Greg Jenkins

Menu:

• Home
• Research
• Instrumentation
• Publications
• Presentations
• Teaching/Service
• Curriculum Vitae
• Contact
• Links

Research

 

Weyl and 3D Dirac semimetals

In condensed-matter systems, Dirac fermions exist in the valence and conduction bands that touch at a pair of points and disperse linearly away from the nodes. These bands are protected against gapping by crystal symmetry. If either crystal inversion or time-reversal symmetry is broken, each Dirac node splits into a pair of opposite chirality Weyl nodes, topological objects that act as a source or sink of Berry's phase curvature. This topological band structure effect is analogous to opposite-polarity magnetic monopoles residing at the nodes in momentum space, which fundamentally alter the semiclassical equations of motion and Maxwell's constitutive relations. The repercussions include new unique electronic properties like Fermi-arc surface states, chiral pumping effects, and magnetoelectric-like effects in plasmonics and optics in the absence of an applied field.

Optical measurements have been reported on the 3D Dirac semimetals Na₃Bi and Cd₃As₂, and the time-symmetry broken Weyl material Eu₂Ir₂O₇. In all systems, signatures of the Dirac dispersion are encoded in optical data through the temperature dependent chemical potential affecting the Drude free carrier response and interband transitions. The effects allow extraction of the Fermi velocity, anisotropy, Lifshitz gap characteristics, intrinsic doping level, and interaction energy. Current work focuses more on directly probing the underlying Berry-phase curvature employing magneto-optical measurement schemes.

In the pyrochlore Eu₂Ir₂O₇, the onset of semimetallic behavior below the magnetic transition temperature and Dirac-like behavior of conductivity at low frequency provides optical evidence that Eu₂Ir₂O₇ (at a specific off-stoichiometry) is a Weyl state system. Since Na₃Bi is highly reactive with air, performing measurements is sufficiently challenging that this was the first reported optical characterization. A few discoveries were made: a bulk mode exists near the plasma edge that may indicate unexpected Weyl state magneto-plasmon effects, the ground state crystal symmetry is likely P3c1, the optical matrix elements are zero at the Lifshitz (saddle point) gap at the G-point, and a non-monotonic strongly temperature dependent plasma edge is indicative of the temperature dependence of Dirac free carriers.

 

Topological Insulators

Three dimensional topological insulators have two-dimensional surface states that exhibit Dirac-like dispersion similar to graphene, but the spins of the electrons are locked perpendicularly to the surface current. Manipulating the current (electric field) effects the spins (magnetic field) and vice versa. This and other emergent exotic properties, with the introduction of magnetically or superconducting induced energy gaps in the surface state, may have far reaching implications ranging from quantum computing to spintronics. The properties of the surface state in the topological insulator Bi₂Se₃ were spectroscopically characterized at terahertz frequencies in magnetic field. A Dirac-like cyclotron mass and evidence of puddling in the vicinity of the Dirac point was observed similar to graphene. When buried beneath the trivial insulator In₂Se₃, the surface state shows evidence of a large shift of the Dirac point towards the conduction band edge. This shift of the Dirac point is tunable through the material properties of the trivial insulator offering new routes to tailoring the surface.

 

Graphene And Carbon Nanotube Emitters & Detectors

I am an active collaborator in research into the use of graphene and carbon nanotubes as detectors in the THz and IR spectral region. Graphene is particularly well suited for use as a bolometer — a device that detects temperature-induced changes in electrical conductivity caused by the absorption of light— since it has a small electron heat capacity and weak electron–phonon coupling. The combined effects lead to large light-induced changes of the electron temperature.

Compared with commercial state-of-the-art semiconducting bolometer detectors, a hot-electron bolometer made of bilayer graphene demonstrated a noise-equivalent power that is several times lower with an intrinsic speed (>1 GHz at 10 K) three to five orders of magnitude higher.

Single carbon nanotube quantum dots coupled to antennas are sensitive, broadband, terahertz quantum detectors with spectral resolution. Their response is due to photon-assisted single-electron tunneling and it is substantially enhanced by a novel radiation-induced non-equilibrium cooling of the electrons.

In single layer graphene, an important avenue of exploration involves asymmetric coupling of contacts to the graphene while leveraging the strong photothermoelectric effect. Coupling antennas to graphene structures permits the investigation of plasmonic detection schemes. Current research focuses on multimode mixing in novel single layer plasmonic detectors.

 

High Temperature Cuprate Superconductors

Since their discovery 25 years ago, the anomalous Hall effect in the cuprates has played a central roll in providing vital clues to the possible mechanisms which describe the electronic properties. By extending the dc Hall measurements to finite frequency, crucial information has been garnered to help solve this long standing problem. The anomalous magneto-transport properties in both n- and p-type cuprates were investigated using Hall measurements at THz frequencies. In overdoped Pr_2−xCe_xCuO₄, the complex Hall angle, extrapolated to low temperatures, shows that inelastic scattering is the culprit which introduces electron-like contributions to the expected simple hole-like Hall response. Prof. H. Kontani calculated the complex Hall angle which include current vertex corrections (CVC) induced by electron interactions mediated by magnetic fluctuations in the Hall conductivity. The results show that CVC effects are the source of the anomalous Hall transport properties in overdoped n-type cuprates.

In underdoped Pr_2−xCe_xCuO₄, fractionalization of the Fermi surface is measured by a rapidly decreasing Hall mass consistent with electron Fermi pockets formed by spin-density wave ordering. For a Fermi surface which has been reconstructed due to magnetic ordering, one would naively expect to recover a large hole-like Fermi surface at high temperature, but the Hall mass remains negative with no signature of a change in Fermi surface topology at or above the Neel temperature and the characteristic temperature at which the optical gap fully closes. However, in the paramagnetic state the observed behavior of the Hall mass may be qualitatively understood as arising from the current vertex corrections to the Hall conductivity due to magnetic fluctuations as similarly observed in overdoped Pr_2−xCe_xCuO₄.

In optimally doped Bi₂Sr₂CaCu₂O_8+x, a p-type cuprate superconductor, the expected THz Hall response above Tc calculated within a Boltzmann formalism using ARPES measured parameters was significantly larger than the measured THz Hall response. This is the THz manifestation of the well-known anomalous dc Hall effect where dc-RH enhancements are much larger than the value expected from Luttinger’s thereom as well as the expected value calculated directly from ARPES data analyzed within the RTA.  These enhancements as well as the frequency and temperature dependence of the dc and THz Hall response are also well described by CVC effects.

The anomalous behavior of the Hall effect, at dc as well as at THz frequencies in both n- and p-type cuprates, appears to be captured by the same physics. Current work is now focused upon the underdoped region in p-type cuprates where antiferromagnetic fluctuations are expected to play a diminishing roll in the Hall phenomenology as doping decreases.

 

Berry-phase Contribution To Photocurrents In GaAs Quantum Well Stacks

Currently, no experimental evidence has been associated with an expected Berry-phase contribution to transport properties in which time reversal symmetry is preserved and inversion symmetry is broken. A recent prediction shows the helicity-dependent photocurrents in zincblende semiconducting quantum wells are controlled by a Berry-phase contribution generated by quantum confinement. A polarization-sensitive photocurrent measurement system utilizing a novel polarization modulation technique to test these predictions in a GaAs (110) quantum well stack has been implemented.