Optical control of topological materials

Materials' optical and electronic properties are determined by the structure of their electrons' bands (the energy of allowed states as a function of momentum), and particularly by the presence or absence of a gap--a range of disallowed energies near the Fermi level. The "topological" materials discovered in recent years, such as Dirac and Weyl semimetals, have a band structure featuring linear Dirac bands that cross gaplessly at a node. We are seeking ways to control these materials' band structure by opening and closing a nodal gap on timescales of just a few hundred femtoseconds.

Image: Two planes of Sb atoms are shown vibrating symmetrically away from a plane of Mn atoms in SrMnSb2. This vibrational mode is predicted to periodically close and open a gap at the material's Dirac point.

A material's band structure derives from its crystalline structure (the periodic arrangement of its atoms), so one method of opening or closing a nodal gap is to rapidly shift the atoms' positions. A laser pulse may do this by exciting a "coherent phonon," in which the atoms' positions all oscillate in sync with each other. A phonon is the quantum unit of atoms' vibration, and a phonon becomes coherent when the vibrations are synchronized rather than random. The usual way to excite a coherent phonon is with a very short laser pulse that starts all the atoms moving at the same instant. As the atoms' positions oscillate periodically, the energies of the electrons' bands will oscillate too. There are many different ways in which the atoms can oscillate--different phonon "modes." If the atoms oscillate in the right mode, and far enough, the band energies may change by enough to create a gap at the Dirac node. The gap would then open and close at the frequency of the phonon mode, typically several times per picosecond.

The Weber lab is seeking to put this idea into practice, using laser pulses to control topological materials via coherent phonons. Of particular interest is the Dirac semimetal SrMnSb2, which suffers a Peierls-like distortion of its crystal lattice. This distortion has two effects: it makes the crystal particularly susceptible to coherent phonons, and it opens a gap of about 200 meV at the Dirac node. We have found that ultrafast pulses readily excite coherent oscillations of one particular phonon mode (see the illustration). According to theoretical calculations, a sufficiently large coherent motion of this mode ought to periodically close and re-open the nodal gap. We are now seeking to measure the atoms' motion and the simultaneous changes in the band energies, and to excite the phonon to an amplitude sufficient to completely close the gap.

Ultrafast thermodynamics of Dirac & Weyl semimetals

Thermodynamics concerns itself with the number (or density) of particles as a function of energy. When a solid absorbs a short pulse of light, its electrons are transferred, almost instantaneously, to high-energy states, leaving behind vacancies ("holes") at low-energy states. Ultrafast thermodynamics asks how these electrons find their way back to their original states: How quickly do they collide with each other to reach a quasi-thermal distribution of energies? How quickly, and to what, do they give up their energy? How do they recombine with holes?

The answers to these questions depend sensitively on the solid's basic electronic properties, with different behaviors for metals, semiconductors, classic semimetals, and topological semimetals. In the Weber lab we have focused on newly-discovered topological materials, the Dirac and Weyl semimetals, measuring Cd3As2, TaAs, NbP, NbAs, SrMnSb2, and ZrSiS. The ultrafast responses of these six materials show remarkable similarities.

In our pump-probe measurements (see Research Methods), we photoexcite electrons with either optical (1.5 eV) or mid-infrared (86-500 meV) photons, and we measure the materials' reflectivity as a function of time. We have found that electrons are rapidly heated by several hundred Kelvin, but cool within just a few picoseconds. In most of these semimetals the reflectivity also shows a sub-picosecond "spike" which is associated with the rapid scattering of electrons to form a thermal distribution. Among these semimetals ZrSiS recovers the most quickly from photoexcitation, suggesting that it may be a promising material for making optical switches and detectors.

Semiconductor spintronics

The field of spintronics seeks to develop a fundamental physical understanding of the behavior of electron's spins in a variety of materials, so that spin may eventually be used for memory, storage, or logic in new generations of computer hardware. The Weber lab has focused on the behavior of spins in III-V semiconductors, where optical selection rules allow us to orient and to measure electron spins using ultrafast optical pulses. We have used transient-grating experiments (see Research Methods) to measure the rate of spin diffusion in GaAs and of electron diffusion in the dilute magnetic semiconductor Ga1-xMnxAs.

Image (Credit Lawrence Berkeley National Laboratory): A representation of electron spins in a GaAs quantum well. The spins' orientation varies with their position in a phenomenon known as a "persistent spin helix."

Chris Weber's contact information

Email: cweber@scu.edu

Phone: none

Office: SCDI, room 2311Q (2nd floor, southeast corner)

Lab: SCDI, rooms 1106 & 1108 (1st floor, north wing)

Visiting SCU? Click on this SCU map and find the "Sobrato Campus for Discovery and Innovation"

Mailing address: Department of Physics // Santa Clara University // 500 El Camino Real // Santa Clara, CA 95053-0315 U.S.A.