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Fundamentals of Quantum Materials

Recent studies have indicated that no known superconductor is able to meet the needs of electric power applications above liquid nitrogen temperature and therefore a new high temperature superconductor is required. Along with three other universities, we have initiated a program to search for high temperature superconductors based on electronic (spin and charge) mechanisms of superconductivity, and their related physics.

My group develops numerical methods and theories of photon-based spectroscopies of strongly correlated materials. The goal of this research is to understand electron dynamics via a combination of analytical theory and numerical simulations to provide insight into new quantum materials and how to better use them in energy-related applications.  My group carries out numerical simulations on high-performance CPU and GPU mid-range computing clusters and supercomputers.

We are interested in studying correlated electronic states, topological phases of matter, and other emergent phenomena in reduced dimensional systems. Our work focuses especially on nanoscale quantum materials and devices, such as those fabricated from atomically thin van der Waals layers. By combining techniques such as transport measurements and scanning single-electron transistor microscopy, we aim to identify the global and local signatures of novel phases that form at cryogenic temperatures in these quantum systems, as well as their tunability in response to applied electromagnetic fields.

Dilute ultracold quantum gases have been of intense interest for the past decade.  Bosonic gases condense to become superfluids, analogous to superfluid helium, but considerably simpler and therefore amenable to theoretical analysis.  Fermionic gases become superfluids through a pairing mechanism analogous to conventional metallic superconductors.  Both of these systems serve as laboratories for the study of quantum superfluids under various extreme or unusual conditions such as fast rotation or confinement in optical lattices.  Current problems of interest are the structure and arrangement of quantized vortices in rapidly rotating Bose-Einstein condensates.

In broad terms, we study materials with unconventional magnetic and electronic properties, with the general aim of obtaining a deeper understanding of the many effects that can emerge from electron correlation. We employ several techniques to grow high-quality single crystals of materials of interest. Experiments probe the thermodynamic and transport properties of these materials, often in high magnetic fields. Current interests include superconductivity, topological insulators, aspects of quantum magnetism, and the behavior of electrons in low-dimensional materials.

The Goldhaber-Gordon group uses advanced fabrication techniques to confine electrons to semiconductor nanostructures, to extend our understanding of quantum mechanics to interacting particles -- when constrained this way, electrons cannot easily avoid each other -- and to provide the basic science that will shape possible designs for future transistors. The Goldhaber-Gordon group makes precision electrical measurements and designs novel scanning probe techniques that allow direct spatial mapping of electron organization and flow. For some of their measurements of exotic quantum states, they cool electrons to a hundredth of a degree above absolute zero, among the coldest temperatures ever achieved for semiconductor nanostructures. They also work to elucidate behavior of related materials such as magnetic topological insulators and graphene, a single atomic layer of carbon atoms.

Harrison has carried out theoretical studies of the electronic structure of solids and molecules for many years.  He is currently focusing on interfaces between metals, semiconductors and ionic solids, understanding how the energy bands on the two sides align and how electrons are transmitted between them.   He is developing realistic quantum descriptions of the electronic structure of the interface for that purpose.   The work is motivated by the need to reduce the resistance arising from the interfaces in semiconductor devices such as field-effect transistors.

Materials physics: Probing correlated electrons at artificial interfaces and in confined systems; Atomic scale synthesis and control of complex oxide heterostructures; Oxide heterostructures for energy applications; Low-dimensional superconductivity; Novel devices based on interface states in oxides.

Professor Kapitulnik studies materials with novel electronic states at low temperatures. The research concentrates on the occurrence and properties of superconductivity, charge-density, or magnetic states in such systems. The group uses a variety of measurements and novel probes such as scanning tunneling microscopy and spectroscopy and high-resolution mageneto-optics.

I am interested in the qualitative understanding of the macroscopic and collective properties of condensed matter systems, and on the relation between this and the microscopic physics at the single electron or single molecule scale. I have been particularly interested in exploring the spectacular consequences of strong correlation effects in electronic materials and devices where the low energy properties are qualitatively different from those of a non interacting electron gas. This field of study has been made particularly rich and exciting by the seemingly nonending sequence of unexpected experimental discoveries that have occurred in this field over the past couple of decades - discoveries which undermine accepted beliefs and raise conceptually deep questions concerning the emergent behavior of systems with many strongly interacting degrees of freedom.

As our experimental understanding of nature has matured, we have come to realize just how artificial the distinction is between fundamental physical law—something that “just is”—and other kinds of physical law that “emerge” through self-organization. Everyday examples of the latter include material rigidity, magnetism, and super-fluidity, but there are countless others. Things become more troubling, however, when we realize that the vacuum of space-time also has symptoms of being emergent. “Fundamental” quantities such as the electron charge defocus and change value as you examine the vacuum at smaller and smaller length scales. Unification of forces becomes mathematically indistinguishable from “quantum phase transitions” of the vacuum. Heats of formation and other collective effects in the vacuum become implicated in inflationary theories of the universe. We are increasingly realizing that finding law – a quantitative relationship among measured quantities that is always true – is not eh same thing as finding fundamental truth. Indeed, when you measure only at “low” energies you simply cannot tell the difference between a law that emerges and a law that “just is”.

The Lee group’s research involves studies of novel electronic and magnetic materials in single crystalline form. The goal is to understand the properties of correlated electron systems and quantum spin systems, with an eye toward discovering new materials or new physical phenomena. Such materials represent a major challenge to our present understanding of condensed matter physics, as they consist of many quantum particles which strongly interact with each other. The delicate interplay between the constituents of these systems (involving the magnetic, charge, orbital, and lattice degrees of freedom) leads to a variety of exotic phases, a famous example of which is high-Tc superconductivity. Specifically, the Lee group seeks to develop a deeper understanding of quantum materials through research activities involving neutron scattering, x-ray scattering, and crystal growth. We are particularly interested in novel states such as quantum spin liquids, exotic superconductivity, and topological phases of matter.

My academic career at Stanford has been focused on research efforts to understand the electronic properties of semiconductor surfaces and interfaces on an atomic scale using synchrotron radiation of importance for the development of modern electronic devices. I have taken a great interest in the development of synchrotron radiation and free electron laser facilities and instrumentation, and research policies for large scale facilities. I have been (and still am) a member/chairperson of a large number of review and advisory committees.

Lindenberg's research is focused on probing the ultrafast dynamics and atomic-scale structure of materials on femtosecond and picosecond time-scales. X-ray techniques are combined with ultrafast laser techniques to provide a new way of taking snapshots of materials in motion. Current research is focused on the dynamics of phase transitions, ultrafast properties of nanoscale materials, photoelectrochemical charge transfer dynamics, and THz nonlinear spectroscopy.

What new science and technologies lurk at the smallest scales of condensed matter? How does physics change in lower dimensions?  Humans have always tried to expand their mastery of the material world. Manipulation of matter has been continuously refined, leading to construction so colossal size and extreme complexity. Progress in the diametric direction of diminishing scale has proved increasingly vital to society. These efforts rely on new tools extending control and measurements to smaller length scales. Instead of this “top-down” approach, what if one proceeds from the bottom and works up? Professor Manoharan seeks to apply the bottom-up approach of atomic and molecular manipulation to outstanding problems in science and technology.

Theory of condensed matter, especially the electronic structure of solids. Examples of recent work include density functional calculations for stability and superconductivity in doped fullerenes, new structures of nitrogen at high pressure, Monte Carlo simulations of many-body electron problems in one-dimensional electron wires, the theory of polarization and localization in insulators, and topological quantum order in Mott insulators.

How does quantum decoherence occur? What is the correct theoretical description of strongly correlated electron materials?  The goal of Professor Moler’s research is to answer these two questions about the fundamental behavior of electrons in materials by:
- creating a toolbox of sensitive, quantitative, high-resolution local magnetic sensors, enabling routine and noninvasive characterization of small magnetic fields in novel quantum materials, and to share the designs for these tools with other scientists
- conducting a systematic survey of the energetic and dynamics of individual quanta of magnetic flux in various superconductors, to elucidate the mechanism of superconductivity
- conducting a systematic survey of persistent currents in mesoscopic normal metals and superconductors, to understand the mechanisms of quantum decoherence in electrons systems, and
- educating a group of creative and highly skilled graduate and undergraduate students.

Pianetta's research is directed towards understanding how the atomic and electronic structure of semiconductor interfaces impacts device technology. His research includes the development of new analytical tools for these studies based on the use of synchrotron radiation. Recent projects include the development of ultrasensitive methods to analyze trace impurities on the surface of silicon wafers at levels as low as 1e-6 monolayer (~1e8 atoms/cm2) and the use of various photoelectron spectroscopies (X-ray photoemission, NEXAFS, X-ray standing waves and photoelectron diffraction) to determine the bonding and atomic structure at the interface between silicon and different passivating layers.

What is the nature of topological phenomena in condensed matter physics and quantum entanglement?  Topological phenomena are the phenomena which are determined by some topological structure in the physical system, which are thus usually universal and robust against perturbations. For example, two famous topological phenomena are the flux quantization in superconductors and Hall conductance quantization in the Quantum Hall states. Recent discovery of topological insulators and topological superconductors in different symmetry classes bring the opportunity to study a large family of new topological phenomena. For example the three-dimensional topological insulator provides a condensed matter realization of the important theoretical concepts in high energy physics such as the ”theta-vacuum” and “axion”. The interplay of topological insulators and superconductors with conventional phases of matter such as ferromagnets and superconductors lead to richer topological phenomena.

I am interested in exploring the ground states and collective properties associated with quantum condensed matter systems.  I have especially been fascinated by correlated electron materials, in which the low energy degrees of freedom behave qualitatively differently than a free electron gas.  The key challenge here lies in understanding the relationship between the microscopic physics and the phenomena which emerge in the long wavelength limit in such systems.  In order to understand this relationship more deeply, it will be essential to confront these problems using a wide variety of theoretical tools, ranging from non-perturbative computational techniques, to low energy effective field theory descriptions of universal properties.

Professor Shen conducts fundamental and applied research on quantum matter. His primary interest is the physics of the “many”, where interactions among multiple constituencies give rise to novel properties not intrinsic to the individual components. His interest also includes ways to utilize the functionality of materials.  He sends electromagnetic waves to probe matter, including X-ray, ultra-violet, and microwave radiation from synchrotron, free-electron laser, and laboratory sources. Insights are gained through precision analysis of ejected particles, either photons or electrons. He also prepares materials and devices for his studies.

My group studies novel ground states and functionality in thin films and heterostructures. We exploit recent advances in atomically precise heteroepitaxy of complex oxides to develop new materials and to probe novel interface phenomena. Many of these phenomena are then incorporated into prototypical device structures. Our recent focus is on strongly correlated materials, especially new spintronic materials, as well as magnetic junction devices and magnetic logic circuits.

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