2,190 research outputs found
Design of an interferometric system for gauge block calibration
We have developed an interferometer for gauge block calibration based on phase shifting algorithms. The measurement process can provide flatness, parallelism, and length. Wavelength values need to be corrected according to the refractive index of air. This correction is obtained indirectly using Edlén’s equation. High-resolution sensors provide the temperature, pressure, and relative humidity readings. To preserve stability, the interferometer is encapsulated in a chamber with active temperature control. Its design, measurement principle, calibration, stability, and reproducibility are analyzed. Since one goal is to employ robust and cheap diode lasers as light sources, we describe the system developed to stabilize a red laser diode using a mode locking technique with a reference gas cell. The instruments and assembly are used to avoid the Doppler effect in the gas cell, which would limit wavelength resolution. Several experiments are carried out to restrict the influence of environmental changes, which affect laser diode frequency.Ministerio de Educación y Ciencia | Ref. DPI2008-06818-C2-0
Quantum geometry in superfluidity and superconductivity
We review the theoretical description of the role of quantum geometry in
superfluidity and superconductivity of multiband systems, with focus on flat
bands where quantum geometry is wholly responsible for supercurrents. This
review differs from previous ones in that it is based on the most recent
understanding of the theory: the dependence of the self-consistent order
parameter on the supercurrent is properly taken into account, and the
superfluid weight in a flat band becomes proportional to the minimal quantum
metric. We provide a recap of basic quantum geometric quantities and the
concept of superfluid density. The geometric contribution of superconductivity
is introduced via considering the two-body problem. The superfluid weight of a
multiband system is derived within mean-field theory, leading to a topological
bound of flat band superconductivity. The physical interpretation of the flat
band supercurrent in terms of Wannier function overlaps is discussed.Comment: 32 pages, 1 figure, Lecture notes for the Proceedings of the
International School of Physics "Enrico Fermi" Course 211 "Quantum Mixtures
with Ultra-Cold Atoms" (Varenna, Italy, 2022
Observation of Fractionally Quantized Anomalous Hall Effect
The integer quantum anomalous Hall (QAH) effect is a lattice analog of the
quantum Hall effect at zero magnetic field. This striking transport phenomenon
occurs in electronic systems with topologically nontrivial bands and
spontaneous time-reversal symmetry breaking. Discovery of its putative
fractional counterpart in the presence of strong electron correlations, i.e.,
the fractional quantum anomalous Hall (FQAH) effect, would open a new chapter
in condensed matter physics. Here, we report the direct observation of both
integer and fractional QAH effects in electrical measurements on twisted
bilayer MoTe. At zero magnetic field, near filling factor (one
hole per moir\'e unit cell) we see an extended integer QAH plateau in the Hall
resistance that is quantized to while the
longitudinal resistance vanishes. Remarkably, at and
we see plateau features in at and
, respectively, while remains small. All these
features shift linearly in an applied magnetic field with slopes matching the
corresponding Chern numbers , , and , precisely as expected for
integer and fractional QAH states. In addition, at zero magnetic field,
is approximately near half filling () and
varies linearly as is tuned. This behavior resembles that of the
composite Fermi liquid in the half-filled lowest Landau level of a
two-dimensional electron gas at high magnetic field. Direct observation of the
FQAH and associated effects paves the way for researching charge
fractionalization and anyonic statistics at zero magnetic field.Comment: 15 pages, 4 figures for main text. 8 extended data figure
Analog of cosmological particle production in moir\'e Dirac materials
Moir\'e materials have recently been established experimentally as a
highly-tunable condensed matter platform, facilitating the controlled
manipulation of band structures and interactions. In several of these moir\'e
materials, Dirac cones are present in the low-energy regime near the Fermi
level. Thus, fermionic excitations emerging in these materials close to the
Dirac cones have a linear dispersion relation near the Fermi surface as
massless relativistic Dirac fermions. Here, we study low-energy fermionic
excitations of moir\'e Dirac materials in the presence of a mass gap that may
be generated by symmetry breaking. Introducing a dynamical Fermi velocity
and/or time-dependent mass gap for the Dirac quasiparticles, we exhibit the
emergence of an analog of cosmological fermion pair production in terms of
observables such as the expected occupation number or two-point correlation
functions. We find that it is necessary and sufficient for quasiparticle
production that only the ratio between the mass gap and the Fermi velocity is
time-dependent. In this way, we establish that moir\'e Dirac materials can
serve as analog models for cosmological spacetime geometries, in particular,
for Friedmann-Lema\^itre-Robertson-Walker expanding cosmologies. We briefly
discuss possibilities for experimental realization.Comment: 15 pages, 4 figure
Nanoscale Property Enhancement of Photovoltaic and 2D Layered Magnetic Materials
Three materials were investigated using two variations of atomic force microscopy and other characterisation techniques. CZTSSe is an earth abundant, non-toxic, semiconductor. Na is often doped into the kesterite structure to improve grain size and overall efficiency. In this thesis Na and other elements are found to migrate in the solar absorber material post deposition. The Na moves preferentially through the grain boundaries, and by applying an external voltage using two capacitor plates, device efficiency is found to be enhanced. The second material investigated is 2D layered perovskite samples BA2Pb(I/Br)4. Using similar external voltage application, nanosized structures form on the surface of the Iodine containing samples. These structures were investigated and found to be highly oriented perovskite structures that enhance crystallinity and conduction attributes. The final material investigated is CGT, a 2D layered magnetic material with a curie temperature of 61K. This material was investigated using a cryogenic MFM system. CGT samples were mechanically buckled forming folds in the 2D sheets. These folds have highly strained areas that were found to show a magnetic contrast at temperatures well above the unstrained curie temperature of 61K. In some samples the enhanced magnetic signal was found even at room temperature, offering future room temperature applicative avenues
"Le present est plein de l’avenir, et chargé du passé" : Vorträge des XI. Internationalen Leibniz-Kongresses, 31. Juli – 4. August 2023, Leibniz Universität Hannover, Deutschland. Band 2
[No abstract available]Deutschen Forschungsgemeinschaft (DFG)/Projektnr. 517991912VGH VersicherungNiedersächsisches Ministerium für Wissenschaft und Kultur (MWK
- …