Pristine Crystal Research Articles

Accumulation of oxygen interstitial-vacancy pairs under irradiation of corundum single crystals with energetic xenon ions

Single crystals of α-Al2O3 with broad sides oriented perpendicular to the c crystal axis have been irradiated by 231-MeV xenon ions with fluence varying from 5×1011 to 1014 ions/cm2. The spectra of radiation-induced optical absorption (absorption of a pristine crystal is subtracted) have been decomposed into Gaussians serving as a measure of oxygen-related Frenkel defects (vacancy-interstitial pairs). The concentration of all structural defects considered – vacancy-type F and F+ centers as well as oxygen interstitials – continuously increases with ion fluence. Therefore, radiation-induced origin of elementary absorption bands at 5.6 and 6.6 eV tentatively ascribed earlier to charged and neutral oxygen interstitials has been proved for the first time. The concentrations of charged interstitials (in the form of superoxide ions) have been directly determined by the EPR method. The evolution of cathodoluminescence bands typical of self-trapped excitons (VUV band at 7.6 eV) and F-type defects (bands peaked around 3.0 and 3.8 eV) with the rise of Xe-ion-irradiation fluence has been measured and analyzed.

Highly luminescent, fast switching electro-optical device based on core-shell bimetallic nanoparticles/ ferroelectric liquid crystal composites.

Owing to the passive nature of liquid crystal (LC) materials, achieving luminous displays using pure LC materials is challenging. In addition, it is difficult to achieve a fast switching time using pristine ferroelectric LC devices without compromising their cell thickness. Herein, we have developed a fast switching and highly luminescent electro-optical device by dispersing a minute concentration of bimetallic nanoparticles (Au@Ag NPs) having a spherical gold core and a silver shell within a ferroelectric liquid crystal (FLC) host matrix, ZLI3654. Au@Ag core-shell NPs having synergic attributes of both counterparts were successfully synthesized by a facile seed-mediated route. The Au core helps to tune the shape of the Ag shell and provides enhanced electron density as well as improved stability against oxidation. Introducing nanoparticles induces little structural modifications to the host FLC, resulting in an improvement in the mesogenic alignment. Interestingly, ∼29-fold enhancement in the photoluminescence (PL) intensity is observed on dispersing 0.25 wt% of Au@Ag NPs into the FLC host matrix. The enhanced electromagnetic field in the FLC-nanocomposite is attributed to the Localized Surface Plasmon Resonance of Au@Ag NPs, which strengthens the photon absorption rates by the FLC molecules, culminating in the massive enrichment of the PL intensity. In addition, the improved localized electric field inside the FLC device led to a noticeable enhancement in the spontaneous polarization, dielectric permittivity, and, most interestingly, ∼53% fastening in the switching time at an optimum concentration (0.25 wt%) of Au@Ag NPs. The improved electro-optical parameters of the Au@Ag NPs/FLC composite have been compared with the performance of both pristine Au NPs/FLC and Ag NPs/FLC composites, respectively, for the comprehensiveness of the study. The present study paves a systematic way to develop FLC-based advanced electro-optical devices with faster switching and higher luminescence properties.

Investigation on crystallographic orientation dependent ion transportation in Li3YBr6 superionic conductor for lithium ion battery

Enhancing ionic conductivity in solid electrolytes poses a significant hurdle in advancing all-solid-state batteries. Researchers have explored a variety of materials and techniques to elevate ionic conductivity. Lithium halide-based materials are emerging as a promising option for solid electrolytes due to their capacity for rapid ion conduction and robust electrochemical stability. Various methods, including substitution strategies and defect chemistry, have been deployed to enhance ionic conductivity within lithium halide solid electrolytes. Ionic conductivity is intricately linked to the substructure of migrating ions; therefore, ionic conductivity can be significantly enhanced by the alteration of the substructure. This work focuses on the impact of crystallographic orientation on ionic conductivity. Halide superionic conductor of structure Li3YX6 (X = halogen) has been used for the study. First, the ion transport mechanism in Li3YBr6 (in the C2/c phase) crystal has been examined and sequentially the impact of the crystallographic orientation has been investigated by creating two surfaces of different crystal orientations of pristine Li3YBr6. Findings of the present work show that among two different crystal-oriented surfaces (010) and (100) the (010) crystal-oriented structure Li3YBr6 has remarkable room temperature ionic conductivity (11.8 mS/cm) which is nearly fourteen times higher than that of pristine Li3YBr6 crystal while (100) oriented structure shows a little increment in room temperature ionic conductivity. To examine the mechanism behind this exceptional ionic conductivity in different structures energy barriers along different migration pathways have been evaluated. For the determination of the energy barrier nudged elastic band simulation was carried out. The results demonstrate that the (010) crystal-oriented surface structure possesses the lowest energy barrier, at 0.16 eV, which is 0.10 eV lower than the (100) oriented structure and 0.19 eV less than pristine Li3YBr6 (C2/c). This reduced energy barrier in the (010) crystal-oriented structure can be attributed to the prevalence of the tetrahedron-to-tetrahedron migration pathway.

Determining thermal activation parameters for ferroelectric domain nucleation in BaTiO3 from molecular dynamics simulations

Polarization switching in ferroelectric hysteresis of BaTiO3 proceeds by localized nucleation and subsequent growth of domains of reverse polarization. While this process is driven by the applied electric field, thermal activation can play an important role in domain nucleation. As a consequence of the stochastic nature of thermal activation, coercive fields in small systems exhibit a significant scatter. It is demonstrated that the statistics of coercive fields observed in molecular dynamics simulations is consistent with the statistical distribution expected for field-assisted thermally activated nucleation of reverse polarization domains. The excellent quantitative agreement between the simulation data and the theory of thermally activated processes provides strong evidence that polarization switching occurs by nucleation-and-growth rather than loss of the local minimum of the Gibbs free energy function. In a pristine crystal, switching is controlled by the field at which thermal fluctuations can create a critical nucleus in the bulk (homogeneous nucleation). The analysis of crystals with various vacancy-type defects demonstrates that such defects may enable heterogeneous nucleation at reduced fields. In both cases, the statistical analysis gives access to the respective activation energies, their field dependence, and the sizes of the critical nuclei.

Initializing intragranular residual stresses within statistically equivalent microstructures for crystal plasticity simulations

We propose a mechanics framework to compute intragranular or type III residual stress within a polycrystalline aggregate for which experimental characterization of residual stress is unknown. The framework is built upon the inter-relationship between intragranular misorientation (IGM), geometrically necessary dislocation (GND), and long-range internal stress. Given a spatial distribution of IGM, we first calculate the spatial distribution of GND density; subsequently, we calculate the internal stress and strain fields due to the GND distribution. For demonstration, we consider a synthetic microstructure of a hexagonal close-packed material, namely Ti-7Al, wherein the grain size and orientation distributions are statistically equivalent to the distributions obtained via experimental characterization. We refer to this microstructure as a statistically equivalent microstructure (SEM). In an SEM, each grain is usually treated as a pristine crystal and therefore, the SEM contains no IGM. We propose a technique to introduce IGM within the SEM and, subsequently, calculate the GND density and residual stress distributions in a consistent manner. In the process, we ensure that statistical distributions of the IGM and residual strain within the SEM are statistically equivalent to the same obtained from the experimental characterization of Ti-7Al. While establishing statistically equivalent residual stress, the thermal effects during processing of Ti-7Al are implicitly taken into account. Next, we perform crystal plasticity finite element simulations to demonstrate the implications of the initialization of IGM and type III residual stress. Although the IGM and residual stress are related, the results suggest that the type III residual stress initialization has a much more pronounced effect on microplasticity-driven damage mechanisms, such as high cycle fatigue, even within a well-annealed material such as Ti-7Al, thereby emphasizing the need for its initialization for microstructure-sensitive fatigue analysis. Finally, we demonstrate that initializing thermally induced residual stress within Ti-7Al SEM through a cooling simulation, accounting for the anisotropic nature of the coefficient of thermal expansion, yields a grain-averaged residual stress distribution statistically similar to that obtained from our framework and is a special case of the type II residual stress implementation.

Studies on thermoelectric performance of pristine and Selenium alloyed Bismuth Sulfide crystals grown by vertical Bridgman technique

By using Vertical Bridgman Technique (VBT), pure and Selenium alloyed Bismuth Sulfide crystals were grown. Effect of Selenium concentration on structural, electrical and thermal transport properties with respect to pristine crystal is thoroughly studied. Grown crystals were characterized by Energy Dispersive analysis of X-rays (EDAX), Field Emission Scanning Electron Microscopy (FE-SEM) and Powder X-ray Diffraction (PXRD) to confirm stoichiometry, surface morphology and structure respectively. Hall effect measurement is done to determine carrier concentration and mobility. Electrical conductivity, Seebeck coefficient and thermal conductivity were measured from 303K to 773K to study electrical and thermal transport mechanism. Electrical conductivity was increased from 3.84 S/cm to 33.34 S/cm with increasing Se concentration while Seebeck coefficient showed an opposite trend. Power factor also showed enhancement from 1.43 μW/cmK2 to 2.70 μW/cmK2 with Se substitution. Pristine S1 (Bi1.97S3.19) showed figure of merit ZT = 0.23 at 683K while maximum ZT = 0.54 at 733K was obtained for S4 (Bi1.97S2.88Se0.3) crystal. This study indicated that appropriate concentration of Selenium is beneficial to improve thermoelectric performance of Bismuth Sulfide.

Remarkable Off-On Tunable Solid-State Luminescence by the Regulation of Pyrene Dimer.

It is always a challenge to achieve "off-on" luminescent switch by regulating non-covalent interactions. Herein, we report a unique strategy for constructing high performance "off-on" tunable luminescent materials utilizing a novel molecule (TFPA) consist of pyrene and cyanostilbene. The pristine crystal of TFPA is almost non-emissive. Upon grinding/UV irradiation, an obvious luminescence enhancement is observed. Theoretical and experimental results revealed the underlying mechanism of this intriguing "off-on" switching behavior. The non-emissive crystal consists of ordered H-aggregates, with adjacent two molecules stacked in an anti-parallel manner and no overlapped area in pyrene moieties. When external force is applied by grinding or internal force is introduced through the photoisomerization, the dimer structures are facilitated with shorter intermolecular distances and better overlapping of pyrene moieties. In addition, the "on" state can recover to "off" state under thermal annealing, showing good reversibility and applicability in intelligence material. The present results promote an in-depth insight between packing structure and photophysical property, and offer an effective strategy for the construction of luminescence "off-on" switching materials, toward the development of stimuli-responsive luminescent materials for anti-counterfeiting.

S^{±}-Wave Pairing and the Destructive Role of Apical-Oxygen Deficiencies in La_{3}Ni_{2}O_{7} under Pressure.

Recently, the bilayer perovskite nickelate La_{3}Ni_{2}O_{7} has been reported to show evidence of high-temperature superconductivity (SC) under a moderate pressure of about 14GPa. To investigate the superconducting mechanism, pairing symmetry, and the role of apical-oxygen deficiencies in this material, we perform a random-phase approximation based study on a bilayer model consisting of the d_{x^{2}-y^{2}} and d_{3z^{2}-r^{2}} orbitals of Ni atoms in both the pristine crystal and the crystal with apical-oxygen deficiencies. Our analysis reveals an s^{±}-wave pairing symmetry driven by spin fluctuations. The crucial role of pressure lies in that it induces the emergence of the γ pocket, which is involved in the strongest Fermi-surface nesting. We further found the emergence of local moments in the vicinity of apical-oxygen deficiencies, which significantly suppresses the T_{c}. Therefore, it is possible to significantly enhance the T_{c} by eliminating oxygen deficiencies during the synthesis of the samples.

Disorder-induced bulk photovoltaic effect in a centrosymmetric van der Waals material

Sunlight is widely seen as one of the most abundant forms of renewable energy, with photovoltaic cells based on pn junctions being the most commonly used platform attempting to harness it. Unlike in conventional photovoltaic cells, the bulk photovoltaic effect (BPVE) allows for the generation of photocurrent and photovoltage in a single material without the need to engineer a pn junction and create a built-in electric field, thus offering a solution that can potentially exceed the Shockley–Queisser efficiency limit. However, it requires a material with no inversion symmetry and is therefore absent in centrosymmetric materials. Here, we demonstrate that breaking the inversion symmetry by structural disorder can induce BPVE in ultrathin PtSe2, a centrosymmetric semiconducting van der Waals material. Homogenous illumination of defective PtSe2 by linearly and circularly polarized light results in a photoresponse termed as linear photogalvanic effect (LPGE) and circular photogalvanic effect (CPGE), which is mostly absent in the pristine crystal. First-principles calculations reveal that LPGE originates from Se vacancies that act as asymmetric scattering centers for the photo-generated electron-hole pairs. Our work emphasizes the importance of defects to induce photovoltaic functionality in centrosymmetric materials and shows how the range of materials suitable for light sensing and energy-harvesting applications can be extended.

Antimony Doping Enabled Photoluminescence Quantum Yield Enhancement in 0D Inorganic Bismuth Halide Crystals.

Owing to the exterior self-trapped excitons (STEs) with adjustable fluorescence beams, low-dimensional ns2-metal halides have recently received considerable attention in solid-state light-emitting applications. However, the photoluminescence (PL) mechanism in metal halides remains a major challenge in achieving high efficiency and controllable PL properties because the excited-state energy of ns2 conformational ions varies inhomogeneously with their coordination environments. Here, a novel zero-dimensional (0D) lead-free bismuth-based Rb3BiCl6·0.5H2O crystal was reported as a pristine crystal to modulate the optical properties. By doping Sb3+ ions with 5s2 electrons into Rb3BiCl6·0.5H2O crystals, bright orange emission at room temperature was obtained with a photoluminescence quantum yield of 39.7%. Optical characterizations and theoretical studies show that the Sb3+ doping can suppress the strong exciton-phonon coupling, optimize the electronic energy band structure, improve the thermal activation energy, soften the structural lattice of the host crystals, deepen the STE states, and ultimately lead to strong photoluminescence. This work manifests a fruitful manipulation in ripening bismuth-based halides with high-efficiency PL properties, and the PL enhancement mechanisms will guide future research in the exploration of emerging luminescent materials.

Chemical Bonding Tuned Lattice Anharmonicity Leads to a High Thermoelectric Performance in Cubic AgSnSbTe3

AbstractComprehension of chemical bonding and its intertwined relation with charge carriers and heat propagation through a crystal lattice is imperative to design compounds for thermoelectric energy conversion. Here, we report the synthesis of large single crystal of new p‐type cubic AgSnSbTe3 which shows an innately ultra‐low lattice thermal conductivity (κlat) of 0.47–0.27 Wm−1 K−1 and a high electrical conductivity (1238 – 800 S cm−1) in the temperature range 294–723 K. We investigated the origin of the low κlat by analysing the nature of the chemical bonding and its crystal structure. The interaction between Sn(5 s)/Ag(4d) and Te(5p) orbitals was found to generate antibonding states just below the Fermi level in the electronic band structure, resulting in a softening of the lattice in AgSnSbTe3. Furthermore, the compound exhibits metavalent bonding which provides highly polarizable bonds with a strong lattice anharmonicity while maintaining the superior electrical conductivity. The electronic band structure exhibits nearly degenerate valence‐band maxima that help to achieve a high Seebeck coefficient throughout the measured temperature range and, as a result, the maximum thermoelectric figure of merit reaches to ≈1.2 at 661 K in pristine single crystal of AgSnSbTe3.

Chemical Bonding Tuned Lattice Anharmonicity Leads to a High Thermoelectric Performance in Cubic AgSnSbTe3.

Comprehension of chemical bonding and its intertwined relation with charge carriers and heat propagation through a crystal lattice is imperative to design compounds for thermoelectric energy conversion. Here, we report the synthesis of large single crystal of new p-type cubic AgSnSbTe3 which shows an innately ultra-low lattice thermal conductivity (κlat ) of 0.47-0.27 Wm-1 K-1 and a high electrical conductivity (1238 - 800 S cm-1 ) in the temperature range 294-723 K. We investigated the origin of the low κlat by analysing the nature of the chemical bonding and its crystal structure. The interaction between Sn(5 s)/Ag(4d) and Te(5p) orbitals was found to generate antibonding states just below the Fermi level in the electronic band structure, resulting in a softening of the lattice in AgSnSbTe3 . Furthermore, the compound exhibits metavalent bonding which provides highly polarizable bonds with a strong lattice anharmonicity while maintaining the superior electrical conductivity. The electronic band structure exhibits nearly degenerate valence-band maxima that help to achieve a high Seebeck coefficient throughout the measured temperature range and, as a result, the maximum thermoelectric figure of merit reaches to ≈1.2 at 661 K in pristine single crystal of AgSnSbTe3 .

Cylindrical void growth vs. grain fragmentation in FCC single crystals: CPFEM study for two types of loading conditions

The crystal plasticity finite element method (CPFEM) is used to investigate the coupling between the cylindrical void growth or collapse and grain refinement in face-centred cubic (FCC) single crystals. A 2D plane strain model with one void is used. The effect of the initial lattice orientation, similarities, and differences between stress- and strain-driven loading scenarios are explored. To this end, boundary conditions are enforced in two different ways. The first one is based on maintaining constant in-plane stress biaxiality via a dedicated truss element, while the second one is imposing a constant displacement biaxiality factor. Uniaxial and biaxial loading cases are studied. For the uniaxial loading case a special configuration, which enforces an equivalent pattern of plastic deformation in the pristine crystal, is selected in order to investigate the mutual interactions between the evolving void and the developed lattice rotation heterogeneity. Next, biaxial loading cases are considered for three crystal orientations, one of which is not symmetric with respect to loading directions. It is analysed how stress or strain biaxility factors and initial lattice orientation influence the void evolution in terms of its size and shape. Moreover, the consequences of variations in the resulting heterogeneity of lattice rotation are studied in the context of the grain refinement phenomenon accompanying the void evolution. Scenarios that may lead to more advanced grain fragmentation are identified.

Flexible organic-intercalated WSe2 hybrid: electronic structure modification and thermoelectric performance optimization

Two-dimensional tungsten selenide (WSe2) has great potential in thermoelectric (TE) applications, but dimensionality reduction results in a dramatic increase in thermal conductivity, which is not beneficial for improving thermoelectric performance. In this work, an electrochemical intercalation method is utilized to fabricate an organic-inorganic hybrid superlattice structure by decoupling the WSe2 layers without damaging the original bulk structure and then intercalating tetrabutylammonium (TBA) cations into the inorganic layers. The few-layer mixed-staging TBA-WSe2 hybrid with alternating stacking organic TBA cations and WSe2 inorganic layers possesses an electronic structure similar to that of the few-layer WSe2, which is beneficial for an enhanced Seebeck coefficient. The electron injection results in a considerable increase in the carrier concentration while the disorder in the mixed-staging structure succeeds in improving phonon scattering, leading to an enhanced figure-of-merit ZT of 0.25 at 340 K, which is three orders of magnitude higher than that of the pristine WSe2 single crystal and one of the highest ZT values among WSe2-based thermoelectric materials in recent years. This work may guide future research on dimensionality reduction in the electronic structures of bulk materials and the fabrication of flexible superlattice hybrids with considerable TE performance.

In-doping induced resonant level and thermoelectric performance enhancement in n-type GeBi2Te4 single crystals with intrinsically low lattice thermal conductivity

Resonant level, the impurity level located inside the valence/conduction band, is an effective strategy for decoupling the electrical conductivity and Seebeck coefficient and enhancing the thermoelectric power factor. In this work, we show that In doping induces resonant level and enhances the thermoelectric performance of GeBi2Te4, a promising n-type thermoelectric material with layered structure. Single crystals grown by Bridgman method were used in this study to take the advantage of its anisotropic transport properties. An ultralow lattice thermal conductivity of ∼0.51 W m−1 K−1 at 323 K was found along the c-axis of GeBi2Te4, driven by intrinsic cation disorder and the low phonon group velocity from the relatively weak chemical bonds as well as the strong lattice anharmonicity caused by hierarchical bond orders. Both enhanced electrical conductivity and Seebeck coefficient were observed in the In-doped samples, indicating that In-doping not only increases the carrier concentration, but also introduces the resonant level that was further confirmed by the first-principles calculation. A maximum zT of ∼0.42 at 523 K and an average zT of 0.36 over 323 to 573 K were achieved in Ge0.97In0.03Bi2Te4 crystal sample, ∼100% and ∼89% enhancement compared to those of pristine single crystal sample, respectively. This study demonstrates another example of resonant level for thermoelectric performance enhancement, which may be applied to other related materials.

Ultralong afterglow and unity quantum yield from a transparent CsCdCl3:Mn crystal

AbstractTransparent afterglow crystals are keenly desired for three‐dimensional information storage. Herein, CsCdCl3 perovskite crystals were grown by a programmable cooling procedure in a hydrothermal reactor. The pristine crystal showed an abnormal optical behavior where the absorption increased by 2.3 folds at high temperature, leading to a fourfold boost of photoluminescence (PL) intensity. After Mn2+ doping, the PL quantum yield was improved to nearly unity. Importantly, the doped crystals exhibited an ultralong afterglow up to 12 h after ceasing UV excitation and a high transmittance up to 75% in the visible region. This work brought a new member to the library of transparent afterglow crystal, opening up many possibilities to advanced applications such as volumetric display and three‐dimensional information encryption.

Bridgman Growth of Mg2Sn1–xBixSingle Crystals and Anisotropic Thermoelectric Properties

Mg2X-based (X = Si, Ge, Sn) phases are promising for thermoelectric applications due to their nontoxicity, low cost, and high performance. Substantial experimental work has already been carried out based on the polycrystalline samples; however, studies on related single-crystal growth and corresponding properties are still very limited owing to the high volatility and reactivity of Mg, which makes it very challenging to obtain high-quality single crystals with big sizes. In this report, for the first time, the Bridgman method was successfully applied in the single-crystal growth of the Bi-doped Mg2Sn materials. The crystals are of high quality and related anisotropic thermoelectric properties were systematically investigated. The Bi-doped Mg2Sn0.99Bi0.01 single crystal exhibited a maximum zT value of 0.58 along the [111] orientation at 623 K, which is about 30 times that of the pristine Mg2Sn single crystal.

Intrinsically Low Lattice Thermal Conductivity and Anisotropic Thermoelectric Performance in In‐doped GeSb2Te4 Single Crystals

AbstractLayer‐structured GeSb2Te4 is a promising thermoelectric candidate, while its anisotropy of thermal and electrical transport properties is still not clear. In this study, Ge1–xInxSb2Te4 single crystals are grown by Bridgman method, and their anisotropic thermoelectric properties are systematically investigated. Lower electrical conductivity and higher Seebeck coefficient are observed in the c‐axis due to the higher effective mass in this direction. Intrinsically low lattice thermal conductivity is also observed in the c‐axis due to the weak chemical bonding and the strong lattice anharmonicity proved by density functional theory calculation. Indium doping introduces an impurity band in the bandgap of GeSb2Te4 and leads to the locally distorted density of states near the Fermi level, which contributes to enhanced Seebeck coefficient and improved power factor. Ultimately, a peak zT value of 1 at 673 K and an average zT value of 0.68 within 323–773 K are obtained in Ge0.93In0.07Sb2Te4 along the c‐axis direction, which are 54% and 79% higher than that of the pristine GeSb2Te4 single crystal, respectively. This study clarified the origin of intrinsic low lattice thermal conductivity and anisotropy transport properties in GeSb2Te4, and shed light on the performance optimization of other layered thermoelectric materials.

Influence of Cu induced crystallographic disorder on the optical and lattice vibrational properties of ZnO nanoparticles.

In this study, the vibrational and optical responses of 0-10% excess Cu incorporated ZnO nanoparticles (NPs) prepared by the low temperature (∼400 °C) wet chemical route were investigated experimentally and were found to be predominantly linked to the formation of both intrinsic and Cu induced crystallographic defects instead of substitutional Cu itself for the first time. For low temperature chemical synthesis, the effective band gap (Eg) of pristine ZnO NPs was found to be as low as 2.84 eV, which was followed by a further reduction by as high as ∼24.4% with gradual Cu inclusion. Excess Cu incorporation was found to drastically restrict the particle growth phenomenon by ∼61% and alter the shape morphology from anisotropic to irregular isotropic. Although all NPs showed a single phase structure with hexagonal P63mc symmetry, the X-ray peak profile analysis along with the characteristic E2(High) Raman peak and electron-phonon coupling strength obtained from the 2nd order Raman mode suggested the presence of appreciable crystallographic disorders pertaining to point defects like Cu induced Zn and O interstitials and vacancies. Optical analysis revealed a gradual increase in Urbach tailing (Eu) from 0.35 to 0.96 eV directly associated with both intrinsic and Cu induced disorders arising from successive Cu incorporation at low processing temperature. A correlation between Eg and Eu predicted a direct-type band-to-band transition energy of ∼3.2 eV for the pristine ZnO crystal, suggesting both intrinsic and excess Cu induced extrinsic disorders to be the primary source of optical modulation of the NPs for the low temperature processing condition.

Tip-induced superconductivity enhancement in single-crystalline PdSb by point-contact spectroscopy

We report our mechanical point-contact spectroscopy (MPCS) study on single-crystalline superconductor PdSb with ${T}_{c}\ensuremath{\sim}$ 1.32 K and ${\ensuremath{\mu}}_{0}{H}_{c2}(0)\ensuremath{\sim}$ 900 Oe, whose ${T}_{c}$ changes very slightly against hydrostatic pressure below 2.5 GPa. For MPCS with a soft Au tip, the Andreev reflection signals suggest the same superconducting temperature ${T}_{c}$ as in the pristine PdSb crystal and the conductance curves show a fully gapped behavior, yielding a superconducting gap value of ${\mathrm{\ensuremath{\Delta}}}_{0}=$ 0.23 meV with $2{\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{\mathrm{B}}{T}_{c}\ensuremath{\sim}$ 4.25 in the strong-coupling regime. However, for MPCS with a hard W tip, an enhanced superconducting ${T}_{c}$ can be observed with a concomitant increase in the superconducting gap $\mathrm{\ensuremath{\Delta}}$ in a linear relation of 0.109 meV/kelvin, whereas $2{\mathrm{\ensuremath{\Delta}}}_{0}/{k}_{\mathrm{B}}{T}_{c}$ decreases from 4.25 to 3.5 in a negative correlation with ${T}_{c}$. From first-principles calculations, we observe a Lifshitz transition related to Pd-$4{d}_{{z}^{2}}$ orbitals under uniaxial pressure, which is absent under hydrostatic pressure. Such Lifshitz transition results in an enhanced density of states near the Fermi level $n({E}_{F})$, which is in line with the enhanced ${T}_{c}$. In addition, a soft phonon mode at $H$ is hardened under uniaxial pressure, suggesting a reduced coupling strength in this case. Such local strain effect provides a natural explanation for our MPCS results on PdSb.