Liangzi Deng

This study investigates the effects of 60 keV proton irradiation on BaTiO3-doped YBa2Cu3O7−δ (YBCO) films using masks with micron-scale holes to create controlled defect patterns aimed at enhancing superconducting properties. Contrary to expectations, masked irradiation resulted in a reduction in the critical current density (Jc), while unmasked irradiation demonstrated improvement, consistent with previous studies. Notably, no improvement was observed at 2 T around liquid nitrogen temperature. These observations highlight the challenges of employing micron-scale masks in defect engineering and underscore the need for further refinement to achieve the desired performance enhancement. Insights from this study contribute to advancing defect engineering techniques for improving YBCO’s performance in high-field applications, including fusion energy systems.

This study investigates the effects of 60 keV proton irradiation on BaTiO3-doped YBa2Cu3O7−δ (YBCO) films using masks with micron-scale holes to create controlled defect patterns aimed at enhancing superconducting properties. Contrary to expectations, masked irradiation resulted in a reduction in the critical current density (Jc), while unmasked irradiation demonstrated improvement, consistent with previous studies. Notably, no improvement was observed at 2 T around liquid nitrogen temperature. These observations highlight the challenges of employing micron-scale masks in defect engineering and underscore the need for further refinement to achieve the desired performance enhancement. Insights from this study contribute to advancing defect engineering techniques for improving YBCO’s performance in high-field applications, including fusion energy systems.

In light of breakthroughs in superconductivity under high pressure, and considering that record critical temperatures (T _c s) across various systems have been achieved under high pressure, the primary challenge for higher T _c should no longer solely be to increase T _c under extreme conditions but also to reduce, or ideally eliminate, the need for applied pressure in retaining pressure-induced or -enhanced superconductivity. The topological semiconductor Bi _0.5 Sb _1.5 Te ₃ (BST) was chosen to demonstrate our approach to addressing this challenge and exploring its intriguing physics. Under pressures up to ~50 GPa, three superconducting phases (BST-I, -II, and -III) were observed. A superconducting phase in BST-I appears at ~4 GPa, without a structural transition, suggesting the possible topological nature of this phase. Using the pressure-quench protocol (PQP) recently developed by us, we successfully retained this pressure-induced phase at ambient pressure and revealed the bulk nature of the state. Significantly, this demonstrates recovery of a pressure-quenched sample from a diamond anvil cell at room temperature with the pressure-induced phase retained at ambient pressure. Other superconducting phases were retained in BST-II and -III at ambient pressure and subjected to thermal and temporal stability testing. Superconductivity was also found in BST with T _c up to 10.2 K, the record for this compound series. While PQP maintains superconducting phases in BST at ambient pressure, both depressurization and PQP enhance its T _c , possibly due to microstructures formed during these processes, offering an added avenue to raise T _c . These findings are supported by our density-functional theory calculations.

A silicon oxycarbide (SiOC) ceramic composite produced via a high-temperature spark plasma sintering process exhibits excellent mechanical robustness, high electrical conductivity, and low thermal conductivity, useful for various applications.

Iron oxide nanoparticles (IONPs) are widely used for biomedical applications due to their unique magnetic properties and biocompatibility. However, the controlled synthesis of IONPs with tunable particle sizes and crystallite/grain sizes to achieve desired magnetic functionalities across single‐domain and multi‐domain size ranges remains an important challenge. Here, a facile synthetic method is used to produce iron oxide nanospheres (IONSs) with controllable size and crystallinity for magnetic tunability. First, highly crystalline Fe₃O₄ IONSs (crystallite sizes above 24 nm) having an average diameter of 50 to 400 nm are synthesized with enhanced ferrimagnetic properties. The magnetic properties of these highly crystalline IONSs are comparable to those of their nanocube counterparts, which typically possess superior magnetic properties. Second, the crystallite size can be widely tuned from 37 to 10 nm while maintaining the overall particle diameter, thereby allowing precise manipulation from the ferrimagnetic to the superparamagnetic state. In addition, demonstrations of reaction scale‐up and the proposed growth mechanism of the IONSs are presented. This study highlights the pivotal role of crystal size in controlling the magnetic properties of IONSs and offers a viable means to produce IONSs with magnetic properties desirable for wider applications in sensors, electronics, energy, environmental remediation, and biomedicine.

We have studied LK-99 [Pb_10−x Cu _x (PO₄)₆O], alleged by Lee et al to exhibit superconductivity above room temperature and at ambient pressure, and have reproduced all anomalies in electric and magnetic measurements that they reported as evidence for the claim of LK-99 being an ambient-pressure, room-temperature superconductor. We found that these anomalies are associated with the structural transition of the Cu₂S impurity in their sample and not with superconductivity.

Studies of vacancy-mediated anomalous transport properties have flourished in diverse fields since these properties endow solid materials with fascinating photoelectric, ferroelectric, and spin-electric behaviors. Although phononic and electronic transport underpin the physical origin of thermoelectrics, vacancy has only played a stereotyped role as a scattering center. Here we reveal the multifunctionality of vacancy in tailoring the transport properties of an emerging thermoelectric material, defective n-type ZrNiBi. The phonon kinetic process is mediated in both propagating velocity and relaxation time: vacancy-induced local soft bonds lower the phonon velocity while acoustic-optical phonon coupling, anisotropic vibrations, and point-defect scattering induced by vacancy shorten the relaxation time. Consequently, defective ZrNiBi exhibits the lowest lattice thermal conductivity among the half-Heusler family. In addition, a vacancy-induced flat band features prominently in its electronic band structure, which is not only desirable for electron-sufficient thermoelectric materials but also interesting for driving other novel physical phenomena. Finally, better thermoelectric performance is established in a ZrNiBi-based compound. Our findings not only demonstrate a promising thermoelectric material but also promote the fascinating vacancy-mediated anomalous transport properties for multidisciplinary explorations.

We report muon spin rotation (µSR) experiments on the microscopic properties of superconductivity and magnetism in the kagome superconductor CeRu₂ with T c ≃ 5  K. From the measurements of the temperature-dependent magnetic penetration depth λ, the superconducting order parameter exhibits nodeless pairing, which fits best to an anisotropic s-wave gap symmetry. We further show that the T c /λ ⁻² ratio is comparable to that of unconventional superconductors. Furthermore, the powerful combination of zero-field (ZF)-µSR and high-field µSR has been used to uncover magnetic responses across three characteristic temperatures, identified as T 1 ∗ ≃ 110  K, T 2 ∗ ≃ 65  K, and T 3 ∗ ≃ 40  K. Our experiments classify CeRu₂ as an exceedingly rare nodeless magnetic kagome superconductor.

We report muon spin rotation (µSR) experiments on the microscopic properties of superconductivity and magnetism in the kagome superconductor CeRu₂ with T c ≃ 5  K. From the measurements of the temperature-dependent magnetic penetration depth λ, the superconducting order parameter exhibits nodeless pairing, which fits best to an anisotropic s-wave gap symmetry. We further show that the T c /λ ⁻² ratio is comparable to that of unconventional superconductors. Furthermore, the powerful combination of zero-field (ZF)-µSR and high-field µSR has been used to uncover magnetic responses across three characteristic temperatures, identified as T 1 ∗ ≃ 110  K, T 2 ∗ ≃ 65  K, and T 3 ∗ ≃ 40  K. Our experiments classify CeRu₂ as an exceedingly rare nodeless magnetic kagome superconductor.

p‐type CaMg₂Sb₂ has not been favorably considered a promising thermoelectric material in comparison to other Zintl phases. However, a series of meticulously designed tactics is successfully implemented here to analyze and improve the thermoelectric performance of a CaMg₂Sb₂‐based material. Band structure calculations show that the pristine CaMg₂Sb₂ has a wide indirect band gap of ≈1.11 eV, which results in its less optimal carrier concentration. To improve its consequently inferior thermoelectric performance, Na doping is first deployed to increase the carrier concentration to an optimal level. Subsequently, the ideal bandwidth and enhanced carrier mobility are simultaneously obtained through Zn alloying. Finally, partial Yb substitution for Ca effectively restrains the bipolar effect in this CaMg₂Sb₂‐based material and augments its power factor while suppressing its lattice thermal conductivity to a minimum. As a result, a 20‐fold‐higher peak figure of merit (zT) of ≈0.85 at 773 K and an average zT value of ≈0.50 from 298 to 773 K are achieved in Ca_0.69Yb_0.3Na_0.01Mg_1.1Zn_0.9Sb₂, the highest for both values among all [Ca,Mg,Eu,Yb]Mg₂Sb₂‐based materials reported to date. Furthermore, such a combination of rationally designed tactics as proposed here may assist in exploring the improvement of other thermoelectric materials that have long been overlooked.

The rich phenomena in the FeSe and related compounds have attracted great interests as it provides fertile material to gain further insight into the mechanism of high temperature superconductivity. A natural follow-up work was to look into the possibility of superconductivity in MnSe. We demonstrated in this work that high pressure can effectively suppress the complex magnetic characters of MnSe, and induce superconductivity with T_c ~ 5 K at pressure ~12 GPa confirmed by both magnetic and resistive measurements. The highest T_c is ~ 9 K (magnetic result) at ~35 GPa. Our observations suggest the observed superconductivity may closely relate to the pressure-induced structural change. However, the interface between the metallic and insulating boundaries may also play an important role to the pressure induced superconductivity in MnSe.

To raise the superconducting-transition temperature (T _c ) has been the driving force for the long-sustained effort in superconductivity research. Recent progress in hydrides with T _c s up to 287 K under pressure of 267 GPa has heralded a new era of room temperature superconductivity (RTS) with immense technological promise. Indeed, RTS will lift the temperature barrier for the ubiquitous application of superconductivity. Unfortunately, formidable pressure is required to attain such high T _c s. The most effective relief to this impasse is to remove the pressure needed while retaining the pressure-induced T _c without pressure. Here, we show such a possibility in the pure and doped high-temperature superconductor (HTS) FeSe by retaining, at ambient pressure via pressure quenching (PQ), its T _c up to 37 K (quadrupling that of a pristine FeSe at ambient) and other pressure-induced phases. We have also observed that some phases remain stable without pressure at up to 300 K and for at least 7 d. The observations are in qualitative agreement with our ab initio simulations using the solid-state nudged elastic band (SSNEB) method. We strongly believe that the PQ technique developed here can be adapted to the RTS hydrides and other materials of value with minimal effort.

Skyrmion materials hold great promise for information technology due to the extremely low current needed to modify the spin configurations and the small size of magnetic domains. To facilitate their application, one great challenge is to break the magnetic field–temperature phase space restriction for the skyrmion state. We found that the temperature region for the skyrmion phase in bulk Cu ₂ OSeO ₃ can be greatly enhanced under physical pressure, making applications more practical by the use of strained heterostructures, for example. The observation of additional structures suggests that the skyrmion state may be insensitive to the underlying crystal structure. This work will stimulate research on finding skyrmion materials with different crystal structures and retaining the room-temperature skyrmion state at ambient condition.

Skyrmion materials hold great promise for information technology due to the extremely low current needed to modify the spin configurations and the small size of magnetic domains. To facilitate their application, one great challenge is to break the magnetic field–temperature phase space restriction for the skyrmion state. We found that the temperature region for the skyrmion phase in bulk Cu ₂ OSeO ₃ can be greatly enhanced under physical pressure, making applications more practical by the use of strained heterostructures, for example. The observation of additional structures suggests that the skyrmion state may be insensitive to the underlying crystal structure. This work will stimulate research on finding skyrmion materials with different crystal structures and retaining the room-temperature skyrmion state at ambient condition.

Bulk Cu transforms into Cu atoms that spontaneously self-intercalate into specific 2D layer TMDs under ambient conditions.

Kagome-nets, appearing in electronic, photonic and cold-atom systems, host frustrated fermionic and bosonic excitations. However, it is rare to find a system to study their fermion–boson many-body interplay. Here we use state-of-the-art scanning tunneling microscopy/spectroscopy to discover unusual electronic coupling to flat-band phonons in a layered kagome paramagnet, CoSn. We image the kagome structure with unprecedented atomic resolution and observe the striking bosonic mode interacting with dispersive kagome electrons near the Fermi surface. At this mode energy, the fermionic quasi-particle dispersion exhibits a pronounced renormalization, signaling a giant coupling to bosons. Through the self-energy analysis, first-principles calculation, and a lattice vibration model, we present evidence that this mode arises from the geometrically frustrated phonon flat-band, which is the lattice bosonic analog of the kagome electron flat-band. Our findings provide the first example of kagome bosonic mode (flat-band phonon) in electronic excitations and its strong interaction with fermionic degrees of freedom in kagome-net materials.

The discovery of ferromagnetism in atomically thin layers at room temperature widens the prospects of 2D materials for device applications. Recently, two independent experiments demonstrated magnetic ordering in two dissimilar 2D systems, CrI₃ and Cr₂Ge₂Te₆, at low temperatures and in VSe₂ at room temperature, but observation of intrinsic room‐temperature magnetism in 2D materials is still a challenge. Here a transition at room temperature that increases the magnetization in magnetite while thinning down the bulk material to a few atom‐thick sheets is reported. DC magnetization measurements prove ferrimagnetic ordering with increased magnetization and density functional theory calculations ascribe their origin to the low dimensionality of the magnetite layers. In addition, surface energy calculations for different cleavage planes in passivated magnetite crystal agree with the experimental observations of obtaining 2D sheets from non‐van der Waals crystals.

The efficiency of sunlight-driven reduction of carbon dioxide (CO₂), a process mimicking the photosynthesis in nature that integrates the light harvester and electrolysis cell to convert CO₂ into valuable chemicals, is greatly limited by the sluggish kinetics of oxygen evolution in pH-neutral conditions. Current non-noble metal oxide catalysts developed to drive oxygen evolution in alkaline solution have poor performance in neutral solutions. Here we report a highly active and stable oxygen evolution catalyst in neutral pH, Brownmillerite Sr₂GaCoO₅, with the specific activity about one order of magnitude higher than that of widely used iridium oxide catalyst. Using Sr₂GaCoO₅ to catalyze oxygen evolution, the integrated CO₂ reduction achieves the average solar-to-CO efficiency of 13.9% with no appreciable performance degradation in 19 h of operation. Our results not only set a record for the efficiency in sunlight-driven CO₂ reduction, but open new opportunities towards the realization of practical CO₂ reduction systems.

The efficiency of sunlight-driven reduction of carbon dioxide (CO₂), a process mimicking the photosynthesis in nature that integrates the light harvester and electrolysis cell to convert CO₂ into valuable chemicals, is greatly limited by the sluggish kinetics of oxygen evolution in pH-neutral conditions. Current non-noble metal oxide catalysts developed to drive oxygen evolution in alkaline solution have poor performance in neutral solutions. Here we report a highly active and stable oxygen evolution catalyst in neutral pH, Brownmillerite Sr₂GaCoO₅, with the specific activity about one order of magnitude higher than that of widely used iridium oxide catalyst. Using Sr₂GaCoO₅ to catalyze oxygen evolution, the integrated CO₂ reduction achieves the average solar-to-CO efficiency of 13.9% with no appreciable performance degradation in 19 h of operation. Our results not only set a record for the efficiency in sunlight-driven CO₂ reduction, but open new opportunities towards the realization of practical CO₂ reduction systems.

Undoped CaFe₂As₂ (Ca122) can be stabilized in two slightly different non-superconducting tetragonal phases, PI and PII, through thermal treatments. Upon proper annealing, superconductivity with a T_c up to 25 K emerges in the samples with an admixture of PI and PII phases. Systematic low-temperature X-ray diffraction studies were conducted on undoped Ca122 samples annealed at 350 °C over different time periods. In addition to the diffraction peaks associated with the single-phase aggregation of PI and PII, a broad intermediate peak that shifts with annealing time was observed in the superconducting samples only. Our simulation of phase distribution suggests that the extra peak is associated with the admixture of PI and PII on the nanometer scale. High-resolution transmission electron microscopy confirms the existence of these nano-scale phase admixtures in the superconducting samples. These experimental results and simulation analyses lend further support for our conclusion that interfacial inducement is the most reasonable explanation for the emergence of superconductivity in undoped Ca122 single crystals.

Undoped CaFe₂As₂ (Ca122) can be stabilized in two slightly different non-superconducting tetragonal phases, PI and PII, through thermal treatments. Upon proper annealing, superconductivity with a T_c up to 25 K emerges in the samples with an admixture of PI and PII phases. Systematic low-temperature X-ray diffraction studies were conducted on undoped Ca122 samples annealed at 350 °C over different time periods. In addition to the diffraction peaks associated with the single-phase aggregation of PI and PII, a broad intermediate peak that shifts with annealing time was observed in the superconducting samples only. Our simulation of phase distribution suggests that the extra peak is associated with the admixture of PI and PII on the nanometer scale. High-resolution transmission electron microscopy confirms the existence of these nano-scale phase admixtures in the superconducting samples. These experimental results and simulation analyses lend further support for our conclusion that interfacial inducement is the most reasonable explanation for the emergence of superconductivity in undoped Ca122 single crystals.

Achieving higher transition temperature (T _c ) is a primary goal in superconductivity research. T _c and doping have been found to have a dome-like universal relation where the peak position is the maximum T _c-max , which is consistent with previous experimental results in the lower pressure range. By using the ultrasensitive magnetization measurement technique under high pressure that we developed, we discovered a universal resurgence of T _c passing the peak predicted by the general T _c - p (doping) or -P (pressure) relation for cuprate high-temperature superconductor and attribute the resurgence to a pressure-induced electronic transition, which is supported qualitatively by our density functional theory calculations. This offers a paradigm to raise the T _c of the layered cuprate high-temperature superconductors to a new height.

Carbon doping can induce unique and interesting physical properties in hexagonal boron nitride (h‐BN). Typically, isolated carbon atoms are doped into h‐BN. Herein, however, the insertion of nanometer‐scale graphene quantum dots (GQDs) is demonstrated as whole units into h‐BN sheets to form h‐CBN. The h‐CBN is prepared by using GQDs as seed nucleations for the epitaxial growth of h‐BN along the edges of GQDs without the assistance of metal catalysts. The resulting h‐CBN sheets possess a uniform distrubution of GQDs in plane and a high porosity macroscopically. The h‐CBN tends to form in small triangular sheets which suggests an enhanced crystallinity compared to the h‐BN synthesized under the same conditions without GQDs. An enhanced ferromagnetism in the h‐CBN emerges due to the spin polarization and charge asymmetry resulting from the high density of CN and CB bonds at the boundary between the GQDs and the h‐BN domains. The saturation magnetic moment of h‐CBN reaches 0.033 emu g⁻¹ at 300 K, which is three times that of as‐prepared single carbon‐doped h‐BN.

The transformation from semiconducting to metallic phase, accompanied by a structural transition in 2D transition metal dichalcogenides has attracted the attention of the researchers worldwide. The unconventional structural transformation of fluorinated WS₂ (FWS₂) into the 1T phase is described. The energy difference between the two phases debugs this transition, as fluorination enhances the stability of 1T FWS₂ and makes it energetically favorable at higher F concentration. Investigation of the electronic and optical nature of FWS₂ is supplemented by possible band structures and bandgap calculations. Magnetic centers in the 1T phase appear in FWS₂ possibly due to the introduction of defect sites. A direct consequence of the phase transition and associated increase in interlayer spacing is a change in friction behavior. Friction force microscopy is used to determine this effect of functionalization accompanied phase transformation.

One of the major goals for scientists in the field of superconductivity and materials science has been to obtain superconductors with higher critical temperatures (T _c ). One way that has long been proposed to achieve enhanced T _c s is to take advantage of artificially or naturally assembled interfaces. The present work clearly demonstrates that high-T _c superconductivity in the well-known nonsuperconducting compound CaFe ₂ As ₂ can be induced by antiferromagnetic/metallic layer stacking and provides the most direct evidence to date for the interface-enhanced T _c in this compound. The observations offer an avenue to higher T _c.

Recently, the detection of non-bulk superconductivity with unexpectedly high onset-T_cs up to 49 K in Pr-doped CaFe₂As₂ [(Ca,Pr)122] single crystals and the report of a T_c up to 65 K in one-unit-cell (1UC) FeSe epi-films, offer an unusual opportunity to seek an answer to the question posed in the title. Through systematic compositional, structural, resistive, and magnetic investigations on (Ca,Pr)122 single crystals, we have observed a doping-level-independent T_c, the simultaneous appearance of superparamagnetism and superconductivity, large magnetic anisotropy, and the existence of mesoscopic-2D structures in these crystals, thus providing clear evidence consistent with the proposed interface-enhanced T_c in these naturally occurring rareearth-doped Fe-based superconductors, (Ca,R)122. Similar resistive and magnetic measurements were also made on the 3–4UC FeSe ultrathin epi-films. We have detected weak links in the Meissner state below 20 K, weakly coupled small superconducting patches between 20–45 K, and collective excitations of spin and/or superconducting nature between 45–80 K. The unusual frequency dependences of the diamagnetic moment observed in the films in different temperature ranges will be presented and their implications discussed.

We report the detection of unusual superconductivity up to 49 K in single crystalline CaFe ₂ As ₂ via electron-doping by partial replacement of Ca by rare-earth. The superconducting transition observed suggests the possible existence of two phases: one starting at 49 K, which has a low critical field < 4 Oe, and the other at 21 K, with a much higher critical field > 5 T. Our observations are in strong contrast to previous reports of doping or pressurizing layered compounds AeFe ₂ As ₂ (or Ae122), where Ae = Ca, Sr, or Ba. In Ae122, hole-doping has been previously observed to generate superconductivity with a transition temperature ( T _c ) only up to 38 K and pressurization has been reported to produce superconductivity with a T _c up to 30 K. The unusual 49 K phase detected will be discussed.