60 research outputs found
Exciton Diamagnetic Shifts and Valley Zeeman Effects in Monolayer WS and MoS to 65 Tesla
We report circularly-polarized optical reflection spectroscopy of monolayer
WS and MoS at low temperatures (4~K) and in high magnetic fields to
65~T. Both the A and the B exciton transitions exhibit a clear and very similar
Zeeman splitting of approximately 230~eV/T (), providing
the first measurements of the valley Zeeman effect and associated -factors
in monolayer transition-metal disulphides. These results complement and are
compared with recent low-field photoluminescence measurements of valley
degeneracy breaking in the monolayer diselenides MoSe and WSe. Further,
the very large magnetic fields used in our studies allows us to observe the
small quadratic diamagnetic shifts of the A and B excitons in monolayer WS
(0.32 and 0.11~eV/T, respectively), from which we calculate exciton
radii of 1.53~nm and 1.16~nm. When analyzed within a model of non-local
dielectric screening in monolayer semiconductors, these diamagnetic shifts also
constrain and provide estimates of the exciton binding energies (410~meV and
470~meV for the A and B excitons, respectively), further highlighting the
utility of high magnetic fields for understanding new 2D materials.Comment: 9 pages, 5 figure
Magneto-reflection spectroscopy of monolayer transition-metal dichalcogenide semiconductors in pulsed magnetic fields
We describe recent experimental efforts to perform polarization-resolved
optical spectroscopy of monolayer transition-metal dichalcogenide
semiconductors in very large pulsed magnetic fields to 65 tesla. The
experimental setup and technical challenges are discussed in detail, and
temperature-dependent magneto-reflection spectra from atomically thin tungsten
disulphide (WS) are presented. The data clearly reveal not only the valley
Zeeman effect in these 2D semiconductors, but also the small quadratic exciton
diamagnetic shift from which the very small exciton size can be directly
inferred. Finally, we present model calculations that demonstrate how the
measured diamagnetic shifts can be used to constrain estimates of the exciton
binding energy in this new family of monolayer semiconductors.Comment: PCSI-43 conference (Jan. 2016; Palm Springs, CA
Magneto-Optics of Exciton Rydberg States in a Monolayer Semiconductor
We report 65 tesla magneto-absorption spectroscopy of exciton Rydberg states
in the archetypal monolayer semiconductor WSe. The strongly field-dependent
and distinct energy shifts of the 2s, 3s, and 4s excited neutral excitons
permits their unambiguous identification and allows for quantitative comparison
with leading theoretical models. Both the sizes (via low-field diamagnetic
shifts) and the energies of the exciton states agree remarkably well with
detailed numerical simulations using the non-hydrogenic screened Keldysh
potential for 2D semiconductors. Moreover, at the highest magnetic fields the
nearly-linear diamagnetic shifts of the weakly-bound 3s and 4s excitons provide
a direct experimental measure of the exciton's reduced mass, .Comment: To appear in Phys. Rev. Lett. Updated version (25 jan 2018) now
includes detailed supplemental discussion of Landau levels, Rydberg exciton
energies, exciton mass, Dirac Hamiltonian, nonparabolicity, and dielectric
effect
Asymmetric magnetic proximity interactions in MoSe/CrBr van der Waals heterostructures
Magnetic proximity interactions (MPIs) between atomically-thin semiconductors
and two-dimensional magnets provide a means to manipulate spin and valley
degrees of freedom in nonmagnetic monolayers, without the use of applied
magnetic fields. In such van der Waals (vdW) heterostructures, MPIs originate
in the nanometer-scale coupling between the spin-dependent electronic
wavefunctions in the two materials, and typically their overall effect is
regarded as an effective magnetic field acting on the semiconductor monolayer.
Here we demonstrate that this picture, while appealing, is incomplete: The
effects of MPIs in vdW heterostructures can be markedly asymmetric, in contrast
to that from an applied magnetic field. Valley-resolved optical reflection
spectroscopy of MoSe/CrBr vdW structures reveals strikingly
different energy shifts in the and valleys of the MoSe, due to
ferromagnetism in the CrBr layer. Strong asymmetry is observed at both the
A- and B-exciton resonances. Density-functional calculations indicate that
valley-asymmetric MPIs depend sensitively on the spin-dependent hybridization
of overlapping bands, and as such are likely a general feature of such hybrid
vdW structures. These studies suggest routes to selectively control
\textit{specific} spin and valley states in monolayer semiconductors.Comment: 12 pages total (including 4 figures + 7 Supplemental Figures
Spin noise spectroscopy to probe quantum states of ultracold fermionic atomic gases
Ultracold alkali atoms provide experimentally accessible model systems for
probing quantum states that manifest themselves at the macroscopic scale.
Recent experimental realizations of superfluidity in dilute gases of ultracold
fermionic (half-integer spin) atoms offer exciting opportunities to directly
test theoretical models of related many-body fermion systems that are
inaccessible to experimental manipulation, such as neutron stars and
quark-gluon plasmas. However, the microscopic interactions between fermions are
potentially quite complex, and experiments in ultracold gases to date cannot
clearly distinguish between the qualitatively different microscopic models that
have been proposed. Here, we theoretically demonstrate that optical
measurements of electron spin noise -- the intrinsic, random fluctuations of
spin -- can probe the entangled quantum states of ultracold fermionic atomic
gases and unambiguously reveal the detailed nature of the interatomic
interactions. We show that different models predict different sets of
resonances in the noise spectrum, and once the correct effective interatomic
interaction model is identified, the line-shapes of the spin noise can be used
to constrain this model. Further, experimental measurements of spin noise in
classical (Boltzmann) alkali vapors are used to estimate the expected signal
magnitudes for spin noise measurements in ultracold atom systems and to show
that these measurements are feasible
A quantitative study of spin noise spectroscopy in a classical gas of K atoms
We present a general derivation of the electron spin noise power spectrum in
alkali gases as measured by optical Faraday rotation, which applies to both
classical gases at high temperatures as well as ultracold quantum gases. We
show that the spin-noise power spectrum is determined by an electron spin-spin
correlation function, and we find that measurements of the spin-noise power
spectra for a classical gas of K atoms are in good agreement with the
predicted values. Experimental and theoretical spin noise spectra are directly
and quantitatively compared in both longitudinal and transverse magnetic fields
up to the high magnetic field regime (where Zeeman energies exceed the
intrinsic hyperfine energy splitting of the K ground state)
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