Condensed Matter Physics

When a large number of atoms condense into a fluid or solid, behaviors emerge that are only indirectly related to the physics of the individual atoms. Superconductivity is one such an emergent behavior, which could not be anticipated from even a detailed study of an isolated atom. The goal of condensed matter physics is not only to measure and explain such emergent phenomena, but also to manipulate these properties to produce the novel effects we desire. This allows us to both investigate fundamental physics and to develop commercially important applications.
Student performs photolithography in our clean room
Student performs photolithography in our clean room

A student in condensed matter physics must have a thorough understanding of both the microscopic quantum mechanics that underlies the system and the classical macroscopic theories of mechanics, electromagnetism, and statistical mechanics that describe its large scale behavior. This broad background enables students to go on to careers in academia, government labs, and industry.

The condensed matter group encompasses semiconductor physics, soft-matter physics, and nanophysics. A major focus of our group is the experimental and theoretical study of highly confined electron systems in artificially structured semiconductors and other low dimensional materials.

We cover all aspects of these systems from fundamental theory to device fabrication. This group operates as part of the Center for Semiconductor Physics in Nanostructures (C-SPIN) one of the National Science Foundation’s few Materials Research, Science and Engineering Centers. C-SPIN is a multi-million dollar, interdisciplinary research collaboration between scientists at the University of Oklahoma and the University of Arkansas. We have theoretical and experimental efforts in nano-scale semiconductor devices, spin transport in semiconductors and high-speed transistors. In addition to semiconductor studies, the group also has research efforts in high-efficiency photovoltaics, graphene, self-assembled monolayers, molecular plasmonics, nanoparticles, and lithium ion conducting polymers. Some of this work is performed in conjunction with researchers in the Departments of Chemistry and Biochemistry, Electrical & Computer Engineering, and Chemical, Biological, & Materials Engineering.

A flat gold nanoparticle grown by Prof. Bumm in solution, imaged by an electron microscope. The symmetric structure of the particle arises spontaneously from the local surface energies during growth. This is an example of a “bottom-up” method.

The majority of our experimental research takes place in the department’s state-of-the-art laboratories.

Our well equipped facilities include: a dual-chamber molecular beam epitaxy (MBE) system for the growth of III-V and IV-VI semiconductors; several scanning tunneling and atomic force microscopes for high resolution imaging and patterning of atomic surfaces; a cleanroom for optical lithography and semiconductor processing; a thin-film laboratory for routine vapor deposition; low temperature (<20 mK) and high magnetic field (15 T) facilities for optical and electrical studies; optical microscopes for single nanoparticle spectroscopy; a grazing angle infrared spectrometer for molecular spectroscopy of monolayers; full characterization techniques for solar cells analysis including a class-A solar simulator, an external quantum efficiency system with capacitance-voltage analysis equipment; and picosecond pulsed laser systems for our polymer studies. Scanning electron and transmission electron microscopes are available in the Samuel Roberts Noble Electron Microscopy Laboratory and are routinely used by our students for their research.

Graphene ripples in two dimensional carbon
Graphene ripples in two dimensional carbon

The condensed matter theory group (Mullen and Uchoa) is interested in many areas of research including new quantum phases in strongly correlated systems, quantum criticality and transport in semiconductor and carbon nanoscale systems.

New quantum states of matter have been found in a variety of materials in which the electronic many-body states are protected by topology. Many of these materials have elementary electronic quasiparticles that behave as Dirac fermions, the relativistic analogs to massless neutrinos. This class of electronic systems is called “Dirac materials” and current examples include graphene, (a new 2 dimensional allotropic form of carbon), topological insulators, and Weyl semimetals. Other novel correlated states can be found in semiconductor nanostructures that have no simple atomic analog, such as in rings or spherical shells. New quantum phases include chiral edge states in topological insulators, novel superconducting condensates in carbon-based systems, and polarized arrays of quantum rings.

Generic phase diagram with a quantum critical point
Generic phase diagram with a quantum critical point

Quantum criticality refers to phase transitions driven by quantum fluctuations rather than temperature. It is unusual in that quantum mechanics can be made manifest on a macroscopic level by tuning the system through its quantum phase transition. Examples of candidate materials for the experimental observation of quantum criticality include high temperature superconductors, heavy fermions, Dirac materials among other systems.

Transport on the nanoscale can be profoundly different from classical transport. Our studies involve currents in heat, charge, energy, spin and pseudospin in mesoscopic systems in general. We have predicted diverse transport results ranging from supercurrents in stressed graphene monolayers to cooling of an electron gas by “blowing off” the hotter parts of the current . Areas of interest include the role of disorder and many body effects into transport. In addition theory is an important partner to the experimental efforts of the Condensed Matter group.

Scattered wavefunction of an electron in the presence of a magnetic field

 

The research activities of the Condensed Matter group include the production of novel materials with fundamentally new properties, fabrication of nanostructures, and the advancement of new theories that can shed a conceptual light into new emergent quantum phenomena. The faculty in condensed matter physics share appointments in the engineering physics program. The engineering physicist provides the link between the pure scientist and the engineer by applying fundamental scientific theories to the solution of technological problems. As the miniaturization of transistors, lasers, and memory elements continues, an understanding of their operation increasingly requires knowledge of the underlying physics.

Research Areas

Faculty

Emeritus Faculty

Research Highlight: Good vibrations

Research Highlight: Good vibrations

On the top is a model of a carbon nanotube (CNT) to which alkane chains have been attached. Below are “bad” and “good” normal modes for conduction of heat through the system, as calculated by Abdellah Ait-Moussa, a student working with Prof. Mullen. A “bad” mode only couples to atoms in the CNT; a good one couples to the side chains as well as the CNT, so that the vibration and the energy it carries goes through the whole system. The goal of the research is to optimize the side chains to maximize the flow of heat. Improving heat conduction into CNT’s may lead to plastics that conduct heat as well as metals.