72,728 research outputs found

### Lagrange multipliers for evolution problems with constraints on the derivatives

We prove the existence of generalized Lagrange multipliers for a class of evolution problems for linear differential operators of different types subject to constraints on the derivatives. Those Lagrange multipliers and the respective solutions are stable for the vanishing of the coercive parameter and are naturally associated with evolution variational inequalities with time-dependent convex sets of gradient type. We apply these results to the sandpile problem, to superconductivity problems, to flows of thick fluids, to problems with the biharmonic operator, and to first order vector fields of subelliptic type.The research of A. Azevedo and L. Santos was partially supported by the Research Centre of Mathematics of the University of Minho with the Portuguese Funds from the “Fundação para a Ciência e a Tecnologia,” through the Project UID/MAT/00013/2013, and the one by J. F. Rodrigues was done partially in the framework of the Project PTDC/MAT-PUR/28686/2017

### Tensor network approach to the fully frustrated XY model on a kagome lattice with a fractional vortex-antivortex pairing transition

We have developed a tensor network approach to the two-dimensional fully
frustrated classical XY spin model on the kagome lattice, and clarified the
nature of the possible phase transitions of various topological excitations.We
find that the standard tensor network representation for the partition function
does not work due to the strong frustrations in the low temperature limit. To
avoid the direct truncation of the Boltzmann weight, based on the duality
transformation, we introduce a new representation to build the tensor network
with local tensors lying on the centers of the elementary triangles of the
kagome lattice. Then the partition function is expressed as a product of
one-dimensional transfer matrix operators, whose eigen-equation can be solved
by the variational uniform matrix product state algorithm accurately. The
singularity of the entanglement entropy for the one-dimensional quantum
operator provides a stringent criterion for the possible phase transitions.
Through a systematic numerical analysis of thermodynamic properties and
correlation functions in the thermodynamic limit, we prove that the model
exhibits a single Berezinskii-Kosterlitz-Thouless phase transition only, which
is driven by the unbinding of $1/3$ fractional vortex-antivortex pairs
determined at $T_{c}\simeq 0.075J_{1}$ accurately. The absence of long-range
order of chirality or quasi-long range order of integer vortices has been
verified in the whole finite temperature range. Thus the long-standing
controversy about the phase transitions in this fully frustrated XY model on
the kagome lattice is solved rigorously, which provides a plausible way to
understand the charge-6e superconducting phase observed experimentally in the
two-dimensional kagome superconductors.Comment: 14 pages, 13 figures, submitted version for publicatio

### Conceptual Design of a Liquid Helium Vertical Test-Stand for 2m long Superconducting Undulator Coils

Superconducting Undulators (SCUs) can produce higher photon flux and cover a wider photon energy range compared to permanent magnet undulators (PMUs) with the same vacuum gap and period length. To build the know-how to implement superconducting undulators for future upgrades of the European XFEL facility, the test stand SUNDAE1 for the characterization of SCU is being developed. The purpose of SUNDAE1 is the training, tuning and development of new SCU coils by means of precise magnetic field measurements. The experimental setup will allow the characterization of magnets up to 2m in length. These magnets will be immersed in a Helium bath at 4K or 2K temperature. In this article, we describe the experimental setup and highlight its expected performances

### Engineering anomalous Floquet Majorana modes and their time evolution in helical Shiba chain

We theoretically explore the Floquet generation of Majorana end modes~(MEMs)
(both regular $0$- and anomalous $\pi$-modes) implementing a periodic
sinusoidal modulation in chemical potential in an experimentally feasible setup
based on one-dimensional chain of magnetic impurity atoms having spin spiral
configuration fabricated on the surface of most common bulk $s$-wave
superconductor. We obtain a rich phase diagram in the parameter space,
highlighting the possibility of generating multiple $0$-/$\pi$-MEMs localized
at the end of the chain. We also study the real-time evolution of these
emergent MEMs, especially when they start to appear in the time domain. These
MEMs are topologically characterized by employing the dynamical winding number.
We also discuss the possible experimental parameters in connection to our
model. Our work paves the way to realize the Floquet MEMs in a
magnet-superconductor heterostructure.Comment: 7.5 Pages + 5 PDF figures (Main Text), 4 Pages + 3 PDF figures
(Supplementary Material), Comments are welcom

### Similarities and Differences in the Fermiology of Kagome Metals AV$_{3}$Sb$_{5}$ (A=K, Rb, Cs) Revealed by Shubnikov-de Haas Oscillations

Materials with AV$_3$Sb$_5$ (A=K, Rb, Cs) stoichiometry are recently
discovered kagome superconductors with the electronic structure featuring a
Dirac band, van Hove singularities and flat bands. These systems undergo
anomalous charge-density-wave (CDW) transitions at $T_{\rm CDW}$~80-100 K,
resulting in the reconstruction of the Fermi surface from the pristine phase.
Although comprehensive investigations of the electronic structure via quantum
oscillations (QOs) have been performed on the sister compounds CsV$_3$Sb$_5$
and RbV$_3$Sb$_5$, a detailed QO study of KV$_3$Sb$_5$ is so far absent. Here,
we report the Shubnikov-de Haas QO study in KV$_3$Sb$_5$. We resolve a large
number of new frequencies with the highest frequency of 2202 T (occupying ~54%
of the Brillouin zone area in the $k_x$-$k_y$ plane). The Lifshitz-Kosevich
analysis further gives relatively small cyclotron effective masses, and the
angular dependence study reveals the two-dimensional nature of the frequencies
with a sufficient signal-to-noise ratio. Finally, we compare the QO spectra for
all three AV$_3$Sb$_5$ compounds collected under the same conditions, enabling
us to point out the similarities and differences across these systems. Our
results fill in the gap of the QO study in KV$_3$Sb$_5$ and provide valuable
data to understand the band structure of all three members of AV$_3$Sb$_5$.Comment: 8 pages, 4 figure

### Bipolar thermoelectricity in S/I/NS and S/I/SN superconducting tunnel junctions

Recent studies have shown the potential for bipolar thermoelectricity in
superconducting tunnel junctions with asymmetric energy gaps. The
thermoelectric performance of these systems is significantly impacted by the
inverse proximity effects present in the normal-superconducting bilayer, which
is utilized to adjust the gap asymmetry in the junction. Here, we identify the
most effective bilayer configurations, and we find that directly
tunnel-coupling the normal metal side of the bilayer with the other
superconductor is more advantageous compared to the scheme used before. By
utilizing quasiclassical equations, we examined the nonlinear thermoelectric
junction performance as a function of the normal metal film thickness and the
quality of the normal-superconducting interface within the bilayer, thereby
determining the optimal design to observe and maximize this nonequilibrium
effect. Our results offer a roadmap to achieve improved thermoelectric
performance in superconducting tunnel junctions, with promising implications
for a number of applications.Comment: 5 pages, 3 figure

### Dynamic dielectric function and phonon self-energy from electrons strongly correlated with acoustic phonons in 2D Dirac crystals

The unique structure of two-dimensional (2D) Dirac crystals, with electronic
bands linear in the proximity of the Brillouin-zone boundary and the Fermi
energy, creates anomalous situations where small Fermi-energy perturbations are
known to critically affect the electron-related lattice properties of the
system. The Fermi-surface nesting (FSN) conditions determining such effects via
electron-phonon interaction, require accurate estimates of the crystal's
response function $(\chi)$ as a function of the phonon wavevector q for any
values of temperature. Numerous analytical estimates of $\chi(q)$ for 2D Dirac
crystals beyond the Thomas-Fermi approximation have been so far carried out
only in terms of dielectric response function $\chi(q,\omega)$, for photon and
optical-phonon perturbations, due to relative ease of incorporating a
q-independent oscillation frequency in their calculation. However, models
accounting for Dirac-electron interaction with ever-existing acoustic phonons,
for which $\omega$ does depend on q and is therefore dispersive, are essential
to understand many critical crystal properties. The lack of such models has
often led to assume that the dielectric response function $\chi(q)$ in these
systems can be understood from free-electron behavior. Here, we show that,
different from free-electron systems, $\chi(q)$ calculated from acoustic
phonons in 2D Dirac crystals using the Lindhard model, exhibits a cuspidal
point at the FSN condition. Strong variability of $\frac{\partial\chi}{\partial
q}$ persists also at finite temperatures, while $\chi(q)$ may tend to infinity
in the dynamic case even where the speed of sound is small, albeit
nonnegligible, over the Dirac-electron Fermi velocity. The implications of our
findings for electron-acoustic phonon interaction and transport properties such
as the phonon line width derived from the phonon self energy will also be
discussed

### Resonant and polarization effects in the processes of quantum electrodynamics in a strong magnetic field

The monograph considers resonance and polarization effects in quantum
electrodynamics processes that take place in a strong external magnetic field.
A method for analyzing spin-polarization effects has been developed. The
factorization of process cross sections in resonant conditions and the
representation of these cross sections in the form of Breit-Wigner are
considered. The possibility of testing these effects in modern international
projects to test quantum electrodynamics in strong fields is shown.Comment: 283 pages, 24 figures, monograp

### Scanning tunneling microscopy and spectroscopy study of layered transition metal chalcogenide thin films tuned by thickness, structural phase, and heterostructure

Layered transition metal chalcogenides (TMCs) have a long and fruitful history, exhibiting diverse properties and interesting quantum phenomena, such as superconductivity, charge density wave (CDW), and non-trivial topological states. Since the discovery that layered materials can be exfoliated to two-dimensional (2D) sheets, such as graphene, 2D TMCs have attracted vast interest for both fundamental study and technological development. More often than not, the thin film samples behave differently from the corresponding bulk single crystals. For example, monolayer WTe2 is a quantum spin Hall insulator, a novel phase of quantum matter, while bulk WTe2 is a Weyl semimetal. Moreover, the electronic and magnetic properties of thin films are able to be tuned by various methods, including chemically doping, electrically gating, varying the structural phase, changing the sample thickness, constructing heterostructures with other 2D materials, and mechanically manipulation.
This thesis presents experimental results on the effect of the number of layers, lattice structure, and heterostructure in several thin-film systems. To synthesize the high-quality samples, a home-built molecular beam epitaxy system was utilized to precisely control the thickness and structural phase. The films were characterized by scanning tunneling microscopy and spectroscopy. Our finds are the following. The 1T-VSe2 samples, grown at low temperatures, display different CDW patterns on the first and second layers. In contrast, a novel phase, i.e. distorted 1T, is obtained in the second layer by elevating the growth temperature by 250 C. No CDW is observed in this phase. The same phase engineering strategy was adopted for NbSe2 growth. While monolayer 1H-NbSe2 is demonstrated to host unconventional superconductivity, the Mott insulating ground state and strong evidence for the presence of spinon Fermi surface are disclosed in metastable 1T-NbSe2 monolayers. Besides, thin films of FeSexTe(1-x) grown on single-crystal Bi2Te3$ were systematically investigated under a low-temperature scanning tunneling microscope. Three different heterostructures coexist in the sample. At 0.3 K, the superconductivity appears in all three kinds of FeSexTe(1-x) islands, but with strikingly different gap sizes and depths. The data analysis from the phase-sensitive quasiparticle interference technique implies an odd-parity s+- pairing in superconducting states.LimitedAuthor requested closed access (OA after 2yrs) in Vireo ETD syste

### The passage of time and top-down causation

It is often claimed that the fundamental laws of physics are deterministic
and time-symmetric and that therefore our experience of the passage of time is
an illusion. This paper will critically discuss these claims and show that they
are based on the misconception that the laws of physics are an exact and
complete description of nature. I will argue that all supposedly fundamental
deterministic and time-symmetric laws have their limitations and are
supplemented by stochastic and irreversible elements. In fact, a deterministic
description of a system is valid only as long as interactions with the rest of
the world can be ignored. The most famous example is the quantum measurement
process that occurs when a quantum system interacts with a macroscopic
environment such as a measurement apparatus. This environment determines in a
top-down way the possible outcomes of the measurement and their probabilities.
I will argue that more generally the possible events that can occur in a system
and their probabilities are the result of top-down influences from the wider
context. In this way the microscopic level of a system is causally open to
influences from the macroscopic environment. In conclusion, indeterminism and
irreversibility are the result of a system being embedded in a wider context.Comment: This paper is based on a talk given at the MG16 conference in July
2021, and it appeared this year in the proceedings of this conference
(online, open access, and print

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