Condensed Matter Physics
What is Condensed Matter Physics?
Condensed matter physics (CMP) is the fundamental science of solids and liquids. As the largest branch of physics, it has the greatest impact on our daily lives by providing foundations for technology developments. For example, the invention of transistors and semiconductor chips have led to the widespread use of a variety of data storage, telecommunication, and multi-media devices (e.g., cellular phones, digital still and video cameras, MP3 players, GPS, flat panel HDTVs, DVDs, hard disk drives, flash drives, Blue-ray players, iPod and iPad, etc.), and personal computers. Almost every aspect of our daily life benefits from research in condensed matter physics: for example, composite materials are used in jet turbines and modern tennis rackets; superconducting magnets are used in MRI tomography for medical diagnostics; various solid state sensors and detectors for space exploration and homeland security, and lasers for entertainment, broadband communication, and various medical procedures.
Today, condensed matter physics is one of the most active and exciting research area in both basic sciences and technological applications. At the fundamental level, condensed matter physics is intellectually stimulating due to the continuing discoveries of many new phenomena and the development of new concepts and tools that are necessary to understand them. It is the field in which advances in theory can most directly be confronted with experiments. It has repeatedly served as a source or testing ground for new ideas (e.g., Josephson effect, integer and fractional quantum Hall effects, Aharanov-Bohm effect, mechanism of high-Tc superconductors, dissipative quantum physics, critical phenomena, mesoscopic physics, nonlinear dynamics, etc.). As a result, over the past 50 years, 22 Nobel Prizes in Physics were awarded to condensed matter physics and related areas, and 5 Nobel Prizes in Chemistry were awarded for subjects in condensed matter physics. In addition to its scientific value, condensed matter physics is intimately connected with industry. Condensed matter physics is one of the foundations of most modern technologies, like energy, information, defense, and manufacturing. A large number of scientists trained in condensed matter physics work in industry and found the training they received in university very rewarding.
Research topics in KU Condensed Matter Physics Program
Superconductivity was discovered in 1911. It is a phenomenon of great intricacy, diversity, and elegance. It is one of the most interesting and challenging subfields of CMP. For instance, the mechanism of high-Tc superconductors remains unsolved despite two decades of persistent efforts by some of the leading scientists in the world. On the other hand, significant progress has been made in the technical application of superconductivity. Large scale applications include the world's fastest experimental magnetically levitated train, superconducting magnets in MRI (Magnetic Resonance Image) systems, the world's largest electromagnets for thermonuclear fusion experiments, and bending and focusing magnets for the world's most powerful particle accelerators. Small-scale electronic applications include the fastest operating and the least-power consuming digital logic devices and circuits - the Rapid Single Flux Quanta (RSFQ) logic family, the most sensitive and lowest-noise electronic and magnetic sensors (SQUIDs), the most accurate voltage standard (the Josephson voltage standard), and the highest resolution x-ray detectors, just to name a few. Superconducting devices are also studied in our group in the context of quantum computing.
Quantum Information Science
Quantum mechanics, which often being described as non-intuitive and weird, is one of the greatest achievements and corner stones of modern physics. On one hand, it is the theoretical foundation to many great scientific discoveries and technological inventions such as atomic structures, transistors, lasers, etc while on the other hand there are still many open questions in our understanding of quantum mechanics. Recently, it is found that applying quantum mechanics to information processing one can exponentially increase computing power with the same amount of resources. In addition, coding information quantum mechanically can make communication absolutely secure against all eavesdropping.
At KU we are developing solid state quantum bits (a.k.a. qubits) as the elementary building blocks for quantum computers. A qubit has two quantum states |0> and |1> which represents a bit of binary information. However, unlike a transistor which can either being in state |0> or state |1> at a time a qubit can be in both |0> and |1> states simultaneously – a direct consequence of quantum superposition. Together with a unique property of multi-partite quantum systems called quantum entanglement a quantum computer can solve certain class of mathematical problems that are intractable to any classical computers human can build.
Our research is focused on design, simulation, fabrication, and characterization of qubits based on Josephson junctions. One of the goals is to increase their coherence time so that they are more robust against external disturbances. Another approach is to couple them to superconducting cavities to investigate quantum electrodynamics (QED) – an very active research area for which we are one of the pioneers. Recently, we have demonstrated coherent state control of a tripartite quantum system using Landau-Zener-Stuckelberg interferometry and quantum interference of photon dressed qubit states. In addition, we are interested in develop hybrid qubit systems that combine and exploit the advantages of nature-made and artificial qubits.
Nanoscience and Nanotechnology
Nanoscience and Nanotechnology are emerging disciplines that seek to understand the nanoscale processes that govern the behaviors of materials and devices. While the bulk properties are well understood the properties of nanostructured materials can differ considerably from them. Considering the trend of devices’ size gets smaller the knowledge of these properties becomes more important. This new direction of nanoscience is rather multi-disciplinary, spanning physics, chemistry, biology, medicine, and materials science, as well as many other fields. KU CMP faculty members are actively involved in interdisciplinary projects in synthesis of nanostructured materials and fabrication of novel nanoscale devices. The department runs a state-of-the-art nanofabrication facility that allows to create structures on this scale. Very recently, some of our faculty members are working together, also with researcher from other disciplines, on developing high efficient and low cost solar cells using nanotechnology.
Thin films (thickness typically in the range of 1 nanometer to hundreds of nanometers) in particular are the basis for a large number of technologies. Growth and characterization of thin films are important subjects for basic as well as applied research. Many examples exist in the literature and technology which prove that the important physical properties of thin film materials are strongly affected by their structure. Advanced approaches have been emerging for the need of controllable growth, in situ characterization and quantitative correlation of structural , morphology, electronic and optical properties. A large number of techniques such as X-ray Diffraction (XRD), Transmission and Scanning Electron Microscopy (TEM and SEM), Scanning Probe Microscopy (SPM), electrical transport, optical spectroscopy, and a variety of surface techniques have been used for many years to characterize the physical properties of thin films.
Development of sufficient clean energy represents the one of society’s foremost challenges in this century. Sunlight is a compelling solution to our need for clean, abundant sources of energy, with potential capacity to supply all the world’s current energy needs. Disappointingly, solar energy accounts for <2% of the current world’s energy use. Reducing this huge gap demands technology developments in high performance and low-cost photovoltaics (PV) and other related technology such as energy storage, distribution and usage (built environment, electrical cars, etc). The KU CMP group has established a broad collaboration through the NSF EPSCoR Kansas Center for Solar Energy Research with primary focuses on: (1) acquiring fundamental understanding of the electronic and photonic behaviors of novel graphene-based plasmonic nanostructured transparent conductors (TCs); and (2) to develop high efficiency, low-cost plasmonic graphene-based thin film and nanostructured PVs.
One of interesting research area is to study the critical phenomena in low-dimensional physics at nanoscale. For example, among eight allotropes of carbon, single-walled carbon nanotubes are one-dimensional conductors, acting as quantum wires; the intrinsic resistance of high-quality samples has been observed to be close to quantum conductance h/4e2. Likewise, graphene is an atomic-layered planar semimetal and its charge carriers propagate in the reduced dimension with effectively zero mass and constant velocity. There are numerous fundamental intriguing properties of these aforementioned carbon structures, i.e., Coulomb blockade phenomena in quantum dot systems, the strong electron-electron interactions in 1D lead to Luttinger liquid behavior and the formation of a 1D Wigner crystal phenomena emerges in a dilute electron system. Recently the anomaly quantum Hall effect in few layers of graphene has been observed and a symmetrically biased graphene p–n junction has been shown theoretically to create a negative refractive index medium analogous to a Veselago lens, which could potentially lead to an innovative electronics. Besides the exquisiteness of fundamental physics study, the group also engages in building frontier technology based on new physics and materials. Therefore, one of the research directions is to develop state-of-the-art sensors for applications in various fields. All in all, we rigorously push the performance of high mobility novel electronics, optoelectronics and nanoelectromechanical system (NEMs). The particular experimental techniques are to perform DC/RF electrical transport measurement at various temperatures and magnetic fields to study device characteristics of these nanostructure materials, and in-situ electrical and optical characterization of optoelectronic nanomaterials for device purposes.
Optical spectroscopy of condensed matter systems
Another research area involves optics and ultrafast lasers. In particular, we are interested in semiconductor nanoelectronics, nanophotonics, and nanospintronics. In these research topics, we use ultrafast laser techniques to manipulate and monitor the motions of charge and spin of electrons in nanoscale devices. The control and detection can be done as fast as 100 fs (1 fs = one millionth of billionth of a second) in time, and as small as 0.02 nm in space. These research activities provide building blocks for the next generation electronic devices that are faster, smaller and more powerful. Furthermore, studies of nanoscale transport provide information and knowledge on quantum and coherent properties of electrons that are of fundamental importance. We are also interested in using the laser techniques to study new research topics in CMP like graphene and topological insulators.
Organics semiconductors have become promising candidates for next generation electronics, ranging from photovoltaics to light emitting devices. These materials are abundant in nature, relatively cheap to produce, and devices can be printed on flexible substrates by roll-to-roll processes. These characteristics make them extremely attractive for producing large scale devices such as a solar cell array. Fundamentally, these materials are also of great interest as elementary excitations in these materials are often highly correlated. For instance, light interacts with molecules or conjugated polymer to form tightly bound electron-hole pairs – the so called excitons instead of free charge carriers. In addition, the electronic excitations are not quite free in these materials and they interact strongly with the vibration mode of individual molecules and the phonon mode of crystal. The complexity of these interactions results in the many unexpected and interesting phenomena observed in these materials. To name a few, electronic excitations can be ‘multiplied’ by processes such as multi-exciton generation or singlet fission; metallic phase can be found at the interface of some organic semiconductors; in some organic crystals, the charge mobility can be changed by a significant amount by applying a small strain to the crystal.
At KU, we study the fundamental electronic processes in these materials using various ultrafast time-resolved techniques. In particular, we use time-resolved photoemission to observe how excited electrons in a solid relax in a fs timescale. These new experimental tools give us a direct view on how electron excitation evolves after photoexcitation, which is instrumental to further our understandings on the fundamental interactions in these materials. We are also using a technique called angle-resolved photoemission (ARPES) to directly determine the band structure of various new materials.
If you are interested in taking part in this research, please check the links for the members of the group in the right column for more information on our research projects.