121,759 research outputs found
Disordered Chern insulator with a two step Floquet drive
We explore the physics of a Chern insulator subjected to a two step Floquet
drive. We analytically obtain the phase diagram and show that the system can
exhibit different topological phases characterized by presence and chirality of
edge-modes in the two bulk gaps of the Floquet quasienergy spectrum, around
and . We find that the phase of the system depends on the mean but not on
the amplitude of the drive. The bulk topological invariants characterizing the
phases can be extracted by mapping the unitary evolution within a time period
to an energetically trivial but topologically non-trivial time evolution. An
extensive numerical study of the bulk topological invariants in the presence of
quenched disorder reveals new transitions induced by strong disorder (i) from
the different topological to trivial insulator phases and (ii) from a trivial
to a topological Anderson insulator phase at intermediate disorder strengths.
Careful analysis of level statistics of the quasienergy spectrum indicates a
`levitation-annihilation' mechanism near these transitions.Comment: 15 pages, 10 figures, version published in Phys. Rev.
Break-Junction Tunneling on MgB_2
Tunneling data on magnesium diboride, MgB_2, are reviewed with a particular
focus on superconductor-insulator-superconductor (SIS) junctions formed by a
break-junction method. The collective tunneling literature reveals two distinct
energy scales, a large gap, Delta_L~7.2 meV, close to the expected BCS value,
and a small gap, Delta_S~2.4 meV. The SIS break junctions show clearly that the
small gap closes near the bulk critical temperature, T_c=39 K. The SIS spectra
allow proximity effects to be ruled out as the cause for the small gap and
therefore make a strong case that MgB_2 is a coupled, two-band superconductor.
While the break junctions sometimes reveal parallel contributions to the
conductance from both bands, it is more often found that Delta_S dominates the
spectra. In these cases, a subtle feature is observed near Delta_S+Delta_L that
is reminiscent of strong-coupling effects. This feature is consistent with
quasiparticle scattering contributions to the interband coupling which provides
an important insight into the nature of two-band superconductivity in MgB_2.Comment: 9 pages, 10 pictures, accepted for publication in a special issue of
Physica C on MgB2, minor change
Zigzag edge modes in Z2 topological insulator: reentrance and completely flat spectrum
The spectrum and wave function of helical edge modes in Z_2 topological
insulator are derived on a square lattice using Bernevig-Hughes-Zhang (BHZ)
model. The BHZ model is characterized by a "mass" term M (k) that is
parameterized as M (k) = Delta - B k^2. A topological insulator realizes when
the parameters Delta and B fall on the regime, either 0 < Delta /B < 4 or 4 <
Delta /B < 8. At Delta /B = 4, which separates the cases of positive and
negative (quantized) spin Hall conductivities, the edge modes show a
corresponding change that depends on the edge geometry. In the (1,0)-edge, the
spectrum of edge mode remains the same against change of Delta /B, although the
main location of the mode moves from the zone center for Delta /B < 4, to the
zone boundary for Delta /B > 4 of the 1D Brillouin zone. In the (1,1)-edge
geometry, the group velocity at the zone center changes sign at Delta /B = 4
where the spectrum becomes independent of the momentum, i.e. flat, over the
whole 1D Brillouin zone. Furthermore, for Delta/B < 1.354..., the edge mode
starting from the zone center vanishes in an intermediate region of the 1D
Brillouin zone, but reenters near the zone boundary, where the energy of the
edge mode is marginally below the lowest bulk excitations. On the other hand,
the behavior of reentrant mode in real space is indistinguishable from an
ordinary edge mode.Comment: 19 pages, 33 figure
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Rethinking α-RuCl3
We argue that several empirical constraints strongly restrict parameters of the effective microscopic spin model describing α-RuCl3. In particular, such constraints dictate a substantial positive off-diagonal anisotropic coupling Γ′>0, not anticipated previously. The renormalization by quantum fluctuations allows to reconcile larger values of the advocated bare parameters with their earlier assessments and provides a consistent description of the field evolution of spin excitations in the paramagnetic phase. We assert that large anisotropic terms inevitably result in strong anharmonic coupling of magnons, necessarily leading to broad features in their spectra due to decays, in accord with the observations in α-RuCl3. Using duality transformations, we explain the origin of the pseudo-Goldstone mode that is ubiquitous to the studied parameter space and is present in α-RuCl3. Our analysis offers a description of α-RuCl3 as an easy-plane ferromagnet with antiferromagnetic further-neighbor and strong off-diagonal couplings, which is in a fluctuating zigzag ground state proximate to an incommensurate phase that is continuously connected to a ferromagnetic one
Signatures of a Pressure-Induced Topological Quantum Phase Transition in BiTeI
We report the observation of two signatures of a pressure-induced topological
quantum phase transition in the polar semiconductor BiTeI using x-ray powder
diffraction and infrared spectroscopy. The x-ray data confirm that BiTeI
remains in its ambient-pressure structure up to 8 GPa. The lattice parameter
ratio c/a shows a minimum between 2.0-2.9 GPa, indicating an enhanced c-axis
bonding through pz band crossing as expected during the transition. Over the
same pressure range, the infrared spectra reveal a maximum in the optical
spectral weight of the charge carriers, reflecting the closing and reopening of
the semiconducting band gap. Both of these features are characteristics of a
topological quantum phase transition, and are consistent with a recent
theoretical proposal.Comment: revised final versio
Modes of magnetic resonance of S=1 dimer chain compound NTENP
The spin dynamics of a quasi one dimensional bond alternating spin-gap
antiferromagnet Ni(CHN)NO(ClO) (abbreviated as NTENP) is
studied by means of electron spin resonance (ESR) technique. Five modes of ESR
transitions are observed and identified: transitions between singlet ground
state and excited triplet states, three modes of transitions between spin
sublevels of collective triplet states and antiferromagnetic resonance
absorption in the field-induced antiferromagnetically ordered phase.
Singlet-triplet and intra-triplet modes demonstrate a doublet structure which
is due to two maxima in the density of magnon states in the low-frequency
range. A joint analysis of the observed spectra and other experimental results
allows to test the applicability of the fermionic and bosonic models. We
conclude that the fermionic approach is more appropriate for the particular
case of NTENP.Comment: 11 pages, 11 figures, published in Phys.Rev.
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