Condens. Matter, Volume 8, Issue 4 (December 2023) – 16 articles

LaBr${}_3$:Ce crystals have good scintillation properties for X-ray spectroscopy. Initially, they were introduced for radiation imaging in medical physics with either a photomultiplier or SiPM readout, and they found extensive applications in homeland security and gamma-ray astronomy. We used 1${}^{″}$ round LaBr${}_3$:Ce crystals to realize compact detectors with the SiPM array readout. The aim was a good energy resolution and a fast time response to detect low-energy X-rays around 100 keV. A natural application was found inside the FAMU experiment, at RIKEN RAL. Its aim is a precise measurement of the proton Zemach radius with impinging muons, to contribute to the solution to the so-called “proton radius puzzle”. Signals to be detected are characteristic X-rays around 130 KeV. A limit for this type of detector, as compared to the ones with a photomultiplier readout, is its poorer timing characteristics due to the large capacity of the SiPM arrays used. In particular, long signal falltimes are a problem in experiments such as FAMU, where a “prompt” background component must be separated from a “delayed” one (after 600 ns) in the signal X-rays to be detected. Dedicated studies were pursued to improve the timing characteristics of the used detectors, starting from hybrid ganging of SiPM cells; then developing a suitable zero pole circuit with a parallel ganging, where an increased overvoltage for the SiPM array was used to compensate for the signal decrease; and finally designing ad hoc electronics to split the 1${}^{″}$ detector’s SiPM array into four quadrants, thus reducing the involved capacitances. The aim was to improve the detectors’ timing characteristics, especially falltime, while keeping a good FWHM energy resolution for low-energy X-ray detection. Full article

We present an innovative X-ray spectroscopy system to address the complex study of the X-ray emissions arising from laser–target interactions, where the emissions occur within extremely brief intervals from femtoseconds to nanoseconds. Our system combines a Gas Electron Multiplier (GEM) detector with a silicon-based Timepix3 (TPX3) detector. These detectors work in tandem, allowing for a spectroscopic radiation analysis along the same line of sight. With an active area of 10 × 10 cm${}^2$, the GEM detector allows for 1D measurements for X-ray energies (2–50 keV) by utilizing the full 10 cm gas depth. The high-energy part of the radiation beam exits through a downstream side window of the GEM without being absorbed in the gas volume. Positioned side-on at the GEM detector’s exit, the TPX3 detector, equipped with a pixelated sensor (55 µm × 55 µm; active area 14 mm × 14 mm), uses its full 14 mm silicon sensor to detect hard X-rays (50–500 keV) and gamma rays (0.5–10 MeV). We demonstrate the correct operation of the entire detection system and provide a detailed description of the Timepix3 detector’s calibration procedure, highlighting the suitability of the combined system to work in laser plasma facilities. Full article

Muonic Atom X-ray Emission spectroscopy (µ-XES) is a novel elemental technique that exploits the high-energy X-rays emitted from the muonic atom cascade process to characterize materials. At the ISIS Neutron and Muon Source, the technique is performed at Port4 of the RIKEN-RAL facility, with a user demand that is increasing every year. To cope with this demand, it is necessary to continue to improve the method, either for the hardware (detectors, acquisition, etc.) or software (data analysis and interpretation). In both cases, Monte Carlo codes play an important role: with a simulation, it is possible to reproduce the experimental setup and provide a reliable quantitative analysis. In this work, we investigate the capabilities of GEANT4 for such applications. From the results, we observed that the generation of X-rays, especially the kα and kβ transition for high Z atoms, are not in agreement with the experimental ones. A solution to this issue, other than an attempt with a small modification of the GEANT4 cascade class, could be provided by a database of transition energy calculated by a Dirac equation software called MuDirac. The software, developed by the UKRI scientific computing department and the ISIS muon group, can compute all the transition energy for a given nuclide. Here, preliminary results of the implementation of the MuDirac database in GEANT4 are reported. Full article

Charge density waves (CDWs) profoundly affect the electronic properties of materials and have an intricate interplay with other collective states, like superconductivity and magnetism. The well-known macroscopic Ginzburg–Landau theory stands out as a theoretical method for describing CDW phenomenology without requiring a microscopic description. In particular, it has been instrumental in understanding the emergence of domain structures in several CDW compounds, as well as the influence of critical fluctuations and the evolution towards or across lock-in transitions. In this context, McMillan’s foundational work introduced discommensurations as the objects mediating the transition from commensurate to incommensurate CDWs, through an intermediate nearly commensurate phase characterised by an ordered array of phase slips. Here, we extended the simplified, effectively one-dimensional, setting of the original model to a fully two-dimensional analysis. We found exact and numerical solutions for several types of discommensuration patterns and provide a framework for consistently describing multi-component CDWs embedded in quasi-two-dimensional atomic lattices. Full article

We review the topological gauge theory description of Josephson junction arrays (JJA), fabricated systems which exhibit the superconductor-to-insulator transition (SIT). This description revealed the topological nature of the phases around the SIT and led to the discovery of a new state of matter, the superinsulator, characterized by infinite resistance, even at finite temperatures, due to linear confinement of electric charges. This discovery is particularly relevant for the physics of superconducting films with emergent granularity, which are modeled with JJAs and share the same phase diagram. Full article

We develop a method for the laser synthesis and deposition of carbon–gold films formed by a net of linear sp-carbon chains and stabilized by gold nanoparticles. The originality of the method is in the simultaneous production of carbon chains and gold nanoparticles due to the laser fragmentation of the amorphous carbon and hydrogen tetrachloroaurate (III) or chloroauric acid. We study how surface resistivity alters the effect of the obtained films via the illumination in the visible spectral range. Full article

We discuss a formalism that allows for the calculation of a higher-harmonic-current response to a strong applied electric field for disordered superconducting systems described on the basis of tight-binding models with on- and/or intersite interactions. The theory is based on an expansion of the density matrix in powers of the field amplitudes, where we solve the equation of motion for the individual components. This allows the evaluation of higher-order response functions on significantly larger lattices than one can achieve with a previously used approach, which is based on a direct temporal integration of the equation of motion for the complete density matrix. In the case of small lattices, where both methods can be applied by including also the contribution of collective modes, we demonstrate the agreement of the corresponding results. Full article

In their applications in electrical machines, high-temperature superconductors (HTSs) are mainly used as inductors in synchronous machines due to the AC losses which can lead to high cryogenic costs. In this work, we show the possibility of their use as armature windings, handling some precautions. The approach is based on the combined use of modeling and measurements. The construction and the preliminary tests of a handmade prototype of an axial field HTS synchronous machine are presented. Several tests have been conducted at liquid nitrogen temperature. The measurements have been confirmed by modeling results. The preliminary tests on the prototype, in both modeling and measurements, are very promising. Full article

Among various “families” of iron-based superconductors, the quite recently discovered AeAFe₄As₄ (where Ae is an alkali-earth metal and A is an alkali metal) has high critical current density, a very high upper critical field, and a low anisotropy, and has recently received much interest for the possibility of high magnetic field applications at the liquid hydrogen temperature. We have performed DC magnetization relaxation and frequency-dependent AC susceptibility measurements on high-quality single crystals of CaKFe₄As₄ with the aim of determining the pinning potential U*. The temperature dependence of U* displays a clear crossover between elastic creep and plastic creep. At temperatures around 27–28 K, U* has a very high value, up to 1200 K, resulting in an infinitesimally small probability of thermally activated flux jumps. From the dependence of the normalized pinning potential on irreversible magnetization, we have determined the creep exponents in the two creep regimes, which are in complete agreement with theoretical models. The estimation of the pinning potential from multifrequency AC susceptibility measurements was possible only near the critical temperature due to equipment limitations, and the resulting value is very close to the one that resulted from the magnetization relaxation data. Magnetic hysteresis loops revealed a second magnetization peak and very high values of the critical current density. Full article

The very high capture cross-section of (epi)thermal neutrons by the boron isotope ¹⁰B makes elemental boron and its compounds and composites prospective for serving as materials intensively interacting with neutron irradiation. In their nanostructured form, boron-rich materials reveal properties that improve their radiation-performance characteristics. In this regard, new technologies have been proposed for the synthesis of nanocomposites with matrices of boron carbide B₄C and hexagonal boron nitride h-BN. For the first time, boron carbide-tungsten and hexagonal boron nitride–(iron,magnetite) composites were obtained, respectively, in the form of layered/sandwich structures of components B₄C and W and h-BN nanopowders coated/intercalated with magnetic nanoclusters of iron Fe or magnetite Fe₃O₄. Studying of their chemical/phase composition, structure/morphology, and some other properties leads to the conclusion that the developed B₄C–W and h-BN–(Fe,Fe₃O₄) composites would be useful for solving important problems of boron-based neutron shielding and BNCT (Boron Neutron Capture Therapy), such as attenuating the gamma-radiation accompanying the absorption of neutrons by ¹⁰B nuclei and targeted delivery of ¹⁰B nuclei, as BNCT therapeutic agents, to tumor tissues using control by an external magnetic field, respectively. Full article

The effect of the demagnetizing factor, regarding the determination of the de-pairing current density $J_{dep}$, has been studied in the case of a Fe(Se,Te) crystal, using DC magnetic measurements as a function of a magnetic field (H) at different temperatures (T). First, the lower critical field $H_{c1}$(T) values were obtained, and the demagnetization effects acting on them were investigated after calculating the demagnetizing factor. The temperature behaviors of both the original $H_{c1}$ values and the ones obtained after considering the demagnetization effects ($H_{c1}^{demag}$) were analyzed, and the temperature dependence of the London penetration depth ${\lambda }_L$(T) was obtained in both cases. In particular, the ${\lambda }_L$(T) curves were fitted with a power law dependence, indicating the presence of low-energy quasiparticle excitations. Furthermore, by plotting ${\lambda }_L^{-2}$ as a function of T, we found that our sample behaves as a multigap superconductor, which is similar to other Fe-11 family iron-based compounds. After that, the coherence length $\mathsf{\xi }$ values were extracted, starting with the $H_{c2}$(T) curve. The knowledge of ${\lambda }_L$ and $\mathsf{\xi }$ allowed us to determine the $J_{dep}$ values and to observe how they are influenced by the demagnetizing factor. Full article

The reliable transmission of secure keys is one of the essential tasks to be efficiently accomplished by quantum information processing, and the use of entangled particles is a very important tool toward that goal. However, efficient production of maximally entangled states is still a challenge for further progress in quantum computing and quantum communication. In the search for optimal sources of entanglement, quantum dots have emerged as promising candidates, but the presence of dephasing in the generated entangled states raises questions about their real usefulness in large-scale quantum networks. In this work, we evaluate the effects of the exciton fine structure splitting, present in most quantum dot samples, on the fidelity of the BBM92 protocol for quantum key distribution. We find that the protocol’s performance is heavily impacted by such splitting and establish an upper limit for the product between the energy splitting and the exciton lifetime to have a dependable distributed key. Full article

In 1T-TaS${}_{2-x}$Se${}_x$, the charge density wave (CDW) state features a star of David lattice that expands across layers as the system becomes commensurate upon cooling. The layers can also order along the c-axis, and different stacking orders have been proposed. Using neutron scattering on powder samples, we compared the stacking order previously observed in 1T-TaS${}_2$ when the system is doped with Se. While at low temperature, a 13c layer sequence stacking was observed in TaS${}_2$; this type of ordering was not evident with doping. Doping with Se results in a metallic state in which the Mott transition is suppressed, which may be linked to the absence of layer stacking. Full article

Photo-induced carrier dynamics were measured in the organic Dirac electron candidate $\alpha$-(BETS)${}_2$I${}_3$ to investigate why resistivity increases below $T_{\mathrm{MI}}$ = 50 K. We found a change in carrier dynamics due to an insulating gap formation below $T^{\prime }$ = 50 K. On the other hand, the relaxation time and polarization anisotropy of the observed dynamics differ from those in the charge-ordering (CO) state of the isostructural salt $\alpha$-(ET)${}_2$I${}_3$. Based on the difference, it can be concluded that the insulating phase has a different origin than the CO state. Full article

This study investigated the impact of samarium and lanthanum fluorides (SmF₃ and LaF₃) on the physical and mechanical properties of Tl_0.8Hg_0.2Ba₂Ca_2−xR_xCu₃O_9−δ−yF_y superconducting phases (specifically the (Tl, Hg)-1223 phase), where R = Sm and La, with $0.00\hspace{0.17em}\le \hspace{0.17em}x \hspace{0.17em}\le \hspace{0.17em}0.10$. The superconducting samples were synthesized using the solid-state reaction method. X-ray diffraction (XRD) verified the formation of the (Tl, Hg)-1223 phase without altering its tetragonal structure. Scanning electron micrographs (SEM) reveal the improvement of the grain size and inter-grain connectivity as Sm and La contents increased up to $x=0.025$. The electrical properties of (Tl, Hg)-1223 were studied using I-V and electrical resistivity measurements. Improved superconducting transition temperature (T_c) and transport critical current density (J_c) were observed up to $x=0.025$, beyond which they decreased substantially. Vickers microhardness (H_v) measurements were performed at room temperature to investigate their mechanical performance with various applied loads ($0.49–9.80$ N) and times ($10–90$s). For both substitutions, the mechanical properties were enhanced up to an optimal value at $x=0.025$. All samples exhibited normal indentation size effect (ISE) behavior. The proportional sample resistance (PSR) model best explained H_v values among five theoretical models. Dislocation creep was the primary creep mechanism in the samples, according to indentation creep studies. Full article

In this study, we investigated, for the first time, the effects of the spatially varying effective mass, asymmetry parameter, and well width on the electronic and optical properties of a quantum well which has an improved Rosen–Morse potential. Calculations were made within the framework of the effective mass and parabolic band approximations. We have used the diagonalization method by choosing a wave function based on the trigonometric orthonormal functions to find eigenvalues and eigenfunctions of the electron confined within the improved Rosen–Morse potential. Our results show that the position dependence mass, asymmetry, and confinement parameters cause significant changes in the electronic and optical properties of the structure we focus on since these effects create a significant increase in electron energies and a blue shift in the absorption spectrum. The increase in energy levels enables the development of optoelectronic devices that can operate at wider wavelengths and absorb higher-energy photons. Through an appropriate choice of parameters, the Rosen–Morse potential offers, among many advantages, the possibility of simulating heterostructures close to surfaces exposed to air or vacuum, thus giving the possibility of substantially enriching the allowed optical transitions given the breaking of the system´s symmetries. Similarly, the one-dimensional Rosen–Morse potential model proposed here can be extended to one- and zero-dimensional structures such as core/shell quantum well wires and quantum dots. This offers potential advancements in fields such as optical communication, imaging technology, and solar cells. Full article