arXiv subjects

Benjamin J. Wieder

Publications and source records attributed to Benjamin J. Wieder.

24 records · Page 2Linked to original sources

arXiv2017

Filling-enforced Magnetic Dirac Semimetals in Two Dimensions

Filling-enforced Dirac semimetals, or those required at specific fillings by the combination of crystalline and time-reversal symmetries, have been proposed and discovered in numerous materials. However, Dirac points in these materials are not generally robust against breaking or modifying time-reversal symmetry. We present a new class of two-dimensional Dirac semimetal protected by the combination of crystal symmetries and a special, antiferromagnetic time-reversal symmetry. Systems in this class of magnetic layer groups, while having broken time-reversal symmetry, still respect the operation of time-reversal followed by a half-lattice translation. In contrast to 2D time-reversal-symmetric Dirac semimetal phases, this magnetic Dirac phase is capable of hosting just a single isolated Dirac point at the Fermi level, and that Dirac point can be stabilized solely by symmorphic crystal symmetries. We find that this Dirac point represents a new quantum critical point, and lives at the boundary between Chern insulating, antiferromagnetic topological crystalline insulating, and trivial insulating phases. We present density functional theoretic calculations which demonstrate the presence of this 2D magnetic Dirac semimetallic phase in FeSe monolayers and discuss the implications for engineering quantum phase transitions in these materials.

cond-mat.mes-hall↗

arXiv2016

Spin-Orbit Semimetals in the Layer Groups

Recent interest in point and line node semimetals has led to the proposal and discovery of these phenomena in numerous systems. Frequently, though, these nodal systems are described in terms of individual properties reliant on specific space group intricacies or band-tuning conditions. Restricting ourselves to cases with strong spin-orbit interaction, we develop a general framework which captures existing systems and predicts new examples of nodal materials. In many previously proposed systems, the three-dimensional nature of the space group has obscured key generalities. Therefore, we show how within our framework one can predict and characterize a diverse set of nodal phenomena even in two-dimensional systems constructed of three-dimensional sites, known as the "Layer Groups". Expanding on an existing discussion by Watanabe, Po, Vishwanath, and Zaletel of the relationship between minimal insulating filling, nonsymmorphic symmetries, and compact flat manifolds, we characterize the allowed semimetallic structures in the layer groups and draw connections to related three-dimensional systems.

cond-mat.mes-hall↗

arXiv2015

Double Dirac Semimetals in Three Dimensions

We study a class of Dirac semimetals that feature an eightfold-degenerate double Dirac point. We show that 7 of the 230 space groups can host such Dirac points and argue that they all generically display linear dispersion. We introduce an explicit tight-binding model for space groups 130 and 135, showing that 135 can host an intrinsic double Dirac semimetal -- one with no additional degeneracies at the Fermi energy. We consider symmetry-lowering perturbations and show that uniaxial compressive strain in different directions leads to topologically distinct insulating phases. In addition, the double Dirac semimetal can accommodate topological line defects that bind helical modes. Potential materials realizations are discussed.

cond-mat.mtrl-sci↗

arXiv2015

Critical Behavior of Four-Terminal Conductance of Bilayer Graphene Domain Walls

Bilayer graphene in a perpendicular electric field can host domain walls between regions of reversed field direction or interlayer stacking. The gapless modes propagating along these domain walls, while not strictly topological, nevertheless have interesting physical properties, including valley-momentum locking. A junction where two domain walls intersect forms the analogue of a quantum point contact. We study theoretically the critical behavior of this junction near the pinch-off transition, which is controlled by two separate classes of non-trivial quantum critical points. For strong interactions, the junction can host phases of unique charge and valley conductances. For weaker interactions, the low-temperature charge conductance can undergo one of two possible quantum phase transitions, each characterized by a specific critical exponent and a collapse to a universal scaling function, which we compute.

cond-mat.mes-hall↗

arXiv2015

Dirac Line Nodes in Inversion Symmetric Crystals

We propose and characterize a new $\mathbb{Z}_2$ class of topological semimetals with a vanishing spin--orbit interaction. The proposed topological semimetals are characterized by the presence of bulk one-dimensional (1D) Dirac Line Nodes (DLNs) and two-dimensional (2D) nearly-flat surface states, protected by inversion and time--reversal symmetries. We develop the $\mathbb{Z}_2$ invariants dictating the presence of DLNs based on parity eigenvalues at the parity--invariant points in reciprocal space. Moreover, using first-principles calculations, we predict DLNs to occur in Cu$_3$N near the Fermi energy by doping non-magnetic transition metal atoms, such as Zn and Pd, with the 2D surface states emerging in the projected interior of the DLNs. This paper includes a brief discussion of the effects of spin--orbit interactions and symmetry-breaking as well as comments on experimental implications.

cond-mat.mtrl-sci↗

arXiv2014

Signatures of Majorana Fermions in Topological Insulator Josephson Junction Devices

We study theoretically the electrical current and low-frequency noise for a linear Josephson junction structure on a topological insulator, in which the superconductor forms a closed ring and currents are injected from normal regions inside and outside the ring. We find that this geometry offers a signature for the presence of gapless 1D Majorana fermion modes that are predicted in the channel when the phase difference ϕ, controlled by the magnetic flux through the ring, is π. We show that for low temperature the linear conductance jumps when ϕ passes through π, accompanied by non-local correlations between the currents from the inside and outside of the ring. We compute the dependence of these features on temperature, voltage and linear dimensions, and discuss the implications for experiments.

cond-mat.mes-hall↗