Quantum Materials

Electrons in solids obey the laws of quantum mechanics. In fact, solid state physics provides a platform for investigating fundamental questions about the quantum nature of our universe. For example, you experience Bose/Einstein or Fermi/Dirac quantum statistics in your everyday life when you observe heat transfer or a metallic conductor, respectively. You encounter the Pauli exclusion principle combined with “classical” electrostatics when you handle a ferromagnetic material. The insightful predictions leading to full understanding of these properties were among the greater triumphs of quantum mechanics in the last century. Today, many of the technological applications you use to view this and other websites result from these advances. There remain, however, many open questions and unresolved problems such as the nature of high-temperature superconductivity. And of course, there is always the prospect of discovering new phenomena that can further expand our understanding of condensed matter physics and our universe.

Quantum Materials We Work On

A particularly interesting class of solids is “strongly correlated materials,” in which their valence-state electrons can feel a mutual Coloumb repulsion (after all, they have the same negative charge) and enter a state of strong interaction. At this point, any of the electrons’ quantum numbers become equally important in governing the macroscopic properties. The spin, orbital, and charge number can combine with the underlying (and always vibrating) crystal lattice to ultimately produce a rich array of macroscopic phenomena such as metal-insulator transitions, superconductivity, magnetism, etc. That’s right: interactions at the quantum scale result in properties we can appreciate and make use of at our life-sized scale.

In our group, we are primarily focused on transition-metal oxides as a platform to study strong correlations. The extended d-orbital of the metal ion, hybridized with the adjacent oxygen p-shell, leads to extraordinarily rich physics. Looking at different material families like copper-oxides, nickelates, manganates, iridates, we aim to understand the nature of their complex phase diagrams that include superconductivity, charge+spin density waves, and frustrated magnetism.

Understanding and manipulating these collective quantum grounds states could eventually take us into a new generation of functionalities, data and/or storage devices, and computational logic frameworks.

Quantum Materials At The Nanoscale

In attempting to push the boundaries of materials science and condensed matter physics, we employ modern thin-film deposition techniques. We aim to grow the materials in geometries and interfacial environments that solid-state chemistry cannot easily afford due to the subtle thermodynamic constraints intrinsic to those growth processes (e.g. flux growth). In fact, some of the materials we investigate are so rare, they can only be observed in “single crystal” form if grown epitaxially on a substrate.

Thin-film geometries provides several easily adjustable parameters. By choosing different substrates, the lattice structure of the film can be strained to yield new properties. We can monitor and adjust their thickness with a precision of a few atomic layers. In some cases, we can reduce the thickness down to just a few atomic layers. A nearly 2-dimensional material can thus be created which displays novel properties its bulk, 3-dimensional counterpart does not.

We can also grow layers of different materials and create an interface between them with new electronic behavior. This bilayer sequence, also known as a ‘heterostructure’, can be repeated several times to create a superlattice with many interfaces. Together, all these parameters can be combined to yield unique materials with new underlying physics.

We are inspired by the phrase coined by Herbert Kroemer. After sharing the 2000 Nobel Prize “for developing semiconductor heterostructures used in high-speed- and opto-electronics”, Kroemer said in his Nobel lecture: “the interface is the device”. We aim to identify new, fundamentally interesting, potentially functional properties in transition-metal oxide interfaces.

Quantum Periodicities

A central research topic in our group is the investigation of quantum mechanical ground states with spatial periodicities inside the thin films, heterostructures, and strained materials that we grow. The self-organization into periodic waves of the charge, spin or orbital character of the electrons is a fascinating phenomenon. Just like when atoms form periodic lattices in crystals, creating long-range ordered periodic patterns could minimize the strong, competing interactions between electrons. And nature loves to minimize energy. Spin or charge density waves, for example, are commonly observed in cuprates and nickelates. In addition, thermal excitations of and competition between these ground states yield macroscopic properties like superconductivity, exotic magnetism, etc.

How We Do It

Familiar x-ray diffraction experiments produce a Fourier transform of the atomic landscape via elastic scattering and are routinely used to study the structure of solids. Resolving the inelastic energy exchange between photons and atomic ions probes the dynamical structure factor, a quantity that reveals collective dynamics of the atomic lattice and their dispersion. These techniques provide unambiguous windows into atomic order in solids. What if the principle could be used analogously to probe electronic order in correlated quantum materials?  

By tuning the x-ray energy to electronic transitions and exciting electrons into states near the Fermi level, spatial modulations and temporal dynamics of electronic wavefunctions can be studied in Fourier space. Propelled by technological advances in synchrotron science over the last decades, Resonant (in)elastic X-Ray Scattering (RXS) provides this exciting and powerful tool to investigate the electronic structure of solids.

Where We Do It

High flux x-rays with tunable energy are generated at synchrotron facilities. Thus, we perform the bulk of our state-of-the-art experiments at the facilities all over the world listed in the links section.

With that said, we do have x-ray diffraction capabilities at UCSD. We are part of the X-ray Center, where we operate a 4-circle Huber diffractometer equipped with a line detector and a low temperature cryostat. That way, we ensure that our samples are fully characterized before we go out to the synchrotron.