Conduction of a typical resistor is described by the Ohm’s law, where current (I) is linearly proportional to applied bias voltage (V) with conductance (I/V) as the coefficient. The more sensitive differential conductance (dI/dV) gives the same constant value of I/V. However, at some interfaces between different materials, the conductance varies depending on V and there is a peak (anomaly) in dI/dV at V = 0, called zero bias anomaly (ZBA). Many ZBAs are due to novel physics induced at the interface and have been utilized as crucial signature evidence to identify some of the recent exciting physics such as topological insulators and superconductors, Majorana fermions, nodal superconductivity, and decagonal quasicrystals. In this work, the Chen Lab in Physics has demonstrated that ZBAs can be observed in conventional superconductors and the observed ZBAs cannot be suppressed by a highly spin-polarized current. By systematically varying the size of the interface, they show the evolution of the ZBAs from conductance dips and determine that the ZBAs are not intrinsic to the SCs but are due to a resistance change induced by the critical field of large interfaces. This work demonstrated that it is crucial to vary the interface systematically to identify the origin of the ZBA. . Courtesy of Tingyong Chen, from: Gifford et al., J. Appl. Phys., 120, 163901 (2016).
Spintronics requires spin of conduction electrons aligned in a direction with the degree of the alignment measured by spin polarization. Despite its importance, very few techniques can measure spin polarization of a material. Andreev reflection spectroscopy (ARS) utilizes a spin configuration of a Cooper pair in a superconductor (SC) to detect the spin polarization. Previously, only conventional SCs have been utilized in ARS, with a typical Tc below 10 K and a critical field up to 1 kOe. Unconventional SCs can have much higher Tc up to 160K and much larger critical field of over 122 T, which can be utilized to explore many spin-related phenomena in a wide temperature and magnetic field range. However, spin detection using an unconventional SC has never been demonstrated in ARS. In this work, the Chen lab in Physics has, for the first time, shown that the spin polarization of a highly spin-polarized material can be measured using an unconventional Fe SC up to 52 K and the measured spin polarization is the same as that measured by a conventional SC. Courtesy of Tingyong Chen, from: Gifford et al., AIP Adv., 6, 115023 (2016).
Magnetic monopoles are hypothesized particles that have a magnetic, rather than electric, charge. Despite a long history of nondetections, monopoles continue to be a topic of study because of their possible role in the unification of the fundamental forces. In new theoretical work, Tanmay Vachaspati from Arizona State University, Tempe, considered whether monopoles could be generated through the scattering of waves. The results suggest one might detect monopoles in collisions between high-intensity, circularly polarized light waves.
Doping a semiconductor induces a change in its lattice parameter. This change is caused not only by the different sizes of the impurity and host atoms, but also by the strain developed when the occupation of electronic energy bands is modified. While these ideas are conceptually simple, experimentalists and theoreticians alike have struggled for decades to separate the size and electronic effects. Following up on the most significant earlier developments, which focused on doped Si, a Physics-Chemistry ASU team collected data for n-type Ge doped with novel precursors and discovered trends as a function of the donor species in both Ge and Si that hold the key to resolving the doping dependence of the lattice parameter into its two fundamental physical components.
This work was published by Physical Review B (doi:10.1103/PhysRevB.93.041201) and highlighted as an Editors' Suggestion.
Dexin Kong, a recently graduated PhD student working in the Drucker group, measured the optical response of epitaxial Ag islands grown on Si(100). These islands host localized surface plasmon resonances (LSPR) that are oscillations in their charge density. The LSPR modes that oscillate parallel to the Si surface have different frequencies than those that oscillate perpendicular to the Si surface. A theoretical model was developed that allowed identification of these modes. The figures show scanning and transmission electron micrographs of the Ag/Si(100) islands and a comparison of the real and imaginary parts of the dielectric functions of Si and the experimentally measured and simulated Ag/Si(100) pseudodielectric functions.
This work recently appeared in the J. Appl. Phys., 118, 213103 (2015).
Former Physics graduate student Luying Li returned to continue her collaboration with Physics faculty Dave Smith and Molly McCartney and address the possibility of detecting polarization fields across polytype interfaces in InAs nanopillars using the technique of electron holography. Part of the investigation included studying the local crystal polarity on an individual column-by-column basis. The false color image shows an aberration-corrected electron micrograph from an InAs nanopillar with irregular stacking disorder, also demonstrating that resolution of individual In and As atomic columns can be achieved.
Courtesy of Luying Li and Dave Smith
From: L. Li, et al., Adv. Mater., 26, 1052 (2014)
In a recent collaboration with UT-Austin, physics faculty Dave Smith and Molly McCartney used advanced electron microscopy techniques to study a classical semiconductor/ferroelectric interface between germanium and barium titanate. Aberration-corrected electron micrographs, as shown in the figure, convincingly demonstrated that the Ge(001) 2x1 surface reconstruction remained intact during subsequent BaTiO3 growth. This information was key to explaining the otherwise unsolved electronic properties of the interface.
Image shows stepped interface between barium titanate epilayer and germanium substrate, together with overlaid structural model.
Courtesy of Dave Smith
From: K.D. Fredrickson, et al., Appl. Phys, Lett., 104, 242908 (2014)
Materials graduate Desai Zhang, working with Physics faculty Molly McCartney, used the technique of electron holography to resolve a longstanding question about the multi-grain nature of magnetic domains in thin spinel ferrite films: a) Lorentz image; b) phase image; b) map of magnetic domains; d) domain overlay on Lorentz image demonstrating the multigrain domains. The composite image was featured as a cover illustration in a recent issue of the Journal of Applied Physics.
Courtesy of Molly McCartney
From: D. Zhang, et al., J. Appl. Phys., 116, 083901 (2014)
In conventional electronics, only an electron current is utilized, whereas a spin-polarized electron current is utilized in spintronic devices. The device performance then depends crucially on the value of the spin polarization. Thus, a current with controllable spin polarization can be used to tune the device. However, the spin polarization depends on the specific band structures of the material and cannot readily be changed. In this work, physics faculty Tingyong Chen and Dave Smith have shown that in a Co/Cu multilayer structure consisting of 1 nm Co and 1nm Cu with large giant magnetoresistance (GMR) value of close to 120%, the spin polarization can be continuously controlled by a modest external magnetic field. This result is the first demonstration of continuous control of the spin polarization of current by a magnetic field. Courtesy of Tingyong Chen, from: J. A. Gifford, et al., Appl. Phys. Lett. 108, 212401 (2016)
Dr Daniel Martin working in the Matyushov group has recently took advantage of one of the fastest high performance supercomputers (Anton, D. E. Shaw) to look at electron tunneling in proteins. Membrane-bound bc1 complex is one of the key elements of biology’s energy production chain in mitochondria of animals and photosynthetic centers of bacteria. Longer than 10 microseconds of fully atomistic computer simulations have allowed for the first time to study the protein's low frequency motions driving electron transfer. A broad range of fluctuations, spanning from picoseconds to microseconds, affects the transition. Surprisingly, slow motions, in the range 0.1-1.6 microseconds turned out to be particularly important. This work recently appeared in the J. Chem. Phys. 142, 161101 (2015).
Zeolites are important silicate-based materials that are related to sand, i.e. quartz. Although their structures are built from the same corner-sharing SiO4 tetrahedra as quartz, they contain pores and tunnels that can absorb water and small hydrocarbon molecules (very much unlike quartz). This microporosity gives zeolites enormous internal surface area, enabling a rich and diverse array of chemical uses. For example, about half of the gasoline in the US has been in contact with a zeolite. At present, there are 231 known zeolite Framework Types. By treating zeolites as periodic graphs, we have discovered many millions of low-energy hypothetical topologies in the computer. In theory, there is an innite number of hypothetical zeolites frameworks types. The mystery is “Why are so few of the hypothetical topologies found in nature?” The key appears to be framework exibility. All of the known zeolite frameworks are exible if we treat the SiO4 tetrahedra as being ideal and rigid, allowing exing only at the vertices. In other words, low framework energy is no enough, the framework must be able to “breathe” too. This important property allows us to identify the smaller subset of realizable hypothetical frameworks within our database. Z. Krist., 212 768–791 (1997); Microporous & Mesoporous Materials 74 121–132. (2004).
The structures of amorphous materials are hard to determine. We know that they are highly disordered, but atoms cannot get arbitrarily close to one another, so we know that there is strong short-range order. The open question is “What is the length range of this order?” Recent graduate student Aram Rezikyan studied amorphous silicon and carbon using the Fluctuation Electron Microscopy (FEM) technique. By studying random diraction speckle in images (measured by the normalized intensity variance V(k)) as a function of diraction vector, and beam voltage, he was able to probethe role of electron beam damage in the disruption of the amorphous structure. Such atom motions give rise to an eective electron decoherence. It turns out that atoms move under the electron beam much more than we thought. Microscopy and Microanalysis 21 1455–1474 (2015).
Combining spectroscopy with the high spatial resolution in the electron microscope is a powerful tool to investigate chemistry, including local bonding changes, at the atomic scale. Traditionally it has only been possible to perform the equivalent of UV and soft X-ray spectroscopy with energy resolution of just under 1 eV. Our new Nion electron microscope, with a specially developed monochromator, gives us 10 meV energy resolution. This allows us to do the equivalent of optical and infrared spectroscopy, still at the nm scale. The spectrum on the left comes from a guanine fish scale. All the peaks arising from vibration of hydrogen attached to C or N atoms are distinguishable. Since the presence of hydrogen is generally inferred in electron microscopy, not directly directed, this represents a major advance for characterization of polymers and materials of biological origin.
Crystal structures of noncentrosymmetric materials don’t have inversion symmetry, a common feature observed in most materials. The lack of the inversion symmetry results in an anti-symmetric Dzyaloshinskii-Moriya interaction, which competes with the usual Heisenberg exchange interaction and plays a key role in creating non-collinear magnetic spin structures such as skyrmions in these materials. Furthermore, in superconductivity, the lack of the inversion symmetry may produce an anti-symmetric Rashba-type spin-orbit coupling, which allows mixing of the spin-singlet and the spin-triplet Cooper pairing states, as opposed to the pure spin singlet state in most of the known superconductors. Collaborating with USTC, JHU and UNH, physics faculty Tingyong Chen and his graduate students have shown that Rh2Mo3N, a noncentrosymmetric material with β-manganese structure is a superconductor with critical temperature Tc ≈ 4.3 K. Andreev reflection spectroscopy measurements using both normal metal and half metal show conclusively s-wave pairing with an isotropic gap. Courtesy of Tingyong Chen, from: Wensen Wei, et al., Phys. Rev. B 94, 104503 (2016).