Search

This study explores the effect of Al-buffered layers on the crystal structure and superconductivity of MgB2. The unique Mg1-xAlxB2 phase formed at a thickness of 190 nm is a promising technique for thin-film fabrication. The upper critical field (Hc2) of Mg1-xAlxB2 is suppressed, which corresponds to a change in interband scattering, as confirmed by X-ray diffraction and extended X-ray absorption fine structure results. By contrast, the thicker samples with the Al-buffer layer, which contain MgB2 on the top surface, show an improvement in Hc2 compared to the pure sample because the grain boundaries act as flux pinning sources. The observed reduction in the electron–phonon coupling constants correlates with Tc suppression in the Al-buffered samples. A Meissner force curve reveal larger London penetration depth (λL) in Al-buffered films than in pure MgB2 samples. Particularly, the Mg1-xAlxB2 sample, exhibits a λL value at 0 K of 1009.68 nm, resulting in a thermodynamic critical field (Hc) of 0.026 T. Higher-thickness Al-buffered film samples, which have a MgB2 layer on the top surface, displays λL and Hc values in close agreement with those of a pure MgB2 film.

Abstract The nature of charge degrees-of-freedom distinguishes scenarios for interpreting the character of a second order magnetic transition at zero temperature, that is, a magnetic quantum critical point (QCP). Heavy-fermion systems are prototypes of this paradigm, and in those, the relevant question is where, relative to a magnetic QCP, does the Kondo effect delocalize their f-electron degrees-of-freedom. Herein, we use pressure-dependent Hall measurements to identify a finite-temperature scale E loc that signals a crossover from f-localized to f-delocalized character. As a function of pressure, E loc(P) extrapolates smoothly to zero temperature at the antiferromagnetic QCP of CeRhIn5 where its Fermi surface reconstructs, hallmarks of Kondo-breakdown criticality that generates critical magnetic and charge fluctuations. In 4.4% Sn-doped CeRhIn5, however, E loc(P) extrapolates into its magnetically ordered phase and is decoupled from the pressure-induced magnetic QCP, which implies a spin-density-wave (SDW) type of criticality that produces only critical fluctuations of the SDW order parameter. Our results demonstrate the importance of experimentally determining E loc to characterize quantum criticality and the associated consequences for understanding the pairing mechanism of superconductivity that reaches a maximum T c in both materials at their respective magnetic QCP.

Abstract Emergent inhomogeneous electronic phases in metallic quantum systems are crucial for understanding high‐Tc superconductivity and other novel quantum states. In particular, spin droplets introduced by nonmagnetic dopants in quantum‐critical superconductors (QCSs) can lead to a novel magnetic state in superconducting phases. However, the role of disorders caused by nonmagnetic dopants in quantum‐critical regimes and their precise relation with superconductivity remain unclear. Here, the systematic evolution of a strong correlation between superconductive intertwined electronic phases and antiferromagnetism in Cd‐doped CeCoIn5 is presented by measuring current–voltage characteristics under an external pressure. In the low‐pressure coexisting regime where antiferromagnetic (AFM) and superconducting (SC) orders coexist, the critical current (Ic) is gradually suppressed by the increasing magnetic field, as in conventional type‐II superconductors. At pressures higher than the critical pressure where the AFM order disappears, Ic remarkably shows a sudden spike near the irreversible magnetic field. In addition, at high pressures far from the critical pressure point, the peak effect is not suppressed, but remains robust over the whole superconducting region. These results indicate that magnetic islands are protected around dopant sites despite being suppressed by the increasingly correlated effects under pressure, providing a new perspective on the role of quenched disorders in QCSs.

Abstract Very high applied pressure induces superconductivity with the transition temperature (T c) exceeding 19 K in elemental yttrium, but relatively little is known about the nature of that superconductivity. From point-contact spectroscopy (PCS) measurements in a diamond anvil cell (DAC), a strong enhancement in the differential conductance is revealed near the zero-biased voltage owing to Andreev reflection, a hallmark of the superconducting (SC) phase. Analysis of the PCS spectra based on the extended Blonder-Tinkham-Klapwijk (BTK) model indicates two SC gaps at 48.6 GPa, where the large gap ΔL is 3.63 meV and the small gap ΔS is 0.46 meV. When scaled against a reduced temperature, both small and large SC gaps collapse on a single curve that follows the prediction from BCS theory. The SC gap-to-T c ratio is 8.2 for the larger gap, and the initial slope of the upper critical field is −1.9 T/K, indicating that Y belongs to a family of strongly coupled BCS superconductors. The successful application of PCS to Y in DAC environments demonstrates its utility for future research on other pressure-induced high-T c superconductors.

Thin-film high-entropy alloy (HEA) superconductors have recently attracted a lot of attention, but their critical current density and potential usefulness in engineering applications has remained unclear. Here, the authors fabricate HEA films with remarkably high critical current density and resistance to radiation damage.

We investigated CeCoIn5 and LaCoIn5 single crystals, which have the same HoCoGa5-type tetragonal crystal structure, using infrared spectroscopy. However, while CeCoIn5 has 4f electrons, LaCoIn5 does not. By comparing these two material systems, we extracted the temperature-dependent electronic evolution of the f electrons of CeCoIn5. We observed that the differences caused by the f electrons are more obvious in low-energy optical spectra at low temperatures. We introduced a complex optical resistivity and obtained a magnetic optical resistivity from the difference in the optical resistivity spectra of the two material systems. From the temperature-dependent average magnetic resistivity, we found that the onset temperature of the Kondo effect is much higher than the known onset temperature of Kondo scattering (≃200 K) of CeCoIn5. Based on momentum-dependent hybridization, the periodic Anderson model, and a maximum entropy approach, we obtained the hybridization gap distribution function of CeCoIn5 and found that the resulting gap distribution function of CeCoIn5 was mainly composed of two (small and large) components (or gaps). We assigned the small and large gaps to the in-plane and out-of-plane hybridization gaps, respectively. We expect that our results will provide useful information for understanding the temperature-dependent electronic evolution of f-electron systems near Fermi level.

Fermi-surface fluctuations and lattice instabilities in the two-dimensional metallic kagome superconductor CsV_{3}Sb_{5} are elucidated via polarization-resolved Raman spectroscopy. The presence of a weak electronic continuum in high-quality samples marks the crossover into the charge-density-wave (CDW) ordered phase, while impurity-rich samples promote strong defect-induced electronic scattering processes that affect the coherence of the CDW phase. CDW-induced phonon anomalies appear below T_{CDW}, with emergent C2 symmetry for one of the CDW amplitude modes, alluding to nematicity. In conjunction with symmetry-breaking lattice distortions, a kinklike hardening of the A_{1g} phonon energy at T_{CDW} signifies a concerted interplay of electronic correlations and electron-phonon coupling in the exotic CDW order.

Exploring the magnetism in the van der Waals materials facilitates two dimensional spintronic devices. Here the authors demonstrate the evolution of magnetic behavior, strong perpendicular magnetic anisotropy and existence of magnetic coupling between atomic layers in Fe3GeTe2 nanoflakes by varying the layer thickness.

Knowledge of critical current may provide important information to understand unconventional superconductivity and quantum critical behavior. Here, Jung et al. observe a peak in the pressure dependence of the zero-field critical current at a hidden quantum critical point in pure and Sn-doped heavy fermion superconductor CeRhIn5.

The transition metal dichalcogenide TiSe_{2} is an ideal correlated system for studying the interplay between superconductivity (SC) and a charge density wave (CDW) because both symmetry-breaking phases can be easily controlled by either Cu intercalation or physical pressure. SC appears in proximity to a CDW quantum critical point (QCP) induced by both Cu intercalation and applied pressure, raising the possibility of CDW-driven SC. Here, we report tuning the CDW QCP by simultaneously controlling Cu intercalation and external pressure and the appearance of a SC dome centered on the tunable QCP. When subjected to pressure, CDW ordering of Cu-intercalated Cu_{0.025}TiSe_{2} is completely suppressed at 2.3 GPa, where the residual resistivity and the resistivity-temperature exponent decrease sharply, indicating the presence of the CDW QCP. The upper critical field of Cu_{0.025}TiSe_{2} is 3.51 kOe, 16 times larger than that of pristine TiSe_{2}, and its temperature dependence is linear, indicating that SC of TiSe_{2} is switched from the two-dimensional- to anisotropic three-dimensional-like by Cu intercalation. These discoveries show that the simultaneous application of Cu intercalation and pressure move the CDW QCP and that the highest SC transition temperature is pinned to the QCP, suggesting that the SC in TiSe_{2} is strongly correlated with CDW quantum criticality.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

The influence of proton irradiation on the fluctuation-induced magnetoconductivity of high quality FeSe _1− _x Te _x ( x = 0.4, 0.55) (FST) thin films has been investigated. The measurements were performed with magnetic fields up to 13 T applied in the two main crystal directions. The results were interpreted in terms of the Ginzburg–Landau approach for three-dimensional materials under a total-energy cutoff. The analysis shows that properly-tuned proton irradiation does not appreciably affect fundamental superconducting parameters like the T _c value, the upper critical fields or the anisotropy. This has important consequences from the point of view of possible applications due to the enhancement of vortex pinning induced by irradiation.

We report the upper critical field (Hc2) and its anisotropy in the pressure-induced superconductor CrAs. At ambient pressure, CrAs shows an antiferromagnetic phase transition at TN ∼ 264 K, where magnetostriction occurs simultaneously. TN is rapidly suppressed with increasing pressure and bulk superconductivity is induced near 7 kbar, above which the signature associated with TN is not visible in electrical resistivity measurements. With further increasing pressure, the superconducting phase shows a broad dome shape centered at the optimal pressure of 12.2 kbar. The Hc2 anisotropy of CrAs at 12.2 kbar, γHc2 = H//a/H⊥a, increases with decreasing temperature and becomes saturated at lower temperatures. Taken together with the positive curvature in Hc2 near Tc, these results suggest that the pressure-induced superconductor CrAs possesses multiple superconducting gaps. [ABSTRACT FROM AUTHOR]

The ac magnetic susceptibility measurements on Li-doped [La.sub.2]Cu[O.sub.4] have shown the frequency-dependent behavior to be below a spin-glass temperature ([T.sub.SG]). [T.sub.SG] obtained from magnetic susceptibility is higher than the charge-glass temperature [T.sub.CG] obtained from dielectric constant measurements, which has shown that spin freezes first and drives charge freezing at a lower temperature.

We report large improvement of the critical current density (Jc) of 400-nm-thick MgB2 thin films through carbon (C) ion irradiation. When 350 keV C ions with various dose levels ranging from 1 × 1013–1 × 1015 C atoms/cm² are irradiated into the films, the c-axis lattice constant gradually expands with increase in the number of irradiated C ions and superconducting transition temperature (Tc) of the MgB2 films is suppressed. On the other hand, the field performance of Jc for the MgB2 films remarkably improves after irradiations, which is accompanied by the crossover of the pinning mechanism from weak collective to strong pinning. In addition, scaling behavior of flux pinning force for irradiated MgB2 films shows a dominance of normal point pinning, indicating that the improved field performance of Jc mainly originates from the point defects, such as vacancies and interstitials, created by C-ion irradiation. Our results suggest that carbon acts as an effective irradiating source as well as a doping source to improve Jc in high magnetic fields. [ABSTRACT FROM AUTHOR]

Two-dimensional van der Waals materials have demonstrated fascinating optical and electrical characteristics. However, reports on magnetic properties and spintronic applications of van der Waals materials are scarce by comparison. Here, we report anomalous Hall effect measurements on single crystalline metallic Fe3GeTe2. nanoflakes with different thicknesses. These nanoflakes exhibit a single hard magnetic phase with a near square-shaped magnetic loop, large coercivity (up to 550 mT at 2 K), a Curie temperature near 200 K and strong perpendicular magnetic anisotropy. Using criticality analysis, the coupling length between van der Waals atomic layers in Fe3GeTe2 is estimated to be ~5 van der Waals layers. Furthermore, the hard magnetic behaviour of Fe3GeTe2. can be well described by a proposed model. The magnetic properties of Fe3GeTe2 highlight its potential for integration into van der Waals magnetic heterostructures, paving the way for spintronic research and applications based on these devices. [ABSTRACT FROM AUTHOR]

We have studied the angle-dependent Hall effect in single-crystalline MgB2 thin films prior to and after Fe ion irradiation. The angle-dependent Hall scaling ρxy = Aρβxx shows a universal behavior, as scaling exponent β = 2 ± 0.2 holds even after various doses of irradiation. By applying the anisotropic mass scaling model, the magnetic field dependent resistivity curves measured under various angles were shown to potentially overlap on a main curve, except for the small angle region (called failed angles). Moreover, it was found that the failed angles decreased with increasing irradiation doses. This behavior could be well interpreted within the frame of theories on the Hall effect and vortex dynamics. [ABSTRACT FROM AUTHOR]

We investigate the influence of carbon-ion irradiation on the superconducting (SC) critical properties of MgB2 thin films. MgB2 films of two thicknesses, 400 nm (MB400nm) and 800 nm (MB800nm), were irradiated by 350 keV C ions having a wide range of fluence, 1 × 1013–1 × 1015 C atoms cm−2. The mean projected range (Rp) of 350 keV C ions in MgB2 is 560 nm, thus the energetic C ions will pass through the MB400nm, whereas the ions will remain into the MB800nm. The SC transition temperature (Tc), upper critical field (Hc2), c-axis lattice parameter, and corrected residual resistivity (ρcorr) of both the films showed similar trends with the variation of fluence. However, a disparate behavior in the SC phase transition was observed in the MB800nm when the fluence was larger than 1 × 1014 C atoms cm−2 because of the different Tcs between the irradiated and non-irradiated parts of the film. Interestingly, the SC critical properties, such as Tc, Hc2, and critical current density (Jc), of the irradiated MgB2 films, as well as the lattice parameter, were almost restored to those in the pristine state after a thermal annealing procedure. These results demonstrate that the atomic lattice distortion induced by C-ion irradiation is the main reason for the change in the SC properties of MgB2 films. [ABSTRACT FROM AUTHOR]

We report the fabrication and physical characteristics of niobium ion (Nb5+)-doped double-crossover DNA (DX-DNA) and salmon DNA (SDNA) thin films. Different concentrations of Nb5+ ([Nb5+]) are coordinated into the DNA molecules, and the thin films are fabricated via substrate-assisted growth (DX-DNA) and drop-casting (SDNA) on oxygen plasma treated substrates. We conducted atomic force microscopy to estimate the optimum concentration of Nb5+ ([Nb5+]O = 0.08 mM) in Nb5+-doped DX-DNA thin films, up to which the DX-DNA lattices maintain their structures without deformation. X-ray photoelectron spectroscopy (XPS) was performed to probe the chemical nature of the intercalated Nb5+ in the SDNA thin films. The change in peak intensities and the shift in binding energy were witnessed in XPS spectra to explicate the binding and charge transfer mechanisms between Nb5+ and SDNA molecules. UV-visible, Raman, and photoluminescence (PL) spectra were measured to determine the optical properties and thus investigate the binding modes, Nb5+ coordination sites in Nb5+-doped SDNA thin films, and energy transfer mechanisms, respectively. As [Nb5+] increases, the absorbance peak intensities monotonically increase until ∼[Nb5+]O and then decrease. However, from the Raman measurements, the peak intensities gradually decrease with an increase in [Nb5+] to reveal the binding mechanism and binding sites of metal ions in the SDNA molecules. From the PL, we observe the emission intensities to reduce them at up to ∼[Nb5+]O and then increase after that, expecting the energy transfer between the Nb5+ and SDNA molecules. The current–voltage measurement shows a significant increase in the current observed as [Nb5+] increases in the SDNA thin films when compared to that of pristine SDNA thin films. Finally, we investigate the temperature dependent magnetization in which the Nb5+-doped SDNA thin films reveal weak ferromagnetism due to the existence of tiny magnetic dipoles in the Nb5+-doped SDNA complex. [ABSTRACT FROM AUTHOR]

We investigate the effects of magnetic impurities on the upper critical field (μ0Hc2) in La-doped CaFe2As2 (LaCa122) single crystals. The magnetic field dependency of the superconducting transition temperature (Tc) for LaCa122 is rapidly suppressed at low fields up to ∼1 kOe despite its large μ0Hc2(0) value on the order of tens of Tesla, resulting in a large positive curvature of μ0Hc2(T) near Tc. The magnetization hysteresis (M–H) loop at temperatures above Tc shows a ferromagnetic-like signal and the M(H) value rapidly increases with increasing magnetic field up to ∼1 kOe. Taken together with the linear suppression of Tc with the magnetization in the normal state, these results suggest that the large upward curvature of μ0Hc2(T) near Tc in La-doped CaFe2As2 mainly originates from the suppression of superconductivity due to the presence of magnetic impurities. [ABSTRACT FROM AUTHOR]

We investigate the pressure dependence of the upper critical fields (μ0Hc2) for FeSe single crystals with pressure up to 2.57 GPa. The superconducting (SC) properties show a disparate behavior across a critical pressure where the pressure-induced antiferromagnetic phase coexists with superconductivity. The magnetoresistance for H//ab and H//c is very different: for H//c, magnetic field induces and enhances a hump in the resistivity close to the Tc for pressures higher than 1.2 GPa, while it is absent for H//ab. Since the measured μ0Hc2 for FeSe samples is smaller than the orbital limited upper critical field estimated by the Werthamer, Helfand and Hohenberg model, the Maki parameter (α) related to Pauli spin-paramagnetic effects is additionally considered to describe the temperature dependence of μ0Hc2(T). Interestingly, the α value is hardly affected by pressure for H//ab, while it strongly increases with pressure for H//c. The pressure evolution of the μ0Hc2(0) for the FeSe single crystals is found to be almost similar to that of Tc(P), suggesting that the pressure-induced magnetic order adversely affects the upper critical fields as well as the SC transition temperature. [ABSTRACT FROM AUTHOR]

We studied NbCl5 and ZnMg alloy intercalated graphite compounds using an optical spectroscopy technique. These intercalated metallic graphite samples were quite challenging to obtain optical reflectance spectra since they were not flat and quite thin. By using both a new method and an in situ gold evaporation technique we were able to obtain reliable reflectance spectra of our samples in the far and mid infrared range (80–7000 cm−1). We extracted the optical constants including the optical conductivity and the dielectric function from the measured reflectance spectra using a Kramers–Kronig analysis. We also extracted the dc conductivity and the plasma frequencies from the optical conductivity and dielectric functions. NbCl5 intercalated graphite samples show similar optical conductivity spectra as bare highly oriented pyrolytic graphite even though there are some differences in detail. ZnMg intercalated samples show significantly different optical conductivity spectra from the bare graphite. Optical spectroscopy is one of the most reliable experimental techniques to obtain the electronic band structures of materials. The obtained optical conductivities support the recent theoretically calculated electronic band structures of NbCl5 and ZnMg intercalated graphite compounds. Our results also provide important information of electronic structures and charge carrier properties of these two new intercalated materials for applications. [ABSTRACT FROM AUTHOR]

We investigated the electrical resistivity and heat capacity of 1% Cd-doped CeIrInc under hydrostatic pressure up to 2.7 GPa, near where long-range antiferromagnetic order is suppressed and bulk superconductivity suddenly reemerges. The pressure-induced Tc is close to that of pristine CeIrIn5 at 2.7 GPa, and no signatures of a quantum critical point under pressure support a local origin of the antiferromagnetic moments in Cd-Celrln5 at ambient pressure. Similarities between superconductors Celrln5 and CeCoIns in response to Cd substitutions suggest a common magnetic mechanism. [ABSTRACT FROM AUTHOR]