Research
Our research areas include
♦ Synthesis of novel materials using modified Bridgman, VLS, electrochemistry, and MBE
♦ New nanofabrication and characterization techniques
♦ Magneto-transport in topological insulator structures
Quantum interference effects and quantum oscillations
Effects of disorder and low-dimensionality
♦ Spin response from topological surfaces
♦ Superconductivity and strong correlations in topological and other materials
♦ Magnetically doped topological structures: anomalous Hall effect and massive Dirac fermions
♦ Quantum behaviors of topological systems tuned using high energy particle irradiation techniques
♦ Optical characterization: Raman, contactless electro-reflectance (CER), photocurrents
MBE growth of TIs and heterostructures
Growth of high-quality Bi2Se3 films is crucial not only for study of topological insulators but also for manufacture of technologically important materials. We study the heteroepitaxy of single-crystal Bi2Se3 thin films grown on GaAs and InP substrates by use of molecular beam epitaxy. Surface topography, crystal structure, and electrical transport properties of these Bi2Se3 epitaxial films are indicative of highly c-axis oriented films with atomically sharp interfaces.
Surfaces of three-dimensional topological insulators (TIs) have been proposed to host quantum phases at the interfaces with other types of materials, provided that the topological properties of interfacial regions remain unperturbed. We report on the molecular beam epitaxy growth of II-VI semiconductor-TI heterostructures using c-plane sapphire substrates. Our studies demonstrate that Zn0.49Cd0.51Se and Zn0.23Cd0.25Mg0.52Se layers have improved quality relative to ZnSe. The structures exhibit a large relative upward shift of the TI bulk quantum levels when the TI layers are very thin (~6nm) , consistent with quantum confinement imposed by the wide bandgap II-VI layers. Our transport measurements show that the characteristic topological signatures of the Bi2Se3 layers are preserved.
Singular robust Dirac spin response
Topological insulators are a class of solids in which the non-trivial inverted bulk band structure gives rise to metallic surface states that are robust against impurity scattering. In three-dimensional (3D) topological insulators, however, the surface Dirac fermions intermix with the conducting bulk, thereby complicating access to the low-energy (Dirac point) charge transport or magnetic response. We use differential magnetometry to probe spin rotation in the 3D topological material family (Bi2Se3, Bi2Te3 and Sb2Te3). We detected a paramagnetic singularity in the magnetic susceptibility at low magnetic fields that persists up to room temperature, and which we demonstrate to arise from the surfaces of the samples. The singularity is universal to the entire family, largely independent of the bulk carrier density, and consistent with the existence of electronic states near the spin-degenerate Dirac point of the 2D helical metal. The exceptional thermal stability of the signal points to an intrinsic surface cooling process, probably of thermoelectric origin and establishes a sustainable platform for the singular field-tunable Dirac spin response.
Emergent superconductivity on the surface of Sb2Te3
Surfaces of three-dimensional topological insulators have emerged as one of the most remarkable states of condensed quantum matter where exotic electronic phases should arise. We discovered surface superconductivity in Sb2Te3 with transition to zero resistance at TCR~ 9K induced through a very slight tuning of growth chemistry which depletes bulk conduction channels witnessed by large (almost two orders of magnitude) reduction of carrier density and by huge (~25,000 cm2/V•s) carrier mobility. We demonstrated from local superconducting tunneling spectroscopy and magnetic response that the superconducting condensate forms in surface Dirac puddles at unprecedentedly higher temperatures, near 60 K and above, with global coherence at TCR then mediated by interpuddle diffusion of quasiparticles. Our discovery suggests that weakly-interacting particle scenario for topological surfaces should be reexamined; they appear on the verge of strongly correlated instability, with emergent superconducting mesoscopic surface structure -- a possible hunting ground for Majorana particles -- that could be tuned by the topological material's parameters through band engineering.
♥ Watch a video of the surface superconductivity in Sb2Te
Irradiation: large-scale route to intrinsic quantum transport in TIs
Topological insulators are an imminently transformative class of quantum solids with immune-to-disorder metallic surface states having Dirac band structure and, ideally, with insulating bulk. The biggest challenge to accessing surface charge transport has been from lattice imperfections donating charge carriers that intermix with surface Dirac particles and pull Fermi energy level into the bulk bands. We demonstrated a new approach to reaching charge neutrality that uses swift (~2:5 MeV energy) electron beams to compensate charged bulk defects and brings the Fermi level back into the bulk gap. By controlling the beam uence we can tune bulk conductivity from p - (holelike) to n -type (electron-like) and back, crossing the Dirac point, while preserving the robust topological signatures of surface channels. We established that at charge neutrality conductance has a two-dimensional (2D) character with a minimum value on the order of ten conductance quanta G0 =e2/h. From quantum interference contribution to 2D conductance we demonstrated in two systems, Bi2Te3 and Bi2Se3, that at charge neutrality only two quantum channels corresponding to two topological surfaces are present. The achieved stable charge neutrality point using electron beams with long penetration range shows a route to intrinsic quantum transport of the topological states unconstrained by the bulk size.
Optical characterization of TIs: Raman, CER, photocurrents
A symmetry specific phonon mode renormalization is observed across an amorphous to crystalline phase transformation in thin films of the topological material Sb2Te3 using Raman spectroscopy. We demonstrated local crystalline symmetry in the amorphous state, eventhough, the q=0 Raman selection rule is broken due to strong structural disorder. At crystallization, the in-plane polarized (Eg2) mode abruptly sharpens while the out-of-plane polarized (A1g) modes are only weakly effected. This effect unique to the Eg symmetry is exceptional considering that polarized spectra and comparison of the single phonon density of states between the amorphous and crystalline phases suggest that short range order of the amorphous phase is, on the average, similar to that of the crystalline material while electrical transport measurements reveal a sharp insulator-to-metal transition. Our findings point to the important role of anisotropic disorder affecting potential applications of topological and phase-change based electronics.
The contactless electro-reflectance (CER) spectra can reveal the electronic properties of the semiconductor layers. CER is a modulated reflectance technique in which an ac modulating voltage (1 kV at a frequency 1000 Hz) is applied to the sample in a condenser-like configuration. The plot of ΔR/R yields a reflectance spectrum with sharp derivative-like features from which the optical transitions can be obtained.
At lower energies a new spectral feature emerges in bilayers where the Bi2Se3 layer is very thin (samples S1 and S3). It appears in the Zn0.23Cd0.25Mg0.52Se/Bi2Se3 structure at 2.24 eV and in Zn0.49Cd0.51Se/Bi2Se3 at 1.99 eV, separated by 0.7 eV from the II-VI spectral peaks. This feature is consistent with upshifted excited states in quantum wells formed in thin TI layers due to quantum confinement by large bandgap semiconductor overlayers.
