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 $0$ and $\pi$. 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

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′&gt;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 $S=1$ bond alternating spin-gap antiferromagnet Ni(C$_9$H$_{24}$N$_4$)NO$_2$(ClO$_4$) (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.