Spin-charge separation and quantum spin Hall effect of [Formula: see text]-bismuthene.

Multiple works suggest the possibility of classification of quantum spin Hall effect with magnetic flux tubes, which cause separation of spin and charge degrees of freedom and pumping of spin or Kramers-pair. However, the proof of principle demonstration of spin-charge separation is yet to be accomplished for realistic, ab initio band structures of spin-orbit-coupled materials, lacking spin-conservation law. In this work, we perform thought experiments with magnetic flux tubes on [Formula: see text]-bismuthene, and demonstrate spin-charge separation, and quantized pumping of spin for three insulating states, that can be accessed by tuning filling fractions. With a combined analysis of momentum-space topology and real-space response, we identify important role of bands supporting even integer invariants, which cannot be addressed with symmetry-based indicators. Our work sets a new standard for the computational diagnosis of two-dimensional, quantum spin-Hall materials by going beyond the [Formula: see text] paradigm and providing an avenue for precise determination of the bulk invariant through computation of quantized, real-space response.

Tuning the Structural and Magnetic Properties in Mixed Cation MnxCo2-xP2S6.

The metal thiophosphates (MTP), M2P2S6, are a versatile class of van der Waals materials, which are notable for the possibility of tuning their magnetic properties with the incorporation of different transition-metal cations. Further, they also offer opportunities to probe the independent and synergistic role of the magnetically active cation sublattice when coupled to P2Q6 polyhedra. Herein, we report the structural, magnetic, and electronic properties of the series of MTPs, MnxCo2-xP2S6 (x = 0.25, 0.5, 1, 1.5, 1.75) synthesized by the P2S5 flux method. Structural and elemental analysis indicates a homogeneous stoichiometry in the MnxCo2-xP2S6 compounds. We observe that a correlation is apparent between the intensities of specific Raman modes and Raman shifts with respect to the alloying ratio between Mn and Co. Magnetic susceptibility measurements indicate that the alloyed systems adopt an ordered antiferromagnetic (AFM) configuration with a dependence of the Néel temperature on the alloying ratio. A possible magnetic frustration behavior was observed for the composition MnCoP2S6 due to magnetic moment compensation as the alloying ratio between Mn and Co approaches parity. Interestingly, mixed oxidation states of the metal cation species are also observed in MnxCo2-xP2S6 along with a linear dependence of the work function on the alloying ratio of Mn and Co.

Anderson Localization and the Quantum Phase Diagram of Three Dimensional Disordered Dirac Semimetals.

We study the quantum phase diagram of a three dimensional noninteracting Dirac semimetal in the presence of either quenched axial or scalar potential disorder, by calculating the average and the typical density of states as well as the inverse participation ratio using numerically exact methods. We show that as a function of the disorder strength a half-filled (i.e., undoped) Dirac semimetal displays three distinct ground states, namely an incompressible semimetal, a compressible diffusive metal, and a localized Anderson insulator, in stark contrast to a conventional dirty metal that only supports the latter two phases. We establish the existence of two distinct quantum critical points, which respectively govern the semimetal-metal and the metal-insulator quantum phase transitions and also reveal their underlying multifractal nature. Away from half-filling the (doped) system behaves as a diffusive metal that can undergo Anderson localization only, which is shown by determining the mobility edge and the phase diagram in terms of energy and disorder.

Superconductivity at the border of electron localization and itinerancy.

The superconducting state of iron pnictides and chalcogenides exists at the border of anti-ferromagnetic order. Consequently, these materials could provide clues about the relationship between magnetism and unconventional superconductivity. One explanation, motivated by the so-called bad metal behaviour of these materials proposes that magnetism and superconductivity develop out of quasi-localized magnetic moments that are generated by strong electron-electron correlations. Another suggests that these phenomena are the result of weakly interacting electron states that lie on nested Fermi surfaces. Here we address the issue by comparing the newly discovered alkaline iron selenide superconductors, which exhibit no Fermi-surface nesting, to their iron pnictide counterparts. We show that the strong-coupling approach leads to similar pairing amplitudes in these materials, despite their different Fermi surfaces. We also find that the pairing amplitudes are largest at the boundary between electronic localization and itinerancy, suggesting that new superconductors might be found in materials with similar characteristics.

Quantum criticality between topological and band insulators in 3+1 dimensions.

Four-component massive and massless Dirac fermions in the presence of long range Coulomb interaction and chemical potential disorder exhibit striking fermionic quantum criticality. For an odd number of flavors of Dirac fermions, the sign of the Dirac mass distinguishes the topological and the trivial band insulator phases, and the gapless semimetallic phase corresponds to the quantum critical point that separates the two. Up to a critical strength of disorder, the semimetallic phase remains stable, and the universality class of the direct phase transition between two insulating phases is unchanged. Beyond the critical strength of disorder the semimetallic phase undergoes a phase transition into a disorder controlled diffusive metallic phase, and there is no longer a direct phase transition between the two types of insulating phases.

Effects of the Berry phase and instantons in one-dimensional Kondo-Heisenberg model.

Motivated by the global phase diagram of antiferromagnetic heavy-fermion metals, we study the Kondo effect from the perspective of a nonlinear sigma model in the one-dimensional Kondo-Heisenberg model away from half-filling. We focus on the effects of the instanton configurations of the sigma-model field and the associated Berry phase. Guided by the results derived using bosonization methods, we demonstrate that the Kondo-singlet formation is accompanied by an emergent Berry phase. This Berry phase also captures the competition between the Kondo-singlet formation and spin-Peierls correlations. Related effects are likely to be realized in Kondo lattice systems in higher dimensions.

Dissipation and criticality in the lowest Landau level of graphene.

The lowest Landau level of graphene is studied numerically by considering a tight-binding Hamiltonian with disorder. The Hall conductance sigma_{xy} and the longitudinal conductance sigma_{xx} are computed. We demonstrate that bond disorder can produce a plateaulike feature centered at nu=0, while the longitudinal conductance is nonzero in the same region, reflecting a band of extended states between +/-E_{c}, whose magnitude depends on the disorder strength. The critical exponent corresponding to the localization length at the edges of this band is found to be 2.47+/-0.04. When both bond disorder and a finite mass term exist the localization length exponent varies continuously between approximately 1.0 and approximately 7/3.

Rounding by disorder of first-order quantum phase transitions: emergence of quantum critical points.

We give a heuristic argument for disorder rounding of a first-order quantum phase transition into a continuous phase transition. From both weak and strong disorder analysis of the N-color quantum Ashkin-Teller model in one spatial dimension, we find that, for N > or =3, the first-order transition is rounded to a continuous transition and the physical picture is the same as the random transverse field Ising model for a limited parameter regime. The results are strikingly different from the corresponding classical problem in two dimensions where the fate of the renormalization group flows is a fixed point corresponding to N-decoupled pure Ising models.

Experimental and theoretical investigation of fluorescence photobleaching and recovery in human breast tissue and tissue phantoms.

Photobleaching and recovery of 488-nm excited fluorescence from resected human breast tissue samples have been studied. Profiles of photobleaching decay were seen to be faster in cancerous tissue than in those of the normal tissue. The reverse behavior was observed in profiles of recovery after photobleaching. A theoretical model based on one-dimensional diffusion theory has been developed to provide insight into the phenomena of fluorescence during photobleaching and recovery in a multiply scattering medium such as tissue. To understand photobleaching and recovery with the help of this theoretical model, we carried out experiments with model media that were prepared with authentic fluorophores, scatterers, and absorbers. The results of these studies suggest that the fluorescence photobleaching profiles are affected more by the absorption than by the scattering properties of a turbid medium such as tissue. In contrast, the scattering properties of the medium are found to affect the fluorescence recovery profiles to a greater extent. These observations could be related to the observed difference in fluorescence photobleaching and recovery profiles of normal and cancerous breast tissues.