arXiv subjects

Sean Howard

Publications and source records attributed to Sean Howard.

6 recordsLinked to original sources

arXiv2021

Observation of a Smoothly Tunable Dirac Point in $Ge(Bi_{x}Sb_{1-x})_{2}Te_{4}$

State-of-the-art topological devices require the use topological surface states to drive electronic transport. In this study, we examine a tunable topological system, $Ge(Bi_{x}Sb_{1-x})_{2}Te_{4}$, for a range of 'x' values from 0 to 1, using a combination of Fourier Transform Scanning Tunneling Spectroscopy (FT-STS) and Angle-Resolved Photoemission Spectroscopy (ARPES). Our results show that the Dirac point shifts linearly with 'x', crossing the Fermi energy near x = 0.7. This novel observation of a smoothly tunable, isolated Dirac point crossing through the topological transport regime and having strong linear dependence with substitution can be critical for future topological spintronics applications.

cond-mat.mtrl-sci↗

arXiv2019

Observation of linearly dispersive edge modes in a magnetic Weyl semimetal Co3Sn2S2

The physical realization of Chern insulators is of fundamental and practical interest, as they are predicted to host the quantum anomalous Hall effect (QAHE) and topologically protected chiral edge states which can carry dissipationless current. The realization of the QAHE state has however been challenging because of the complex heterostructures and sub-Kelvin temperatures required. Time-reversal symmetry breaking Weyl semimetals, being essentially stacks of Chern insulators with inter-layer coupling, may provide a new platform for the higher temperature realization of robust QAHE edge states. In this work we present a combined scanning tunneling spectroscopy and theoretical investigation of a newly discovered magnetic Weyl semimetal, Co3Sn2S2. Using modeling and numerical simulations we find that chiral edge states can be localized on partially exposed Kagome planes on the surface of a Weyl semimetal. Correspondingly, our STM dI/dV maps on narrow kagome Co3Sn terraces show linearly dispersing quantum well like states, which can be attributed to hybridized chiral edge modes. Our experiment and theory results suggest a new paradigm for studying chiral edge modes in time-reversal breaking Weyl semimetals. More importantly, this work leads a practical route for realizing higher temperature QAHE.

cond-mat.mtrl-sci↗

arXiv2019

Microscopic evidence for a chiral superconducting order parameter in the heavy fermion superconductor UTe2

Spin-triplet superconductivity is a condensate of electron pairs with spin-1 and an odd-parity wavefunction. A particularly interesting manifestation of triplet pairing is a chiral p-wave state which is topologically non-trivial and a natural platform for realizing Majorana edge modes. Triplet pairing is however rare in solid state systems and so far, no unambiguous identification has been made in any bulk compound. Since pairing is most naturally mediated by ferromagnetic spin fluctuations, uranium based heavy fermion systems containing f electron elements that can harbor both strong correlations and magnetism are considered ideal candidate spin-triplet superconductors. In this work we present scanning tunneling microscopy (STM) studies of the newly discovered heavy fermion superconductor, UTe2 with a T$_{SC}$ of 1.6 K. We find signatures of coexisting Kondo effect and superconductivity which show competing spatial modulations within one unit-cell. STM spectroscopy at step edges show signatures of chiral in-gap states, predicted to exist at the boundaries of a topological superconductor. Combined with existing data indicating triplet pairing, the presence of chiral edge states suggests that UTe2 is a strong candidate material for chiral-triplet topological superconductivity.

cond-mat.mtrl-sci↗

arXiv2019

Visualizing 1D zigzag Wigner crystallization at domain walls in the Mott insulator TaS$_2$

In a certain regime of low carrier densities and strong correlations, electrons can crystallize into a periodic arrangement of charge known as Wigner crystal. Such phases are particularly interesting in one dimension (1D) as they display a variety of charge and spin ground states which may be harnessed in quantum devices as high-fidelity transmitters of spin information. Recent theoretical studies suggest that the strong Coulomb interactions in Mott insulators and other flat band systems, may provide an attractive higher temperature platform for Wigner crystallization, but due to materials and device constraints experimental realization has proven difficult. In this work we use scanning tunneling microscopy at liquid helium temperatures to directly image the formation of a 1D Wigner crystal in a Mott insulator, TaS$_2$. Charge density wave domain walls in TaS$_2$ create band bending and provide ideal conditions of low densities and strong interactions in 1D. STM spectroscopic maps show that once the lower Hubbard band crosses the Fermi energy, the charges rearrange to minimize Coulomb energy, forming zigzag patterns expected for a 1D Wigner crystal. The zigzag charge patterns show characteristic noise signatures signifying charge or spin fluctuations induced by the tunneling electrons, which is expected for this more fragile condensed state. The observation of a Wigner crystal at orders of magnitude higher temperatures enabled by the large Coulomb energy scales combined with the low density of electrons, makes TaS$_2$ a promising system for exploiting the charge and spin order in 1D Wigner crystals.

cond-mat.str-el↗

arXiv2019

Automated data validation: an industrial experience report

There has been a massive explosion of data generated by customers and retained by companies in the last decade. However, there is a significant mismatch between the increasing volume of data and the lack of automation methods and tools. The lack of best practices in data science programming may lead to software quality degradation, release schedule slippage, and budget overruns. To mitigate these concerns, we would like to bring software engineering best practices into data science. Specifically, we focus on automated data validation in the data preparation phase of the software development life cycle. This paper studies a real-world industrial case and applies software engineering best practices to develop an automated test harness called RESTORE. We release RESTORE as an open-source R package. Our experience report, done on the geodemographic data, shows that RESTORE enables efficient and effective detection of errors injected during the data preparation phase. RESTORE also significantly reduced the cost of testing. We hope that the community benefits from the open-source project and the practical advice based on our experience.

cs.DB↗

arXiv2019

Signature of Dispersing 1D Majorana Channels in an Iron-based Superconductor

The possible realization of Majorana fermions as quasiparticle excitations in condensed matter physics has created much excitement. Most recent studies have focused on Majorana bound states which can serve as topological qubits. More generally, akin to elementary particles, Majorana fermions can propagate and display linear dispersion. These excitations have not yet been directly observed, and can also be used for quantum information processing. One route to realizing this is in a line junction between two phase-shifted superconductors coupled to topological surface states. Recent theory indicates that in iron-based superconductors, a particular type of crystalline defect, i.e., a domain wall (DW) between two regions with a half-unit cell shift between them, should create a $\pi$-phase shift in the superconducting order parameter. Combined with recent data showing topological surface states in FeSe$_x$Te$_{1-x}$ we find that this is the ideal system to realize helical 1D-dispersing Majorana modes. Here we report scanning tunneling spectroscopic (STS) measurements of crystalline DWs in FeSe$_{0.45}$Te$_{0.55}$. By analyzing large-area superconducting gap maps, we identify the gap in the topological surface state, demonstrating that our sample is an effective Fu-Kane proximitized topological system. We further locate DWs across which the atoms shift by half a unit cell. STS data on these DWs reveal a flat density of states inside the superconducting gap, a hallmark of linearly dispersing modes in 1D. This unique signature is absent in DWs in the related superconductor, FeSe which is not in the topological phase. Our combined data are consistent with the observation of dispersing Majorana states at a $\pi$-phase shift DW in a proximitized topological material.

cond-mat.supr-con↗