research

topological quantum materials

Topological quantum materials

Topological electronic structure of MnBi4Te7

Combinations of nontrivial band topology and long-range magnetic order hold promise for realizations of novel spintronic phenomena, such as the quantum anomalous Hall effect and the topological magnetoelectric effect. Following theoretical advances, material candidates are emerging. Yet, so far a compound that combines a band-inverted electronic structure with an intrinsic net magnetization remains unrealized. MnBi2Te4 has been established as the first antiferromagnetic topological insulator and constitutes the progenitor of a modular (Bi2Te3)n(MnBi2Te4) series. Here, for n=1, we confirm a nonstoichiometric composition proximate to MnBi4Te7. We establish an antiferromagnetic state below 13 K followed by a state with a net magnetization and ferromagnetic-like hysteresis below 5 K. Angle-resolved photoemission experiments and density-functional calculations reveal a topologically nontrivial surface state on the MnBi4Te7(0001) surface, analogous to the nonmagnetic parent compound Bi2Te3. Our results establish MnBi4Te7 as the first band-inverted compound with intrinsic net magnetization providing a versatile platform for the realization of magnetic topological states of matter.


Phys. Rev. X 9, 041065 (2019).

Band inversion phenomena in Mn147 (GGA+U+SOC). (a) Band structure in the ferromagnetic configuration. The symbol size in each k point and band is proportional to the overlap between the corresponding Bloch state and the Te and Bi p orbitals, respectively, depicted in different colors. Filled (empty) dots correspond to spin-down (spin-up). The black arrow indicates the energy of the Weyl node of lowest energy in the conduction band. (b) Band structure for the antiferromagnetic AFM1 configuration. (c) Wannier center evolution in the kz=0 plane. kx is the crystal momentum along the primitive lattice vector ¯b1 and ¯b2 is the second primitive vector in the kz=0 plane. (d) Mn147(0001) surface spectral density along the ¯Γ − ¯M direction for a quintuple layer termination.
Crystal structures of RMC2. (a,c,d) Compounds with non-centrosymmetric Amm2 space group, and (b) Cmcm space group. (a) and (b) illustrate the layered structure, formed by a quasi-planar network spanned by the MC2 complex while the rare-earth (R) ions occupy the interstitial space between the layers. In (d), reflection symmetry planes are depicted in green while the arrows indicate directions [001] (black), [111] (red) and [−111] (blue). The conventional unit cells are indicated by black solid lines.
Energy of Weyl nodes in NdRhC2 as a function of the canting angle θ. Starting from the magnetization along the direction [001], on the left the canting is towards [−111], while on the right is towards [111]. Blue and red lines correspond to Weyl nodes of positive and negative chirality, respectively. The colored region highlights a wide angle range in which near the Fermi surface Weyl fermions of one chirality are majority.

Tunable chirality of the noncentrosymmetric Weyl semimetals

Even if Weyl semimetals are characterized by quasiparticles with well-defined chirality, exploiting this experimentally is severely hampered by Weyl lattice-fermions coming in pairs with opposite chirality, typically causing the net chirality picked up by experimental probes to vanish. Here we show this issue can be circumvented in a controlled manner when both time-reversal- and inversion- symmetry are broken. To this end, we investigate chirality-disbalance in the carbide family RMC2 (R a rare-earth and M a transition metal), showing several members to be Weyl semimetals. Using the noncentrosymmetric ferromagnet NdRhC2 as an illustrating example, we show that an odd number of Weyl nodes can be stabilized at its Fermi surface by properly tilting its magnetization. The tilt direction determines the sign of the resulting net chirality, opening up a simple route to control it.


arXiv pre-print

Large shift current in cubic and hexagonal LiZnX semiconductors


The rectified bulk photovoltaic effect (BPVE) in noncentrosymmetric semiconductors, also called shift current, is considered a promising alternative to the conventional p-n junction based solar cell applications. A clear understanding of the shift current mechanism and search for materials with large shift current is, therefore, of immense interest. ABC semiconductors LiZnX (X = N, P, As and Sb) can be stabilized in cubic as well as hexagonal morphologies lacking inversion symmetry -- an ideal platform to investigate the significant contributing factors to shift current, such as the role of structure and the chemical species. Using density-functional calculations properly accounting for the electronic bandgaps, the shift current conductivities in LiZnX (X = P, As, Sb) are found to be approximately an order of magnitude larger than the well-known counterparts and peak close to the maximum solar radiation intensity. Notably, hexagonal LiZnSb shows a peak shift current conductivity of ~75 microA/V2, comparable to highest predicted values in literature. Our comparative analysis of the shift current response not only posits Li-Zn-based ABC semiconductors as viable material candidates for potential applications, but also elucidates the structure-BPVE relationship and paves way for accelerated development in this direction.


arXiv preprint arXiv:2302.04861


Bulk energy bands of the monolayer-WTe2 tight-binding model: (a) along high-symmetry lines in the Brillouin zone. We have indicated the parities ξ of states at the time-reversal invariant momenta. (b) Bulk Brillouin zone. (c) Energy bands close to Γ along ΓX in a small energy window around the bulk energy gap. The dashed lines indicate the edges of the bulk energy gap.
Energy dispersion of ribbons with different terminations close to the Fermi level without magnetic field. The dashed lines indicate the bulk energy gap. The insets show the respective terminations using the same color code as in the top figure.

Influence of edge terminations and disorder in the QSHI monolayer 1T'-WTe2

We study the influence of sample termination on the electronic properties of the novel quantum spin Hall insulator monolayer 1T′−WTe2. For this purpose, we construct an accurate, minimal four-orbital tight-binding model with spin-orbit coupling by employing a combination of density-functional theory calculations, symmetry considerations, and fitting to experimental data. Based on this model, we compute energy bands and two-terminal conductance spectra for various ribbon geometries with different terminations, with and without a magnetic field. Because of the strong electron-hole asymmetry, we find that the edge Dirac point is buried in the bulk bands for most edge terminations. In the presence of a magnetic field, an in-gap edge Dirac point leads to exponential suppression of conductance as an edge Zeeman gap opens, whereas the conductance stays at the quantized value when the Dirac point is buried in the bulk bands. Finally, we find that disorder in the edge termination drastically changes this picture: the conductance of a sufficiently rough edge is uniformly suppressed for all energies in the bulk gap regardless of the orientation of the edge.


Phys. Rev. Mater. 3, 054206 (2019).

The optical conductivity (a) σxx and (b) σzz from GGA+U and GGA+Δ (Δ = 0.36 eV). The shift-current conductivity (c) σxyz and (d) σzxy from GGA+U and GGA+Δ. (e) Both the optical and shift-current conductivity from GGA+U are plotted on the same scale.

First-principles calculation of shift current in chalcopyrite semiconductor ZnSnP2

The bulk photovoltaic effect generates intrinsic photocurrents in materials without inversion symmetry. Shift current is one of the bulk photovoltaic phenomena related to the Berry phase of the constituting electronic bands: photoexcited carriers coherently shift in real space due to the difference in the Berry connection between the valence and conduction bands. Ferroelectric semiconductors and Weyl semimetals are known to exhibit such nonlinear optical phenomena. Here we consider the chalcopyrite semiconductor ZnSnP2, which lacks inversion symmetry, and calculate the shift-current conductivity. We find that the magnitude of the shift current is comparable to the recently measured values on other ferroelectric semiconductors and an order of magnitude larger than bismuth ferrite. The peak response for both optical and shift-current conductivity, which mainly comes from P-3p and Sn-5p orbitals, is several eV above the band gap.


Phys. Rev. Mater. 4, 064602 (2020).