Magnetic transition metal chalcogenides form an emerging platform for exploring spin-orbit driven Berry phase phenomena owing to the nontrivial interplay between topology and magnetism. Here we show that the anomalous Hall effect in pristine Cr2Te3 thin films manifests a unique temperature-dependent sign reversal at nonzero magnetization, resulting from the momentum-space Berry curvature as established by first-principles simulations. The sign change is strain tunable, enabled by the sharp and well-defined substrate/film interface in the quasi-two-dimensional Cr2Te3 epitaxial films, revealed by scanning transmission electron microscopy and depth-sensitive polarized neutron reflectometry. This Berry phase effect further introduces hump-shaped Hall peaks in pristine Cr2Te3 near the coercive field during the magnetization switching process, owing to the presence of strain-modulated magnetic domains. The versatile interface tunability of Berry curvature in Cr2Te3 thin films offers new opportunities for topological electronics.

Pb1-xSnxTe has been shown to be an interesting tunable topological crystalline insulator system. We present a magnetoterahertz spectroscopic study of thin films of Pb0.5Sn0.5Te. The complex Faraday rotation angle and optical conductivity in the circular basis are extracted without any additional assumptions. Our quantitative measures of the THz response allow us to show that the sample studied contains two types of bulk carriers. One is p type and originates in 3D Dirac bands. The other is n type and appears to be from more conventional 3D bands. These two types of carriers display different cyclotron resonance dispersions. Through simulating the cyclotron resonance of hole carriers, we can determine the Fermi energy and Fermi velocity. Furthermore, the scattering rates of p-type and n-type carriers were found to show opposite field dependences, which can be attributed to their different Landau level broadening behaviors under magnetic field. Our work provides a new way to isolate real topological signatures of bulk states in Dirac and Weyl semimetals.

We have studied the collective properties of two-dimensional (2D) excitons immersed within a quantum well which contains 2D excitons and a two-dimensional electron gas (2DEG). We have also analyzed the excitations for a system of 2D dipole excitons with spatially separated electrons and holes in a pair of quantum wells (CQWs) when one of the wells contains a 2DEG. Calculations of the superfluid density and the Kosterlitz–Thouless (K–T) phase transition temperature for the 2DEG-exciton system in a quantum well have shown that the K–T transition temperature increase with increasing exciton density and that it might be possible to have fast long-range transport of excitons. The superfluid density and the K–T transition temperature for dipole excitons in CQWs in the presence of a 2DEG in one of the wells increases with increasing inter-well separation.

We report measurements of the low-frequency photon noise and the relative intensity noise of an interband cascade laser as a function of laser current at 30 and 100 K. Away from threshold, the laser primarily exhibits a frequency-independent photon noise spectral density in agreement with theory. At threshold, the observed photon noise spectral density exhibits large fluctuations at closely spaced discrete frequencies. Thermal effects at 100 K result in a large increase in the photon noise above threshold.

Excitons which coexist with a degenerate two-dimensional electron gas (2DEG) in the same quantum well subband have been observed in the photoluminescence (PL) from the recombination of electrons with localized photoexcited holes. Under pulsed photoexcitation at a critical applied voltage, the screening response of the 2DEG/exciton system to the appearance of a remote photocurrent filament in the 2DEG induces an increase in the integrated PL intensity, the formation of long-lifetime excitons in the excitation region, and long-lifetime redshifted PL from the excitation region and the region around the filament.

: Excitons that coexist with a degenerate two-dimensional electron gas in the same quantum well subband have been observed in the photoluminescence from the recombination of electrons with localized photoexcited holes, At a critical electron density, an abrupt decrease in the exciton radiative recombination rate is observed, along with the formation of biexcitons. With increased excitation intensity, photoluminescence spectra are observed that verify theory on the radiative renormalization of biexcitons and strongly indicate the occurrence of a Bose-Einstein condensation of biexcitons.

Low temperature photoluminescence (PL) measurements on pseudomorphic modulation-doped transistors with a low-temperature (LT) GaAs layer in the GaAs buffer layer clearly show a decrease in the quantum well PL transition energies compared to a structure with no LT GaAs. Self-consistent calculations of the electron and hole bandstructure suggest that the observed increase in the redshift in PL energies with increasing quantum well-LT GaAs spacing can be attributed to band bending induced by the Fermi level pinning at the undoped GaAs/LT GaAs interface and a novel carrier compensation effect of LT GaAs.