A5060973787- Semiconductor Quantum Structures and Devices
- Quantum and electron transport phenomena
- Quantum Dots Synthesis And Properties
- Chalcogenide Semiconductor Thin Films
- Magnetic properties of thin films
- 2D Materials and Applications
- Physics of Superconductivity and Magnetism
- Perovskite Materials and Applications
- Atomic and Subatomic Physics Research
- Cold Atom Physics and Bose-Einstein Condensates
- Quantum optics and atomic interactions
- Advanced Condensed Matter Physics
- Graphene research and applications
- Electronic and Structural Properties of Oxides
- Magnetic and transport properties of perovskites and related materials
- ZnO doping and properties
- Advancements in Semiconductor Devices and Circuit Design
- Advanced Semiconductor Detectors and Materials
- Molecular Junctions and Nanostructures
- Mechanical and Optical Resonators
- Semiconductor materials and devices
- Multiferroics and related materials
- Carbon Nanotubes in Composites
- Semiconductor materials and interfaces
- Heusler alloys: electronic and magnetic properties
Los Alamos National Laboratory
2015-2024
National High Magnetic Field Laboratory
2015-2024
High Magnetic Field Laboratory
2017
University of Minnesota
2005-2013
Center for Integrated Nanotechnologies
2010-2011
Pennsylvania State University
2009
Los Alamos Medical Center
2007
Laser Operations (United States)
2005
Los Alamos National Security (United States)
2002
University of California, Santa Barbara
1994-1998
Ruddlesden-Popper halide perovskites are 2D solution-processed quantum wells with a general formula A2A'n-1M n X3n+1, where optoelectronic properties can be tuned by varying the perovskite layer thickness (n-value), and have recently emerged as efficient semiconductors technologically relevant stability. However, fundamental questions concerning nature of optical resonances (excitons or free carriers) exciton reduced mass, their scaling well thickness, which critical for designing devices,...
We report on the dynamics of resonant energy transfer in monodisperse, mixed-size, and energy-gradient (layered) assemblies CdSe nanocrystal quantum dots. Time-resolved spectrally resolved photoluminescence directly reveals energy-dependent rate excitons from smaller to larger dots via electrostatic coupling. The data show a rapid (0.7-1.9 ns) across large tens-of-meV gap (i.e., between disparate size), suggest that interdot can approach picosecond time scales structurally optimized systems.
We investigate the strongly temperature-dependent radiative lifetime of electron–hole excitations in colloidal CdSe nanocrystal quantum dots over nearly three orders magnitude temperature (300 K to 380 mK). These studies reveal an intrinsic, upper limit ∼1 μs for storage excitons below 2 K. At higher temperatures, exciton lifetimes are consistent with thermal activation from dark-exciton ground state, but two different thresholds.
Abstract In bulk and quantum-confined semiconductors, magneto-optical studies have historically played an essential role in determining the fundamental parameters of excitons (size, binding energy, spin, dimensionality so on). Here we report low-temperature polarized reflection spectroscopy atomically thin WS 2 MoS high magnetic fields to 65 T. Both A B exhibit similar Zeeman splittings approximately −230 μeV T −1 ( g -factor ≃−4), thereby quantifying valley effect monolayer transition-metal...
We report 65 tesla magneto-absorption spectroscopy of exciton Rydberg states in the archetypal monolayer semiconductor WSe$_2$. The strongly field-dependent and distinct energy shifts 2s, 3s, 4s excited neutral excitons permits their unambiguous identification allows for quantitative comparison with leading theoretical models. Both sizes (via low-field diamagnetic shifts) energies $ns$ agree remarkably well detailed numerical simulations using non-hydrogenic screened Keldysh potential 2D...
A strong electron–hole exchange interaction (EI) in semiconductor nanocrystals (NCs) gives rise to a large (up tens of meV) splitting between optically active ('bright') and passive ('dark') excitons. This dark–bright has significant effect on the optical properties band-edge excitons leads pronounced temperature magnetic field dependence radiative decay. Here we demonstrate nanoengineering-based approach that provides control over EI while maintaining nearly constant emission energy. We...
Excitons in atomically-thin semiconductors necessarily lie close to a surface, and therefore their properties are expected be strongly influenced by the surrounding dielectric environment. However, systematic studies exploring this role challenging, part because most readily accessible exciton parameter -- exciton's optical transition energy is largely \textit{un}affected medium. Here we show that of environment revealed through its influence on \textit{size} exciton, which can directly...
In semiconductor physics, many essential optoelectronic material parameters can be experimentally revealed via optical spectroscopy in sufficiently large magnetic fields. For monolayer transition-metal dichalcogenide semiconductors, this field scale is substantial-tens of teslas or more-due to heavy carrier masses and huge exciton binding energies. Here we report absorption [Formula: see text], text] very high fields 91 T. We follow the diamagnetic shifts valley Zeeman splittings not only...
Ultrafast optical pulses are used to initiate and measure free-induction decays of coherent conduction electron spins embedded magnetic ${\mathrm{Mn}}^{2+}$ ions in a series magnetic-semiconductor quantum wells. These time-resolved Faraday rotation experiments transverse applied fields complement previous studies spin dynamics longitudinal by unambiguously distinguishing between the relaxation electrons holes, identifying mechanism which angular momentum is transferred from spin-polarized...
We directly imaged electrical spin injection and accumulation in the gallium arsenide channel of lateral spin-transport devices, which have ferromagnetic source drain tunnel-barrier contacts. The emission spins from was observed, a region near contact. Both injected accumulated same orientation (antiparallel to contact magnetization), we show that polarization flows away (against net electron current), indicating are polarized by reflection conductance can be modulated controlling optically...
We study spectrally resolved dynamics of Förster energy transfer in single monolayers and bilayers semiconductor nanocrystal quantum dots assembled using Langmuir−Blodgett (LB) techniques. For a monolayer, we observe distribution times from ∼50 ps to ∼10 ns, which can be quantitatively modeled assuming that the is dominated by interactions donor with acceptor nanocrystals first three "shells" surrounding donor. also detect an effective enhancement absorption cross section (up factor 4) for...
Femtosecond-resolved measurements of induced Faraday rotation reveal exchange coupling between the spin angular momenta charge carriers and a sublattice magnetic ions in semiconductor quantum wells. In transverse fields, we observe terahertz precession photoinjected electrons, rapid relaxation holes, coherent transfer momentum to via ultrafast local moments. The perturbed undergo free-induction decay, enabling time-domain all-optical electron-spin-resonance single planes.
We have developed a new class of colloidal nanocrystals composed Cu-doped ZnSe cores overcoated with CdSe shells. Via spectroscopic and magneto-optical studies, we conclusively demonstrate that Cu impurities represent paramagnetic +2 species serve as source permanent optically active holes. This implies the Fermi level is located below Cu2+/Cu1+ state, is, in lower half forbidden gap, which signature p-doped material. It further suggests activation optical emission due to requires injection...
We show that the accumulation of spin-polarized electrons at a forward-biased Schottky tunnel barrier between Fe and n-GaAs can be detected electrically. The spin leads to an additional voltage drop across is suppressed by small transverse magnetic field, which depolarizes spins in semiconductor. dependence electrical signal on bias current, temperature good agreement with predictions drift-diffusion model for transport.
Using scanning Kerr microscopy, we directly acquire two-dimensional images of spin-polarized electrons flowing laterally in bulk epilayers n:GaAs. Optical injection provides a local dc source polarized electrons, whose subsequent drift and/or diffusion is controlled with electric, magnetic, and - particular strain fields. Spin precession induced by uniaxial stress along the <110> axes demonstrates direct k-linear spin-orbit coupling electron spin to shear (off-diagonal) components tensor.
We measure the frequency spectra of random spin fluctuations, or ``spin noise,'' in ensembles $(\mathrm{In},\mathrm{Ga})\mathrm{As}/\mathrm{GaAs}$ quantum dots (QDs) at low temperatures. employ a noise spectrometer based on sensitive optical Faraday rotation magnetometer that is coupled to digitizer and field-programmable gate array, average from 0--1 GHz continuously real time with $\mathrm{\text{subnanoradian}}/\sqrt{\mathrm{Hz}}$ sensitivity. Both electron hole fluctuations generate...
Strong geometrical frustration in magnets leads to exotic states such as spin liquids, supersolids, and complex magnetic textures. SrCu 2 (BO 3 ) , a spin-1/2 Heisenberg antiferromagnet the archetypical Shastry–Sutherland lattice, exhibits rich spectrum of magnetization plateaus stripe-like textures applied fields. The structure these is still highly controversial due intrinsic complexity associated with competing length scales. We discover via magnetostriction magnetocaloric measurements...
Using time-resolved Kerr rotation, we measure the spin/valley dynamics of resident electrons and holes in single charge-tunable monolayers archetypal transition-metal dichalcogenide (TMD) semiconductor WSe2. In n-type regime, observe long (70 ns) polarization relaxation that is sensitive to in-plane magnetic fields $B_y$, indicating spin relaxation. marked contrast, extraordinarily (2 microsecond) revealed p-type unaffected by directly confirming long-standing expectations strong spin-valley...