Non-Fermi Liquids as Ersatz Fermi Liquids: General Constraints on Compressible Metals
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75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
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Results for “Fermi surface”
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Observation of topological superconductivity on the surface of an iron-based superconductor
Topological superconductors are predicted to host exotic Majorana states that obey non-Abelian statistics and can be used to implement a topological quantum computer. Most of the proposed topological superconductors are realized in difficult-to-fabricate heterostructures at very low temperatures. By using high-resolution spin-resolved and angle-resolved photoelectron spectroscopy, we find that the iron-based superconductor FeTe 1–x Se x (x = 0.45; superconducting transition temperature T c = 14.5 kelvin) hosts Dirac-cone–type spin-helical surface states at the Fermi level; the surface states exhibit an s-wave superconducting gap below T c . Thus, our study shows that the surface states of FeTe 0.55 Se 0.45 are topologically superconducting, providing a simple and possibly high-temperature platform for realizing Majorana states.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Surface superconductivity in the type II Weyl semimetal TaIrTe 4
The search for unconventional superconductivity in Weyl semimetal materials is currently an exciting pursuit, since such superconducting phases could potentially be topologically non-trivial and host exotic Majorana modes. The layered material TaIrTe 4 is a newly predicted time-reversal invariant type II Weyl semimetal with the minimum number of Weyl points. Here, we report the discovery of surface superconductivity in Weyl semimetal TaIrTe 4 . Our scanning tunneling microscopy/spectroscopy (STM/STS) visualizes Fermi arc surface states of TaIrTe 4 that are consistent with the previous angle-resolved photoemission spectroscopy results. By a systematic study based on STS at ultralow temperature, we observe uniform superconducting gaps on the sample surface. The superconductivity is further confirmed by electrical transport measurements at ultralow temperature, with an onset transition temperature (T c ) up to 1.54 K being observed. The normalized upper critical field h*(T/T c ) behavior and the stability of the superconductivity against the ferromagnet indicate that the discovered superconductivity is unconventional with the p-wave pairing. The systematic STS, and thickness- and angular-dependent transport measurements reveal that the detected superconductivity is quasi-1D and occurs in the surface states. The discovery of the surface superconductivity in TaIrTe 4 provides a new novel platform to explore topological superconductivity and Majorana modes.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Imaging Dirac-mass disorder from magnetic dopant atoms in the ferromagnetic topological insulator Cr x (Bi 0.1 Sb 0.9 ) 2-x Te 3
To achieve and use the most exotic electronic phenomena predicted for the surface states of 3D topological insulators (TIs), it is necessary to open a “Dirac-mass gap” in their spectrum by breaking time-reversal symmetry. Use of magnetic dopant atoms to generate a ferromagnetic state is the most widely applied approach. However, it is unknown how the spatial arrangements of the magnetic dopant atoms influence the Dirac-mass gap at the atomic scale or, conversely, whether the ferromagnetic interactions between dopant atoms are influenced by the topological surface states. Here we image the locations of the magnetic (Cr) dopant atoms in the ferromagnetic TI Cr₀.₀₈(Bi₀.₁Sb₀.₉)₁.₉₂Te₃. Simultaneous visualization of the Dirac-mass gap Δ(r) reveals its intense disorder, which we demonstrate is directly related to fluctuations in n(r), the Cr atom areal density in the termination layer. We find the relationship of surface-state Fermi wavevectors to the anisotropic structure of Δ(r) not inconsistent with predictions for surface ferromagnetism mediated by those states. Moreover, despite the intense Dirac-mass disorder, the anticipated relationship Δ(r)∝n(r) is confirmed throughout and exhibits an electron–dopant interaction energy J* = 145 meV·nm². In addition, these observations reveal how magnetic dopant atoms actually generate the TI mass gap locally and that, to achieve the novel physics expected of time-reversal symmetry breaking TI materials, control of the resulting Dirac-mass gap disorder will be essential.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Fermi-Thomas response of a metal surface to an external point charge.
Dielectric response of semiinfinite metal surface to external point charge in linearized Fermi- Thomas approximation, discussing potential in metal and vacuum
Newns, D. M.↗
Surface states and electronic structure of polar and nonpolar InN - An in situ photoelectron spectroscopy study
Valence band structure and surface states of InN with (0001), (000-1), (1-100), and (11-20) orientation were investigated in situ after growth using photoelectron spectroscopy. Depending on surface orientation, different occupied surface states are identified and differentiated from bulk contributions. For N-polar, m-plane, and a-plane InN, the surface states are located at the valence band maximum, while In-polar InN features surface states close to the Fermi level. The surface band alignment correlates with the position of surface states. For InN(0001), a much larger surface downward band bending is observed compared to N-polar, m-plane, and a-plane InN, where almost flat band conditions occur.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Beyond Conventional Charge Density Wave for Strongly Enhanced 2D Superconductivity in 1H‐TaS 2 Superlattices
Noncentrosymmetric transition metal dichalcogenide (TMD) monolayers offer a fertile platform for exploring unconventional Ising superconductivity (SC) and charge density waves (CDWs). However, the vulnerability of isolated monolayers to structural disorder and environmental oxidation often degrade their electronic coherence. Herein, an alternative approach is reported for fabricating stable and intrinsic monolayers of 1H-TaS2 sandwiched between SnS blocks in a (SnS) 1.15 TaS 2 van der Waals (vdW) superlattice. The SnS block layers not only decouple individual 1H-TaS 2 sublayers to endow them with monolayer-like electronic characteristics, but also protect the 1H-TaS 2 layers from electronic degradation. The results reveal the characteristic 3 × 3 CDW order in 1H-TaS 2 sublayers associated with electronic rearrangement in the low-lying sulfur p band, which uncovers a previously undiscovered CDW mechanism rather than the conventional Fermi surface-related framework. Additionally, the (SnS) 1.15 TaS 2 superlattice exhibits a strongly enhanced Ising-like SC with a layer-independent T c of ≈3.0 K, comparable to that of the isolated monolayer 1H-TaS 2 sample, presumably attributed to their monolayer-like characteristics and retained Fermi states. These results provide new insights into the long-debated CDW order and enhanced SC of monolayer 1H-TaS 2 , establishing bulk vdW superlattices as promising platforms for investigating exotic collective quantum phases in the 2D limit.
2D superconductivity↗
The Critical Role of AlInP Window Design in III–V Rear-Emitter Solar Cells
This article highlights the critical role of window design on short circuit carrier collection in rear-emitter solar cells, as demonstrated through modeling and experiment using metamorphic GaAs y P 1-y . Ultimately, if the window design is not carefully considered, surface depletion caused by Fermi level pinning at the window/air interface can extend into regions of active collection resulting in a large increase in effective window/base interface recombination velocity. This was experimentally shown here to result in a potential AM1.5G photocurrent loss of 8.1 mA/cm 2 , or nearly 50%, based on integrated internal quantum efficiency (IQE). Associated IQE modeling and curve fitting indicate that the effective IRV at the GaAs 0.75 P 0.25 /Al 0.64 In 0.36 P interface is increased by multiple orders of magnitude when the window is not sufficiently thick or doped to fully contain the surface depletion to within the window layer. Calculations of the surface depletion depth as a function of doping and the surface Fermi pinning energy level provides insight into the fundamental limits of window thickness. As a result, this allows the estimation of the Al 0.64 In 0.36 P surface pinning level to be at least 1.25 eV below the conduction band edge, or in the bottom half of the bandgap.
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Imaging electronic states on topological semimetals using scanning tunneling microscopy
Following the intense studies on topological insulators, significant efforts have recently been devoted to the search for gapless topological systems. These materials not only broaden the topological classification of matter but also provide a condensed matter realization of various relativistic particles and phenomena previously discussed mainly in high energy physics. Weyl semimetals host massless, chiral, low-energy excitations in the bulk electronic band structure, whereas a symmetry protected pair of Weyl fermions gives rise to massless Dirac fermions. We employed scanning tunneling microscopy/spectroscopy to explore the behavior of electronic states both on the surface and in the bulk of topological semimetal phases. By mapping the quasiparticle interference (QPI) and emerging Landau levels at high magnetic field in Dirac semimetals Cd 3 As 2 and Na 3 Bi, we observed extended Dirac-like bulk electronic bands. QPI imaged on Weyl semimetal TaAs demonstrated the predicted momentum dependent delocalization of Fermi arc surface states in the vicinity of the surface projected Weyl nodes.
36 MATERIALS SCIENCE↗
Anisotropic surface state in a topological semimetal candidate Ta 3 SiTe 6
Using high-resolution angle-resolved photoemission spectroscopy, we studied the surface state of Ta 3 SiTe 6 , a candidate of topological semimetal protected by nonsymmorphic symmetry. Through photon-energy dependent measurements, the surface state near the Fermi level was detected. By determining the band dispersions, we found that the surface state is topological trivial. Around the surface Brillouin zone center, the surface bands disperse linear-like, forming a band crossing (zero energy gap) point at about 0.2 eV below the Fermi level. In Ta 3 SiTe 6 's rectangular surface Brillouin zone, the surface bands only cross the Fermi level along one axis, forming two hole-like Fermi pockets near surface Brillouin zone boundaries. Although the observed surface state is not topological, we proposed that its anisotropic Fermi surface topography could be potentially useful for future applications.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
Charged Defects in UO 2 Bulk and Surface: A First-Principles Study
Uranium dioxide (UO 2 ) is the primary fuel used in nuclear reactors. Under the extreme heat and radiation inside a reactor, this material inevitably develops defects in its crystal structure. To investigate the nature and behavior of these defects, DFT+U calculations were employed to investigate charged point defects in both bulk UO 2 and its most stable surface, the (111) plane. The formation of defects and their impact on the electronic structure were systematically examined. The results reveal that these defects introduce localized electronic states, alter magnetic behavior, and modify the structural properties. In general, such defects act as deep traps capable of capturing and retaining charge carriers. The stability of these defects depends strongly on the chemical environment and the position of the Fermi level. Surface defect calculations reveal that oxygen vacancies form more readily at the surface than in the bulk over a wide range of electron chemical potential, with subsurface oxygen vacancies being more stable than those in the top layer. Overall, the findings demonstrate how charged defects influence magnetism, transport, and stability in UO 2 , providing insights that may guide improvements in the safety and efficiency of nuclear fuel.
36 MATERIALS SCIENCE↗
Weak antilocalization effect due to topological surface states in Bi 2 Se 2.1 Te 0.9
In this work, we have investigated the weak antilocalization (WAL) effect in the p -type Bi 2 Se 2.1 Te 0.9 topological system. The magnetoconductance shows a cusp-like feature at low magnetic fields, indicating the presence of the WAL effect. The WAL curves measured at different tilt angles merge together when they are plotted as a function of the normal field components, showing that surface states dominate the magnetoconductance in the Bi 2 Se 2.1 Te 0.9 crystal. We have calculated magnetoconductance per conduction channel and applied the Hikami-Larkin-Nagaoka formula to determine the physical parameters that characterize the WAL effect. The number of conduction channels and the phase coherence length do not change with temperature up to T = 5 K. In addition, the sample shows a large positive magnetoresistance that reaches 1900% under a magnetic field of 35 T at T = 0.33 K with no sign of saturation. The magnetoresistance value decreases with both increasing temperature and tilt angle of the sample surface with respect to the magnetic field. The large magnetoresistance of topological insulators can be utilized in future technology such as sensors and memory devices.
36 MATERIALS SCIENCE↗
Superconducting density of states and band structure at the surface of the candidate topological superconductor Au 2 Pb
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75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Weyl and Dirac semimetals in three-dimensional solids
Weyl and Dirac semimetals are three-dimensional phases of matter with gapless electronic excitations that are protected by topology and symmetry. As three-dimensional analogs of graphene, they have generated much recent interest. Deep connections exist with particle physics models of relativistic chiral fermions, and, despite their gaplessness, to solid-state topological and Chern insulators. Their characteristic electronic properties lead to protected surface states and novel responses to applied electric and magnetic fields. The theoretical foundations of these phases, their proposed realizations in solid-state systems, and recent experiments on candidate materials as well as their relation to other states of matter are reviewed.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Dirac nodal lines and flat-band surface state in the functional oxide RuO 2
The efficiency and stability of RuO 2 in electrocatalysis has made this material a subject of intense fundamental and industrial interest. The surface functionality is rooted in its electronic and magnetic properties, determined by a complex interplay of lattice-, spin-rotational, and time-reversal symmetries, as well as the competition between Coulomb and kinetic energies. This interplay was predicted to produce a network of Dirac nodal lines (DNLs), where the valence and conduction bands touch along continuous lines in momentum space. Here we uncover direct evidence for three DNLs in RuO 2 by angle-resolved photoemission spectroscopy. These DNLs give rise to a flat-band surface state that is readily tuned by the electrostatic environment, and that presents an intriguing platform for exotic correlation phenomena. Our findings support high spin-Hall conductivities and bulk magnetism in RuO 2 , and are likely related to its catalytic properties.
75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
Growth of PdCoO 2 films with controlled termination by molecular-beam epitaxy and determination of their electronic structure by angle-resolved photoemission spectroscopy
Utilizing the powerful combination of molecular-beam epitaxy (MBE) and angle-resolved photoemission spectroscopy (ARPES), we produce and study the effect of different terminating layers on the electronic structure of the metallic delafossite PdCoO 2 . Attempts to introduce unpaired electrons and synthesize new antiferromagnetic metals akin to the isostructural compound PdCrO 2 have been made by replacing cobalt with iron in PdCoO 2 films grown by MBE. Using ARPES, we observe similar bulk bands in these PdCoO 2 films with Pd-, CoO 2 -, and FeO 2 -termination. Nevertheless, Pd- and CoO 2 -terminated films show a reduced intensity of surface states. Additionally, we are able to epitaxially stabilize PdFe x Co 1– x O 2 films that show an anomaly in the derivative of the electrical resistance with respect to temperature at 20 K, but do not display pronounced magnetic order.
36 MATERIALS SCIENCE↗
Angle-Resolved Spectroscopy of Positronium Emission from a Cu(110) Surface
We report the affinity A Ps of positronium (Ps) atoms for a metal is the negative of the maximum kinetic energy with which Ps is emitted into vacuum when thermalized positrons in a metal encounter the surface. When this quantity is measured by ground state Ps time of flight (TOF), the precision is severely limited by the short triplet state lifetime of 142 ns. By quickly converting the emitted Ps atoms into long-lived Rydberg states, we are able to dramatically increase the TOF to allow precision measurements of A Ps . From our measurements made on a Cu(110) sample at T = 128 K, we find A Ps (128 K) = (-2.476±0.010 stat ±0.013 syst ) eV, compared with the result A Ps (128 K) = (-2.545±0.010 stat ±0.010 syst ) eV found using highly accurate generalized gradient approximations for both electrons and positrons within density functional theory. Such precision opens up opportunities in the quest for an improved density functional.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
QMCPACK: Advances in the development, efficiency, and application of auxiliary field and real-space variational and diffusion quantum Monte Carlo
We review recent advances in the capabilities of the open source ab initio Quantum Monte Carlo (QMC) package QMCPACK and the workflow tool Nexus used for greater efficiency and reproducibility. The auxiliary field QMC (AFQMC) implementation has been greatly expanded to include k-point symmetries, tensor-hypercontraction, and accelerated graphical processing unit (GPU) support. These scaling and memory reductions greatly increase the number of orbitals that can practically be included in AFQMC calculations, increasing the accuracy. Advances in real space methods include techniques for accurate computation of bandgaps and for systematically improving the nodal surface of ground state wavefunctions. Results of these calculations can be used to validate application of more approximate electronic structure methods, including GW and density functional based techniques. To provide an improved foundation for these calculations, we utilize a new set of correlation-consistent effective core potentials (pseudopotentials) that are more accurate than previous sets; these can also be applied in quantum-chemical and other many-body applications, not only QMC. These advances increase the efficiency, accuracy, and range of properties that can be studied in both molecules and materials with QMC and QMCPACK.
71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗