We develop a theory for the three-terminal nonlocal conductance in Majorana nanowires as existing in the superconductor-semiconductor hybrid structures in the presence of superconducting proximity, spin-orbit coupling, and Zeeman splitting. The key question addressed is whether such nonlocal conductance can decisively distinguish between trivial and topological Majorana scenarios in the presence of chemical potential inhomogeneity and random impurity disorder. We calculate the local electrical as well as nonlocal electrical and thermal conductance of the pristine nanowire (good zero-bias conductance peaks), the nanowire in the presence of quantum dots and inhomogeneous potential (bad zero-bias conductance peaks), and the nanowire in the presence of large disorder (ugly zero-bias conductance peaks). The local conductance by itself is incapable of distinguishing the trivial states from the topological states since zero-bias conductance peaks are generic in the presence of disorder and inhomogeneous potential. The nonlocal conductance, which in principle is capable of providing the bulk gap closing and reopening information at the topological quantum phase transition, is found to be far too weak in magnitude to be particularly useful in the presence of disorder and inhomogeneous potential. Therefore, we focus on the question of whether the combination of the local, nonlocal electrical, and thermal conductance can separate the good, bad, and ugly zero-bias conductance peaks in finite-length wires. Our paper aims to provide a guide to future experiments, and we conclude that a combination of all three measurements would be necessary for a decisive demonstration of topological Majorana zero modes in nanowires-positive signals corresponding to just one kind of measurements are likely to be false positives arising from disorder and inhomogeneous potential.

}, issn = {2469-9950}, doi = {10.1103/PhysRevB.103.014513}, author = {Pan, Haining and Sau, Jay D. and Sankar Das Sarma} } @article {pan_band_2020, title = {Band topology, {Hubbard} model, {Heisenberg} model, and {Dzyaloshinskii}-{Moriya} interaction in twisted bilayer {WSe2}}, journal = {Phys. Rev. Res.}, volume = {2}, number = {3}, year = {2020}, note = {Place: ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA Publisher: AMER PHYSICAL SOC Type: Article}, month = {jul}, abstract = {We present a theoretical study of single-particle and many-body properties of twisted bilayer WSe2. For single-particle physics, we calculate the band topological phase diagram and electron local density of states (LDOS), which are found to be correlated. By comparing our theoretical LDOS with those measured by scanning tunneling microscopy, we comment on the possible topological nature of the first moire valence band. For many-body physics, we construct a generalized Hubbard model on a triangular lattice based on the calculated single-particle moire bands. We show that a layer potential difference, arising, for example, from an applied electric field, can drastically change the noninteracting moire bands, tune the spin-orbit coupling in the Hubbard model, control the charge excitation gap of the Mott insulator at half-filling, and generate an effective Dzyaloshinskii-Moriya interaction in the effective Heisenberg model for the Mott insulator. Our theoretical results agree with transport experiments on the same system in several key aspects, and establish twisted bilayer WSe2 as a highly tunable system for studying and simulating strongly correlated phenomena in the Hubbard model.

}, doi = {10.1103/PhysRevResearch.2.033087}, author = {Pan, Haining and Wu, Fengcheng and Das Sarma, Sankar} } @article { ISI:000506582800005, title = {Generic quantized zero-bias conductance peaks in superconductor-semiconductor hybrid structures}, journal = {Phys. Rev. B}, volume = {101}, number = {2}, year = {2020}, month = {JAN 8}, pages = {024506}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {We show theoretically that quantized zero-bias conductance peaks should be ubiquitous in superconductor-semiconductor hybrids by employing a zero-dimensional random matrix model with continuous tuning parameters. We demonstrate that a normal metal-superconductor (NS) junction conductance spectra can be generically obtained in this model replicating all features seen in recent experimental results. The theoretical quantized conductance peaks, which explicitly do not arise from spatially isolated Majorana zero modes, are easily found by preparing a contour plot of conductance over several independent tuning parameters, mimicking the effect of Zeeman splitting and voltages on gates near the junction. This suggests that, even stable apparently quantized conductance peaks need not correspond to isolated Majorana modes; rather, the a priori expectation should be that such quantized peaks generically occupy a significant fraction of the high-dimensional tuning parameter space that characterizes the NS tunneling experiments.

}, issn = {2469-9950}, doi = {10.1103/PhysRevB.101.024506}, author = {Pan, Haining and Cole, William S. and Sau, Jay D. and Das Sarma, Sankar} } @article {pan_physical_2020, title = {Physical mechanisms for zero-bias conductance peaks in {Majorana} nanowires}, journal = {Phys. Rev. Res.}, volume = {2}, number = {1}, year = {2020}, note = {Place: ONE PHYSICS ELLIPSE, COLLEGE PK, MD 20740-3844 USA Publisher: AMER PHYSICAL SOC Type: Article}, month = {mar}, abstract = {Motivated by the need to understand and simulate the ubiquitous experimentally observed zero-bias conductance peaks in superconductor-semiconductor hybrid structures, we theoretically investigate the tunneling conductance spectra in one-dimensional nanowires in proximity to superconductors in a systematic manner taking into account several different physical mechanisms producing zero-bias conductance peaks. The mechanisms we consider are the presence of quantum dots, inhomogeneous potential, random disorder in the chemical potential, random fluctuations in the superconducting gap, and in the effective g factor with the self-energy renormalization induced by the parent superconductor in both short (L similar to 1 mu m) and long nanowires (L similar to 3 mu m). We classify all foregoing theoretical results for zero-bias conductance peaks into three types: the good, the bad, and the ugly, according to the physical mechanisms producing the zero-bias peaks and their topological properties. We find that, although the topological Majorana zero modes are immune to weak disorder, strong disorder ({\textquotedblright}ugly{\textquotedblright}) completely suppresses topological superconductivity and generically leads to trivial zero-bias peaks. Compared qualitatively with the extensive existing experimental results in the superconductor-semiconductor nanowire structures, we conclude that most current experiments are likely exploring trivial zero-bias peaks in the {\textquotedblleft}ugly{\textquotedblright} situation dominated by strong disorder. We also study the nonlocal end-to-end correlation measurement in both the short and long wires, and point out the limitation of the nonlocal correlation in ascertaining topological properties particularly when applied to short wires. Although we present results for {\textquotedblleft}good{\textquotedblright} and {\textquotedblleft}bad{\textquotedblright} zero-bias peaks, arising respectively from topological Majorana bound states and trivial Andreev bound states, strictly for the sake of direct comparison with the {\textquotedblleft}ugly{\textquotedblright} zero-bias conductance peaks arising from strong disorder, the main goal of the current work is to establish with a very high confidence level the real physical possibility that essentially all experimentally observed zero-bias peaks in Majorana nanowires are most likely ugly, i.e., purely induced by strong disorder, and are as such utterly nontopological. Our work clearly suggests that an essential prerequisite for any future observation of topological Majorana zero modes in nanowires is a substantialmaterials improvement of the semiconductor-superconductor hybrid systems leading to much cleaner wires.}, doi = {10.1103/PhysRevResearch.2.013377}, author = {Pan, Haining and Das Sarma, S.} } @article {ISI:000458854900004, title = {Curvature of gap closing features and the extraction of Majorana nanowire parameters}, journal = {Phys. Rev. B}, volume = {99}, number = {5}, year = {2019}, month = {FEB 15}, pages = {054507}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {Recent tunneling conductance measurements of Majorana nanowires show a strong variation in the magnetic-field dependence of the superconducting gap among different devices. Here, we theoretically study the magnetic field dependence of the gap closing feature and establish that the degree of convexity (or concavity) of the gap closing as a function of Zeeman field can provide critical constraints on the underlying microscopic parameters of the semiconductor-superconductor hybrid system model. Specifically, we show that the gap closing feature is entirely concave only for strong spin-orbit coupling strength relative to the chemical potential. Additionally, the nonlinearity (i.e., concavity or convexity) of the gap closing as a function of magnetic field complicates the simple assignment of a constant effective g-factor to the states in the Majorana nanowire. We develop a procedure to estimate the effective g-factor from recent experimental data that accounts for the nonlinear gap closing, resulting from the interplay between chemical potential and spin-orbit coupling. Thus, measurements of the magnetic field dependence of the gap closure on the trivial side of the topological quantum phase transition can provide useful information on parameters that are critical to the theoretical modeling of Majorana nanowires.}, issn = {2469-9950}, doi = {10.1103/PhysRevB.99.054507}, author = {Pan, Haining and Sau, Jay D. and Stanescu, Tudor D. and S. Das Sarma} } @article {ISI:000455163900004, title = {Two-kind boson mixture honeycomb Hamiltonian of Bloch exciton-polaritons}, journal = {Phys. Rev. B}, volume = {99}, number = {4}, year = {2019}, month = {JAN 8}, pages = {045302}, publisher = {AMER PHYSICAL SOC}, type = {Article}, abstract = {The electronic band structure of a solid is a collection of allowed bands separated by forbidden bands, revealing the geometric symmetry of the crystal structures. Comprehensive knowledge of the band structure with band parameters explains intrinsic physical, chemical, and mechanical properties of the solid. Here we report the artificial polaritonic band structures of two-dimensional honeycomb lattices for microcavity exciton-polaritons using GaAs semiconductors in the wide-range detuning values, from cavity photonlike (red-detuned) to excitonlike (blue-detuned) regimes. In order to understand the experimental band structures and their band parameters, such as gap energies, bandwidths, hopping integrals, and density of states, we originally establish a polariton band theory within an augmented plane wave method with two-kind bosons, cavity photons trapped at the lattice sites, and freely moving excitons. In particular, this two-kind band theory is absolutely essential to elucidate the exciton effect in the band structures of blue-detuned exciton-polaritons, where the flattened excitonlike dispersion appears at larger in-plane momentum values captured in our experimental access window. We reach an excellent agreement between theory and experiments in all detuning values.}, issn = {2469-9950}, doi = {10.1103/PhysRevB.99.045302}, author = {Pan, Haining and Winkler, K. and Powlowski, Mats and Xie, Ming and Schade, A. and Emmerling, M. and Kamp, M. and Klembt, S. and Schneider, C. and Byrnes, Tim and Hoefling, S. and Kim, Na Young} } @article { ISI:000447297200006, title = {Metamorphosis of Andreev bound states into Majorana bound states in pristine nanowires}, journal = {PHYSICAL REVIEW B}, volume = {98}, number = {14}, year = {2018}, month = {OCT 15}, pages = {144511}, issn = {2469-9950}, doi = {10.1103/PhysRevB.98.144511}, author = {Huang, Yingyi and Pan, Haining and Liu, Chun-Xiao and Sau, Jay D. and Stanescu, Tudor D. and S. Das Sarma} }