In quantum materials, strong interactions between particles and quantum entanglement lead to the emergence of exotic phenomena such as high temperature superconductivity. Materials with such effects are either in the bulk form or with reduced dimensionality. Research in this area seeks to develop materials with unusual electronic, magnetic and optical properties, characterize such properties with spectroscopy techniques, understand the properties using many-body theory, and explore the materials functionality in energy and information technology. At the present time, RCQM involves nearly 20 faculty from both Natural Sciences and Engineering Schools.
RCQM Focus Areas
(1) Unconventional Superconductivity
One of the most intriguing phenomena in the scientific world is superconductivity, characterized by zero electrical resistance and expulsion of a magnetic field (Meissner effect). It occurs whenever an attractive interaction compels electrons to bind into pairs, which in turn form a coherent condensate below the superconducting transition temperature. In conventional superconductors such as aluminum or lead, the origin of the attractive force is well understood and stems from the interaction of electrons with the lattice vibrations – phonons. However in 1986, physicists were forced to reconsider this concept when a new class of copper-based high-temperature superconductors were discovered. Ever since, elucidating the mechanism of this unconventional superconductivity has become a primary goal of modern condensed matter physics. Several other families of unconventional superconductors have also been found, including heavy fermion and organic materials and, most recently, iron-based compounds. Scientists hope that understanding the origin of this phenomenon will pave way for discovering even higher-temperature superconductors relevant to technological applications.
Rice University is at the very forefront of research into unconventional superconductors, with a multi-disciplinary team engaged in crystal growth, physical property characterization, optical and neutron spectroscopies, and theoretical analysis. This research ties in to the other research foci of RCQM. Efforts to search for new superconductors often end up with the discovery of non-superconducting materials with exotic electronic properties. Understanding the mechanism of unconventional superconductivity is strongly coupled to the overall studies of the role of strong correlations in many-body physics in general.
(2) Quantum criticality
By raising or lowering the temperature of a liquid, one can induce evaporation into a gas, or freezing into a solid. Physicists refer to the states of matter (solid, liquid, gas) as phases, and the changes of state between them as phase transitions. In the usual case of melting or evaporation, the transition occurs due to heating of the lower temperature phase, which increases the average kinetic energy of the constituent particles. At a continuous phase transition, the low and high temperature phases "dissolve" into a third, highly fluctuating transition state in which energy is exchanged on all length scales. In a universe described by classical mechanics, all such fluctuations would be frozen out at sufficiently low temperatures. In reality, phase transitions are possible between states of matter even at zero temperature, due to quantum mechanics. A continuous zero temperature transition is driven by quantum fluctuations. Quantum criticality influences physical properties over a wide parameter range at nonzero temperatures.
Quantum criticality is an established route that leads to unusual excitations and emergent phases in electronic systems. It might hold the key to properties such as high temperature superconductivity. Rice university is prominent worldwide on this subject. Our studies are conducted in a variety of contexts, including heavy fermion systems, iron-based materials and itinerant magnets.
(3) Ultracold Matter
The most important questions in many-body physics can now be addressed with powerful new tools developed to control and characterize gases of ultracold atoms and molecules. Using techniques of laser and evaporative cooling developed in the atomic physics community, temperatures as low as a billionth of a degree above absolute zero are accessible. In this regime, weak interactions between particles such as short-range contact interactions, superexchange, and long-range dipolar forces can become dominant, leading to realization of analogs of quantum magnetism and superconductivity, for example. Trapping potentials formed with interfering laser beams can create artificial lattices that mimic defect-free crystalline solids, but with the added ability to vary dimensionality, lattice depth, and interaction strength by simply changing the laser intensity or frequency or a magnetic field. Lattice defects or impurities can be controllably introduced to address outstanding questions on localization. The ability to isolate and perturb the system allow studies of non-equilibrium many-body physics that has previously been inaccessible, and artificial gauge fields and spin-orbit coupling can be created with properly tailored laser beams.
These advances have made studies of ultracold atomic physics one of the most exciting areas of research in all of science, with recent Nobel Prizes granted for laser cooling (1997), realization of Bose-Einstein condensation in dilute gases (2001), ultra-precise tools of laser spectroscopy (2005), and the manipulation of individual quantum systems (2012). Rice is one of the leading institutions in the world in this area. Researchers here are pushing theoretical understanding of these novel systems and the experimental ability to realize them. The goals are to uncover the underlying mechanisms that give rise to the fascinating properties of quantum materials and to discover entirely new phenomena to inspire new generations of technological applications.
(4) Materials out of Equilibrium
Studying the out of equilibrium dynamics of quantum many-particle systems is an emerging field that has been enabled by experimental advances on different fronts. Ultrasmall systems like quantum dots and single molecule devices probe the quantum limits of electronic devices. The small size means that such devices are easily driven far from equilibrium by an external voltage. The energy gained by electrons traversing the device can be transferred to quantized molecular vibrations or other quantum degrees of freedom. Ultracold experiments on atoms and molecules isolate quantum matter from its environment. This allows experimentalists to study how statistical properties of a complex system emerge from its dynamics. An alternative way to avoid the effects of coupling to the environment is to probe a material on very short time scales, as in ultrafast optical pump-probe spectroscopy. Rice researchers are at the forefront, studying phenomena ranging from solitons in Bose-Einstein condensates to conduction in single molecule transistors, and revealing superfluorescent bursts from quantum-degenerate electron-hole gases and coherent quantum dynamics in two-dimensional electron gases and carbon nanotubes.
A weakly interacting system can be described by a distribution function, which assigns particular particles to particular states. This idea is not helpful when the system is strongly interacting, however, for the same reason that it is not helpful to characterize individual H2O molecules in describing the flow of liquid water. What is the analogous framework for non-equilibrium quantum dynamics, and what are the intrinsic departures from classical hydrodynamics? Theoretical results by Rice researchers suggest that ultrafast and ultracold experiments can realize new quantum states of matter that only exist out of equilibrium.
(5) Materials at Low Dimensions
In many of the materials that form the core of digital electronics and other technologies, reduced dimensionality plays a crucial role. The transistors that are the building blocks of computer chips flow current through two-dimensional electron gases created at the interface of two materials. While the operation of conventional transistors can be understood classically, the past 30 years have seen a revolution in low-dimensional quantum phenomena, both in semiconductors and new materials systems. The quantum Hall effect, first observed in silicon, is an example. In the presence of a strong magnetic field and at low temperatures, the dimensionless electric conductance is exactly equal to an integer. This integer encodes the winding of a quantum knot in the bulk electronic structure. Even more remarkable is the fractional quantum Hall effect, in which quantum knots can be tied and untied. These "braiding" operations manipulate quantum information, and might allow the realization of a quantum computer. Graphene, a single atomic layer of graphite first isolated in 2005, has provided a new material in which to study the quantum Hall effects and other phenomena. The recently-discovered topological insulators are materials that exhibit unbreakable edge or surface states as in the quantum Hall effect, but without a magnetic field. Rice researchers are at the forefront, pioneering new topological materials, developing the theories of edge and surface states, and creating devices to exploit their properties.
Materials at the nanoscale can also be structured to confine electrons in one dimension (quantum wires, including carbon nanotubes) and "zero" dimensions (quantum dots, or artificial atoms). Electrons under these constraints can exhibit fascinating phenomena, including collective motions that act as if electrons are split so that their spin and charge can move separately. Quantum dots and related systems, including molecular electronics devices, can act as tunable environments where electrons are manipulated at an individual level, and entangled with each other and other degrees of freedom. These can be model systems for examining the fundamental physics of electron-electron interactions, spin dynamics, electron-phonon couplings, and quantum entanglement, phenomena relevant to the frontiers of information technologies. Rice researchers have expertise and experience in the materials and techniques that enable these investigations.
(6) Functional materials for energy
Materials science and nanotechnology are intimately connected. Continuous advances in nanofabrication, synthesis and self-assembly of nanoscale materials architectures now allow us to control the flow of light, to control charge carrier dynamics, and phonon excitations. By combining nanoscale building blocks - one at a time - low-dimensional materials allow us to create so-called metamaterials, materials with properties that cannot be found in nature. Rice University is known worldwide for its pioneering role in many of these developments. A large group of committed students and faculty are working towards a profound understanding of novel nanomaterials, and their application to solve the energy problem. Recent achievements include the creation of vertically stacked two-dimensional materials or in-plane heterostructures of two-dimensional materials. These atomically engineered metamaterials allow for the fundamental study of quantum effects and their exploitation in energy applications. RCQM will foster interdisciplinary collaborations between experimentalists and theorists in the sciences and engineering to create state-of-the art nanomaterials and device architectures and to characterize their fundamental properties.
Areas of interest include the exploitation of quantum effects in low-dimensional materials to control chemical reactions at their surfaces, the atomic-scale engineering of photocatalysts for solar fuel generation, and the study of quantum effects in photocatalysis that arise from the granularity of the electromagnetic fields on atomic scales. Our center is also at the forefront of developing novel ultrafast laser spectroscopy techniques and nano-characterization tools to gain enhanced insights into quantum materials for energy and sustainability. Researchers in the center have significant experience in transferring fundamental science into energy applications ranging from solar energy conversion, energy storage, thermal management and environmental remediation. By concentrating this expertise, RCQM will create a stimulating environment that will inspire new classes of applications in energy and sustainability.