Heremans Group Heremans Group - Quantum Matter and Nanodevices Lab
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Condensed Matter Experiment: Quantum Matter, Quantum Electronic Devices, Low-Dimensional Systems, Nanoelectronics

Overview
The Quantum Matter and Nanodevices Lab is located at the Department of Physics at Virginia Tech. Most of the Lab’s facilities are housed in Robeson Hall on the Blacksburg campus. Researchers in the group investigate the fundamental properties of low-dimensional electronic systems in advanced devices and in quantum materials, and work on the realization of future nanoscale solid-state electronic devices with new functionalities for computing and communications technologies. The word cloud picture illustrates some of the physics and materials involved in the research. The research covers a number of aspects of nanometer scale semiconductor devices and spintronics devices, and of correlated-electron materials and quantum materials (in quantum materials the device properties are mostly determined by quantum-mechanical effects and not by classical physics). A variety of electronic probes at very low temperatures and in strong magnetic fields are used in the research. The Lab fabricates all of its own quantum devices using advanced nanofab techniques. Topics in solid-state physics and quantum electronic phenomena that are covered include new electron hydrodynamic effects and ballistic transport effects, spintronics, nuclear spintronics, correlated-electron effects in quantum materials, electron quantum coherence, electron spin coherence, quantum and nanoelectronic devices, and plasmonics in nanoscale devices. The Lab has collaborations worldwide with experimental groups and theoretical groups.

Lab
The research uses low-dimensional electron systems in a variety of materials, typically in thin film form (heterostructures GaAs/AlGaAs, InGaAs/InAlAs, InAs/(Al)GaSb and InSb/InAlSb, thin film InSb, thin film bismuth, thin films of other quantum materials, magnetic thin films, ...). Using the nanofabrication techniques in the lab we fashion the diverse materials into measurable nanoscale devices to study the underlying physics. Pictured as examples are the photolithography setup in the lab’s own cleanroom, and the lab’s electron beam lithography and scanning electron microscope system. The lab houses several thin film deposition systems and other processing and characterization equipment. The group also uses Virginia Tech’s central cleanroom for microfabrication. Central to the group’s research are the electronic characterization probes. Magnetotransport and quantum transport measurements are performed in several cryostats (3He to reach below 1 K, and variable temperature) under magnetic fields up to about 10 T. A 3He cryostat and associated low-noise electronics are shown in the picture.

Current Projects
Examples of current research projects are: hydrodynamic transport in low-dimensional electronic materials, using transverse magnetic focusing of electrons to characterize electron-electron interactions, nuclear spintronics and dynamic nuclear polarization, quantum nuclear spin torque, developing non-local quantum sensors based on Aharonov-Bohm quantum interference, spin-dependent quantum magnetotransport to measure electron spin coherence, developing nanoscale plasmonic antennas for optical sensing, electronic and magnetic properties of bismuth iridates and of other quantum materials.

Hydrodynamic and ballistic transport, electron-electron interactions
While the scattering of electrons in materials with impurities and phonons is well-known, coming to terms with electron-electron scattering has proven more challenging, even though electron-electron interactions play a central role in solid-state physics. Electron-electron interactions e.g. determine the quasiparticle lifetime, important in Fermi liquid theory, which describes the normal state of metals at low temperatures. Electron-electron interactions are also responsible for the formation of new quantum states of matter. It has recently been realized that electron-electron interactions further produce an effective viscosity in an electron fluid, by promoting momentum exchange between different layers of the fluid. This leads to the new field of solid-state electron hydrodynamics, of great current fundamental and applied interest. Hydrodynamic effects become important in ultra-clean materials where the mobile carriers at low temperatures have mean-free-paths which can reach several microns, such as GaAs/AlGaAs heterostructures, graphene, and dellafossites. In such materials, only recently achievable, electron momentum does not rapidly dissipate to the lattice, and effects of electron-electron interactions dominate. We have used GaAs/AlGaAs heterostructures to study low-temperature hydrodynamic transport and ballistic transport (where even electron-electron interactions are assumed weak). Using transverse magnetic focusing (a ballistic effect in small devices) we have introduced a precision method to measure electron-electron interactions. The picture illustrates this work, showing the device and principle, and the computer-intensive simulations of the current streamlines by our theoretical collaborators. The simulations extensively use the Advanced Computing Resources at Virginia Tech. We found that previous theory is only approximately followed, a far-reaching result we published in Nature Communications. In a Physical Review Letters we compare hydrodynamic and ballistic transport, and find that typical hydrodynamic phenomena such as vortices are also produced in ballistic transport, a surprising finding. Work is ongoing, with theoretical collaborators, to explore these newly achievable electron transport regimes from both a fundamental point of view, as well as from the point of view of applications for low-power and/or high-frequency electronics and ultra-quiet amplifiers. Here is a movie we made to show the evolution of hydrodynamic-like effects in ballistic transport.

Nuclear spintronics, dynamic nuclear polarization and quantum nuclear spin torque in bismuth
Bismuth is an interesting semimetal, with naturally long carrier mean-free-paths, low-dimensional surface states holding highly mobile metallic electrons, strong spin-orbit interaction in its surface states, and strong hyperfine interaction coupling the electron spin to a high nuclear spin. Bi can become a 2D topological insulator in ultrathin-film form. With such properties Bi is both a quantum material and a spintronics material. We have developed a unique van der Waals epitaxy method to deposit high-quality Bi thin films. We have used the films to study the generation of spin-polarized electrons in Bi(111) surface states using the strong spin-orbit interaction, and then we used the spin-polarized electrons to spin-polarize the Bi nuclei in a dynamic nuclear polarization step. This was achieved all-electrically, and is illustrated in the figure. This spintronics and nuclear spintronics work was published in a Physical Review Letters paper, an Editors' Suggestion. The research demonstrates nuclear spintronics in Bi, where the electron spin is used to manipulate the nuclear spin, and where the nuclear spin can serve as a long-lived memory of spin information. The work also demonstrates steps towards an extension of the spin torque concept to quantum nuclear spin torque, as we are exploring with collaborating groups. Our findings can also be used to mitigate electron spin decoherence from hyperfine interaction in devices for quantum information processing. Work is ongoing on nuclear spintronics using dynamic nuclear polarization, and on quantum nuclear spin torque.

Aharonov-Bohm quantum interferometers for quantum sensing
Quantum sensing involves the use of a quantum property to perform a physical measurement, and is an important component of quantum information technologies. The Aharonov-Bohm effect has its origin in the quantum mechanical phase (expression in the picture) that a particle acquires from the presence of a non-local magnetic field flux. Constructive and destructive interference of partial electron quantum waves in a ring-like interferometer (shown in the picture), results in oscillations in resistance measured over the ring at low temperatures (shown in the picture). Fabricating functional Aharonov-Bohm electron interferometers is not easy and not often achieved, but is now routine in the group as shown in the picture. The Aharonov-Bohm effect is famous for demonstrating the non-local effects of electromagnetic fluxes in quantum mechanics, but also has surprisingly powerful implications for quantum sensing of magnetic qubits. In ongoing projects, the group uses its unique ability to reliably fabricate and measure Aharonov-Bohm interferometers to develop their use for quantum sensing.

Nanoscale optical antennas
We are all familiar with the wireless communications antennas on rooftops, on cell towers, and in cell phones. These antennas are large, because they detect electromagnetic waves of wavelengths of about 10 m to 10 mm. The group studies very small nanofabricated antennas of micrometer or nanometer scales, that can detect electromagnetic radiation with wavelengths of infrared or visible light. These nanoantennas are sensitive, can be directional, and can detect phases hence can be used in phased arrays etc. Plasmonic effects due to mobile carriers in the substrate enter as design parameters not available for free-space antennas. Applications for such photon detectors are plentiful.

Other quantum materials projects: bismuth iridates, tantalum trisulfide, tungsten disulfide, ...
In collaboration with other experimental and theoretical groups, research is ongoing on electronic and magnetic measurements on pyrochlore bismuth iridates, quantum materials where the energy scales of electron-electron interactions, spin-orbit interaction and kinetic energy are similar and combine in interesting ways to generate new quantum states of electronic matter. It is predicted that exotic electronic and magnetic quantum ground states emerge from the interplay of the energy scales and geometrical magnetic frustration in this correlated-electron materials system. The system can e.g. be driven into a topological Weyl semimetal state. The group performs the low-temperature quantum magnetotransport measurements in this collaboration, on e.g. microfabricated geometries as shown in the picture. Another electronic material of interest is the trichalcogenide TaS3, which is an environmentally stable layered material with pronounced crystalline anisotropy that supports charge density waves. Also the dichalcogenide WS2 is of interest, as a layered 2D electronic material with appreciable spin-orbit interaction and promising as channel material in field-effect transistors with new functionalities.