36 research outputs found
Current-Voltage Characteristics of Weyl Semimetal Semiconducting Devices, Veselago Lenses and Hyperbolic Dirac Phase
The current-voltage characteristics of a new range of devices built around
Weyl semimetals has been predicted using the Landauer formalism. The potential
step and barrier have been reconsidered for a three-dimensional Weyl
semimetals, with analogies to the two-dimensional material graphene and to
optics. With the use of our results we also show how a Veselago lens can be
made from Weyl semimetals, e.g. from NbAs and NbP. Such a lens may have many
practical applications and can be used as a probing tip in a scanning tunneling
microscope (STM). The ballistic character of Weyl fermion transport inside the
semimetal tip, combined with the ideal focusing of the Weyl fermions (by
Veselago lens) on the surface of the tip may create a very narrow electron beam
from the tip to the surface of the studied material. With a Weyl semimetal
probing tip the resolution of the present STMs can be improved significantly,
and one may image not only individual atoms but also individual electron
orbitals or chemical bonding and therewith to resolve the long-term issue of
chemical and hydrogen bond formation. We show that applying a pressure to the
Weyl semimental, having no centre of spacial inversion one may model matter at
extreme conditions such as those arising in the vicinity of a black hole. As
the materials Cd3As2 and Na3Bi show an asymmetry in their Dirac cones, a
scaling factor was used to model this asymmetry. The scaling factor created
additional regions of no propagation and condensed the appearance of
resonances. We argue that under an external pressure there may arise a
topological phase transition in Weyl semimetals, where the electron transport
changes character and becomes anisotropic. There a hyperbolic Dirac phases
occurs where there is a strong light absorption and photo-current generation
Pressure Induced Changes in the Antiferromagnetic Superconductor YbPd2Sn
Low temperature ac magnetic susceptibility measurements of the coexistent
antiferromagnetic superconductor YbPd2Sn have been made in hydrostatic
pressures < 74 kbar in moissanite anvil cells. The superconducting transition
temperature is forced to T(SC) = 0 K at a pressure of 58 kbar. The initial
suppression of the superconducting transition temperature is corroborated by
lower hydrostatic pressure (p < 16 kbar) four point resisitivity measurements,
made in a piston cylinder pressure cell. At ambient pressure, in a modest
magnetic field of ~ 500 G, this compound displays reentrant superconducting
behaviour. This reentrant superconductivity is suppressed to lower temperature
and lower magnetic field as pressure is increased. The antiferromagnetic
ordering temperature, which was measured at T(N) = 0.12 K at ambient pressure
is enhanced, to reach T(N) = 0.58 K at p = 74 kbar. The reasons for the
coexistence of superconductivity and antiferromagnetism is discussed in the
light of these and previous findings. Also considered is why superconductivity
on the border of long range magnetic order is so much rarer in Yb compounds
than in Ce compounds. The presence of a new transition visible by ac magnetic
susceptibility under pressure and in magnetic fields greater than 1.5 kG is
suggested.Comment: 5 pages, 6 figure
The emergence of quantum capacitance in epitaxial graphene
We found an intrinsic redistribution of charge arises between epitaxial
graphene, which has intrinsically n-type doping, and an undoped substrate. In
particular, we studied in detail epitaxial graphene layers thermally elaborated
on C-terminated - (- ()). We have investigated
the charge distribution in graphene-substrate systems using Raman spectroscopy.
The influence of the substrate plasmons on the longitudinal optical phonons of
the substrates has been detected. The associated charge redistribution
reveals the formation of a capacitance between the graphene and the substrate.
Thus, we give for the first time direct evidence that the excess negative
charge in epitaxial monolayer graphene could be self-compensated by the
substrate without initial doping. This induced a previously unseen
redistribution of the charge-carrier density at the substrate-graphene
interface. There a quantum capacitor appears, without resorting to any
intentional external doping, as is fundamentally required for epitaxial
graphene. Although we have determined the electric field existing inside the
capacitor and revealed the presence of a minigap () for
epitaxial graphene on - face terminated carbon, it remains small in
comparison to that obtained for graphene on face terminated . The
fundamental electronic properties found here in graphene on substrates
may be important for developing the next generation of quantum technologies and
electronic/plasmonic devices.Comment: 26 pages, 8 figures, available online as uncorrected proof, Journal
of Materials Chemistry C (2016
Bistability and relaxor ferrimagnetism in off-stoichiometric NiCrO3
© 2017 Elsevier B.V. NiCrO 3 has been proposed as a likely candidate for antiferromagnetic half metallic behaviour. A sample prepared at high pressure adopts the corundum structure with Ni/Cr cation disorder, and is found to have off-stoichiometric composition Ni 0.80 Cr 1.20 O 3 . This material shows complex local magnetic ordering phenomena at temperatures below 120 K but without any long range spin order observed by neutron diffraction. The transition to local ferrimagnetism occurs at 50–100 K, with two distinct regimes at T C1  = 95 and T C2  = 53 K evidencing electronic phase separation driven by variations in local composition. At low temperature the system undergoes a further transition at T C3  = 22 K, assigned to potential freezing of a cluster glass-like state, that results in a substantial increase in magnetic anisotropy. Ni 0.80 Cr 1.20 O 3 is a bistable relaxor ferrimagnet where magnetic properties are linked to the lattice strain manifold that is determined by the peculiarities in the local chemical composition
Superconductivity Induced by Bond Breaking in the Triangular Lattice of IrTe2
IrTe2, a layered compound with a triangular iridium lattice, exhibits a
structural phase transition at approximately 250 K. This transition is
characterized by the formation of Ir-Ir bonds along the b-axis. We found that
the breaking of Ir-Ir bonds that occurs in Ir1-xPtxTe2 results in the
appearance of a structural critical point in the T = 0 limit at xc = 0.035.
Although both IrTe2 and PtTe2 are paramagnetic metals, superconductivity at Tc
= 3.1 K is induced by the bond breaking in a narrow range of x > xc in
Ir1-xPtxTe2. This result indicates that structural fluctuations can be involved
in the emergence of superconductivity.Comment: 10 pages, 4 figure
Possible high-pressure orbital quantum criticality and an emergent resistive phase in PbRuO<sub>3</sub>
The orbital ordering transition in the metallic perovskite PbRuO3 is suppressed from 90 K at ambient pressure towards zero temperature at 50 kbar, where non-Fermi liquid resistivity with a temperature exponent n = 1.6 is observed. This evidences a possible quantum critical point brought about by orbital fluctuations, rather than spin fluctuations as observed in Sr3Ru2O7 and heavy fermion conductors. An anomalous increase of resistivity is observed at pressures above ∼100 kbar, and a transition to a more resistive, possibly semiconducting, phase is observed at 300 kbar and ambient temperature. © 2013 American Physical Society
Control of Mooij correlations at the nanoscale in the disordered metallic Ta - nanoisland FeNi multilayers
Localisation phenomena in highly disordered metals close to the extreme
conditions determined by the Mott-Ioffe-Regel (MIR) limit when the electron
mean free path is approximately equal to the interatomic distance is a
challenging problem. Here, to shed light on these localisation phenomena, we
studied the dc transport and optical conductivity properties of nanoscaled
multilayered films composed of disordered metallic Ta and magnetic FeNi
nanoisland layers, where ferromagnetic FeNi nanoislands have giant magnetic
moments of 10^3-10^5 Bohr magnetons (\mu_B). In these multilayered structures,
FeNi nanoisland giant magnetic moments are interacting due to the indirect
exchange forces acting via the Ta electron subsystem. We discovered that the
localisation phenomena in the disordered Ta layer lead to a decrease in the
Drude contribution of free charge carriers and the appearance of the low-energy
electronic excitations in the 1-2 eV spectral range characteristic of
electronic correlations, which may accompany the formation of electronic
inhomogeneities. From the consistent results of the dc transport and optical
studies we found that with an increase in the FeNi layer thickness across the
percolation threshold evolution from the superferromagnetic to ferromagnetic
behaviour within the FeNi layer leads to the delocalisation of Ta electrons
from the associated localised electronic states. On the contrary, we discovered
that when the FeNi layer is discontinuous and represented by randomly
distributed superparamagnetic FeNi nanoislands, the Ta layer normalized dc
conductivity falls down below the MIR limit by about 60%. The discovered effect
leading to the dc conductivity fall below the MIR limit can be associated with
non-ergodicity and purely quantum (many-body) localisation phenomena, which
need to be challenged further.Comment: 29 pages, 8 figures. This is a post-peer-review, precopyedit version
of an article published in Scientific Reports. The final authenticated
version is available online at http://dx.doi.org/10.1038/s41598-020-78185-