate the observed CDW properties with stoichiometry and the nature of defects. In addition, we have optimized a method to grow ultrahigh-purity 1T-VSe2 crystals with a CDW transition temperature TCDW = (112.7 \ub1 0.8) K and maximum residual resistance ratio RRR 48 49, which is the highest reported thus far. This work highlights the sensitivity of the CDW in 1T-VSe2 to defects and overall stoichiometry and the importance of controlling the crystal growth conditions of strongly correlated transition metal dichalcogenides
Coherent phonons and the interplay between charge density wave and Mott phases in 1<i>T</i>-TaSe<sub>2</sub>
1-TaSe is host to coexisting strongly-correlated phases including
charge density waves (CDWs) and an unusual Mott transition at low temperature.
Here, we investigate coherent phonon oscillations in 1-TaSe using a
combination of time- and angle-resolved photoemission spectroscopy (TR-ARPES)
and time-resolved reflectivity (TRR). Perturbation by a femtosecond laser pulse
triggers a modulation of the valence band binding energy at the -point,
related to the Mott gap, that is consistent with the in-plane CDW amplitude
mode frequency. By contrast, TRR measurements show a modulation of the
differential reflectivity comprised of multiple frequencies belonging to the
distorted CDW lattice modes. Comparison of the temperature dependence of
coherent and spontaneous phonons across the CDW transition shows that the
amplitude mode intensity is more easily suppressed during perturbation of the
CDW state by the optical excitation compared to other modes. Our results
clearly identify the relationship of the in-plane CDW amplitude mode with the
Mott phase in 1-TaSe and highlight the importance of lattice degrees
of freedom.Comment: 7 pages, 4 figures, supplemental materia