Down the rabbit hole: How electrons travel through exotic new material.

Sounds like they are observing the Charge Field?
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Three-dimensional image using scanning tunneling electron microscopy of electrons on the surface of a Weyl semi-metal, a kind of crystal with unusual conducting and insulating properties.

(More at link...)
https://www.sciencedaily.com/releases/2016/03/160310143910.htm

Credit: Yazdani et al., Princeton University.

Researchers at Princeton University have observed a bizarre behavior in a strange new crystal that could hold the key for future electronic technologies. Unlike most materials in which electrons travel on the surface, in these new materials the electrons sink into the depths of the crystal through special conductive channels.

"It is like these electrons go down a rabbit hole and show up on the opposite surface," said Ali Yazdani, the Class of 1909 Professor of Physics. "You don't find anything else like this in other materials."

The research was published in the journal Science.

Yazdani and his colleagues discovered the odd behavior while studying electrons in a crystal made of layers of tantalum and arsenic. The material, called a Weyl semi-metal, behaves both like a metal, which conducts electrons, and an insulator, which blocks them. A better understanding of these and other "topological" materials someday could lead to new, faster electronic devices.

The team's experimental results suggest that the surface electrons plunge into the crystal only when traveling at a certain speed and direction of travel called the Weyl momentum, said Yazdani. "It is as if you have an electron on one surface, and it is cruising along, and when it hits some special value of momentum, it sinks into the crystal and appears on the opposite surface," he said.

These special values of momentum, also called Weyl points, can be thought of as portals where the electrons can depart from the surface and be conducted to the opposing surface. The theory predicts that the points come in pairs, so that a departing electron will make the return trip through the partner point.

The team decided to explore the behavior of these electrons following research, published in Science last year by another Princeton team and separately by two independent groups, revealing that electrons in Weyl semi-metals are quite unusual. For example, their experiments implied that while most surface electrons create a wave pattern that resembles the spreading rings that ripple out when a stone is thrown into a pond, the surface electrons in the new materials should make only a half circle, earning them the name "Fermi arcs."

To get a more direct look at the patterns of electron flow in Weyl semi-metals, postdoctoral researcher Hiroyuki Inoue and graduate student András Gyenis in Yazdani's lab, with help from graduate student Seong Woo Oh, used a highly sensitive instrument called a scanning tunneling microscope, one of the few tools that can observe electron waves on a crystal surface. They obtained the tantalum arsenide crystals from graduate student Shan Jiang and assistant professor Ni Ni at the University of California-Los Angeles.

The results were puzzling. "Some of the interference patterns that we expected to see were missing," Yazdani said.

To help explain the phenomenon, Yazdani consulted B. Andrei Bernevig, associate professor of physics at Princeton, who has expertise in the theory of topological materials and whose group was involved in the first predictions of Weyl semi-metals in a 2015 paper published in Physical Review X.

...
Story Source:

The above post is reprinted from materials provided by Princeton University. Note: Materials may be edited for content and length.

Journal Reference:

   Hiroyuki Inoue, András Gyenis, Zhijun Wang, Jian Li, Seong Woo Oh, Shan Jiang, Ni Ni, B. Andrei Bernevig,and Ali Yazdani. Quasiparticle interference of the Fermi arcs and surface-bulk connectivity of a Weyl semimetal. Science, 2016 DOI: 10.1126/science.aad8766

Arsenic is unbalanced with a single top slot open:
http://www.users.on.net/~Nevyn/science/physics/AtomicWebViewer/V_0_7/AtomicViewer.html?background=None&metadata=false&element=33

Tantalum has an unbalanced structure. Two single slots on the carousel?
http://www.users.on.net/~Nevyn/science/physics/AtomicWebViewer/V_0_7/AtomicViewer.html?background=None&metadata=false&element=73

This is just a quick thought, but if the top slot of Arsenic bonded with Ta on one the carousel sides to form Tantalum Arsenide -- like wouldn't it hang charge flows on one side or the other? Wouldn't flipping be possible with such a structure.  


Long-sought subatomic particle ‘seen’ at last - Scientists detect new subatomic particle in a ‘semimetal’ material

https://student.societyforscience.org/article/long-sought-subatomic-particle-%E2%80%98seen%E2%80%99-last?mode=topic&context=6

The material is called tantalum arsenide. The newfound particles’ behavior gives this material metal-like features. Called a “semimetal,” it shares features with materials such as graphene, which is a sheet of carbon that’s just one atom thick. Its novel structure gives graphene unusual superstrong characteristics that have excited researchers over the last decade or so. “There are a lot of reasons to be interested in these materials,” notes Balents, who was not involved with the new fermion discovery.

Some scientists think that like graphene, tantalum arsenide could change the future of electronics. It could let devices use a fast-moving electrical current that easily evades any bumps or valleys in its path. Physicists can also use tantalum arsenide to learn more about Weyl fermions. These particles are stuck inside the material. But some physicists suspect free-floating Weyl fermions might also exist.

Princeton University.
"After 85-year search, massless particle with promise for next-generation electronics found."
ScienceDaily. ScienceDaily, 16 July 2015. <www.sciencedaily.com/releases/2015/07/150716160325.htm>.

https://www.sciencedaily.com/releases/2015/07/150716160325.htm
(more at link)


A detector image (top) signals the existence of Weyl fermions. The plus and minus signs note whether the particle's spin is in the same direction as its motion -- which is known as being right-handed -- or in the opposite direction in which it moves, or left-handed. This dual ability allows Weyl fermions to have high mobility. A schematic (bottom) shows how Weyl fermions also can behave like monopole and antimonopole particles when inside a crystal, meaning that they have opposite magnetic-like charges can nonetheless move independently of one another, which also allows for a high degree of mobility.

Credit: Image by Su-Yang Xu and M. Zahid Hasan, Princeton Department of Physics

Tantalum arsenide is the first “Weyl semimetal” scientists have found. In some ways, such a semimetal is similar to “topological insulators.” Those are relatively newfound materials that don’t conduct electricity well on the inside, but let electrons run laps around their surface. Tantalum arsenide does not have the same kind of interior. But it does have high-speed electron superhighways on its surface.

The new twist with the Weyl semimetal, Xu says, is that its surface electrons don’t race around a closed track. Instead, they move from one side to the other. Then they disappear into the material and pop back out on the opposite surface.

Weyl semimetals also are similar to graphene, Balents says. Both materials let electrons zip around at extreme speeds and act like they have no mass. All these features make Weyl semimetals an exciting possibility for future electronics, Hasan says.

https://student.societyforscience.org/article/long-sought-subatomic-particle-%E2%80%98seen%E2%80%99-last?mode=topic&context=6

Found a few papers that may tie with Mathis' recent papers on the Hall Effect and Stark Effect:
The Hall Effect: a Charge Field Explanation. I ditch electron holes, offering a logical and mechanical theory for the Hall Voltage.
http://milesmathis.com/hall.pdf

The Stark Effect: Not only do I show charge field causes of both the shift and the split, I show a new cause of spectral lines—one that has nothing to do with electron orbitals.
http://milesmathis.com/stark.pdf

Phonon  engineering  in  graphene  and van der Waals materials
Alexander A. Balandin

http://ndl.ee.ucr.edu/MRS-Review-2014.pdf

   The following article is based on the MRS Medal Award presentation given by Alexander A. Balandin on
December 4, 2013, at the Materials Research Society Fall Meeting in Boston. Balandin was recognized
for his  “ discovery of the extraordinary high intrinsic thermal conductivity of graphene, development of
an original optothermal measurement technique for investigation of thermal properties of graphene,
and theoretical explanation of the unique features of the phonon transport in graphene .”          
Phonons—quanta of crystal lattice vibrations—reveal themselves in electrical, thermal,
optical, and mechanical phenomena in materials. Phonons carry heat, scatter electrons,
and affect light–matter interactions. Nanostructures opened opportunities for tuning the
phonon spectrum and related properties of materials for specific applications, thus realizing
what was termed phonon engineering. Recent progress in graphene and two-dimensional
van der Waals materials has led to a better understanding of phonon physics and created
additional opportunities for controlling phonon interactions and phonon transport at room
temperature. This article reviews the basics of phonon confinement effects in nanostructures,
describes phonon thermal transport in graphene, discusses phonon properties of van der
Waals materials, and outlines practical applications of low-dimensional materials that rely
on phonon properties.
========
PHONON ENGINEERING IN GRAPHENE AND VAN DER WAALS MATERIALS  
820

(where  S is the cross-sectional area of the laser spot), which defines  I(Eg²) for  H smaller than the light penetration depth in a given material. This dependence makes possible non-destructive, rapid, and reliable nano-metrology of FQL materials after calibration with atomic force microscopy (AFM).

The graphene-like exfoliation of thin films from Bi2Te3 crystals followed by re-assembly into “pseudo-superlattices” of
the stacks of such van der Waals films resulted in an improved thermoelectric figure of merit.  
 The in-plane thermal conductivity of the stacks decreased by a factor of  ∼2.4 at RT as compared to the bulk, or by  ∼3.5 for the cross-plane value.
It was concluded that the reduction of the phonon thermal
conductivity without degradation of the electrical transport
properties was responsible for the observed improvement of
the thermoelectric efficiency. The film thinning to FQL and tuning of the Fermi level can
potentially lead to achieving high thermoelectric efficiency.  


Van der Waals forces in Graphene systems
http://www.ifm.liu.se/courses/TFYY67/Graphene.pdf

An electron in graphene behaves as a massless particle.

The band structure of graphene is special.
There are cone-structures at six points at
the boundary of the Brillouin zone.
Effectively there are two of these per zone.
The Fermi level is at the point where the
cones meet. So the Fermi surface is just a
point.

Modeling van der Waals forces in graphite
http://www.me.umn.edu/~dtraian/TonySlides.pdf
(Claim: Van der Waals dispersion forces hold graphite together)

Lenard Jones (6-12) Potential
● Energy vs. Bond length
● Steep repulsion due to the
Pauli exclusion principle

Graphite Interlayer energy
● Graphene layers are basically closed shell
systems (no covalent bonding between layers)
● Energy between layers is a balance between
●  Attractive dispersion forces
● Corrugated repulsive overlap forces
● Energy vs. interlayer separation similar to LJ pot.

Also:

Surface lattice vibration and electron-phonon interaction in topological insulator Bi2Te3 (111) films from first principles

G. Q. Huang

Published 12 October 2012 • Copyright EPLA, 2012 • EPL (Europhysics Letters), Volume 100, Number 1
Abstract

Surface phonon and electron-phonon (EP) interaction in Bi2Te3 (111) films are calculated by including spin-orbit coupling from the density-functional perturbation theory. We present a detailed analysis of the zone-center phonons by considering the interlayer relaxation of the films. The phonon density of states and the localized surface phonon modes along the $\overline {\Gamma M}$ and $\overline {\Gamma K}$ directions in the surface Brillouin zone are given. Our results clearly show that softening and stiffening of the surface phonons coexist in Bi2Te3 ultrathin films. Furthermore, we report on a first-principles study of the EP coupling constant for Bi2Te3 films. Our calculated EP interaction in Bi2Te3 films is very weak, which is in favor of application for room temperature electronic devices.

https://iopscience.iop.org/article/10.1209/0295-5075/100/17001/meta

(more at link...)
Synopsis: A van der Waals Tuning Knob
December 1, 2015
By adding dopant atoms to a graphene sheet, researchers are able to control the van der Waals attraction that the surface exerts on molecules.

http://physics.aps.org/synopsis-for/10.1103/PhysRevLett.115.236101

Synopsis figure

Nicolae Atodiresei/Forschungszentrum Jülich

The van der Waals attraction occurs when two objects come close enough to induce an electric polarization in one another. Researchers have now demonstrated a way to tune the van der Waals force exerted by graphene on a molecule. The technique, which is based on doping the graphene from the back side, could be used to control adsorption to graphene for a wide range of molecules and raises the prospect of electrical control of adsorption.

Tunability is nothing new to graphene. Its unique cone-shaped electronic band structure allows one to control the potential energy (or Fermi level) of its electrons by simply adding a gate electrode or dopant atoms. Recent work has used this Fermi-level tuning to manipulate the strength of ionic bonding between atoms and the graphene surface. Felix Huttmann from the University of Cologne, Germany, and his colleagues have extended this surface control to the weaker, but more general, van der Waal interactions.

The researchers produced a graphene sheet on a metal substrate and then added either electron-donor atoms (n-type dopants) or electron-acceptor atoms (p-type dopants). In both cases, the dopants nestled between the graphene and metal, leaving a “clean slate” on the top surface of the graphene. To explore van der Waals interactions, the team exposed the graphene to naphthalene molecules and observed their adsorption on the surface with scanning tunneling microscopy. When the samples were heated, the temperature at which naphthalene desorbed was higher for n doping than for p doping, implying that the van der Waals attraction was stronger in the former. Theoretical calculations confirmed this picture by demonstrating that n doping causes the electron orbitals around carbon atoms to extend out further spatially, making the atoms more easily polarizable.

This research is published in Physical Review Letters.

–Michael Schirber

Charge Density Waves in Exfoliated Films of van der Waals Materials:
Evolution of Raman Spectrum in TiSe2

Pradyumna Goli, Javed Khan, Darshana Wickramaratne, Roger K. Lake, and Alexander A. Balandin
*
Department of Electrical Engineering and Materials Science and Engineering Program, Bourns College of Engineering, University of
California
Riverside, Riverside, California 92521, United States

ABSTRACT:
A number of the charge-density-wave materials
reveal a transition to the macroscopic quantum state around
200 K. We used graphene-like mechanical exfoliation of TiSe2 crystals to prepare a set of films with different thicknesses. The transition temperature to the charge-density-wave state was determined via modification of Raman spectra of TiSe2 films. It was established that the transition temperature can increase
from its bulk value to ∼240 K as the thickness of the van der Waals films reduces to the nanometer range. The obtained results are important for the proposed applications of such materials in the collective-state information processing, which require room-temperature operation.

http://ndl.ee.ucr.edu/Balandin-CDW-Nano-Letter.pdf