The ability to control and manipulate time-reversal (T) symmetry-breaking phases with near-zero net magnetization is a sought-after goal in spintronic devices. The recently discovered hexagonal altermagnet manganese telluride (α-MnTe) is a prime example. It has a compensated altermagnetic ground state where the magnetic moments are aligned in each layer and stacked antiparallel along the c axis, yet it exhibits a spontaneous anomalous Hall effect (AHE) that breaks the T-symmetry with a vanishingly small c-axis ferromagnetic (FM) moment. However, the presence of three 120^∘ separated in-plane magnetic domains presents a challenge in understanding the origin of the AHE and the effective control of the altermagnetic state. Here we use neutron scattering to show that symmetry breaking anisotropic strain, induced by compressive uniaxial pressure along the nearest-neighbor (NN) Mn-Mn bond directions, detwins α-MnTe into a single in-plane magnetic domain. This control over in-plane domains allows us to unambiguously establish that the in-plane moments are aligned along the NNN Mn-Mn bond direction, irrespective of the applied strain directions. Mounting the sample on a piezoelectric strain cell along both NN and NNN directions can drive the sample into a single-domain state that significantly sharpens the AHE hysteresis loop and extends the AHE to lower temperatures. Furthermore, tuning the uniaxial strain reverses the sign of the AHE near room temperature. Remarkably, this is achieved without altering the altermagnetic phase-transition temperature or substantially changing the small c-axis FM moment. Combined with our phenomenological model, we argue that these effects result from the modification of the electronic Berry curvature by a combination of both spin-orbit coupling and strain. (See the full abstract in the PDF.)

In light of breakthroughs in superconductivity under high pressure, and considering that record critical temperatures (T_cs) across various systems have been achieved under high pressure, the primary challenge for higher Tc 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_0.5Sb_1.5Te_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.

In light of breakthroughs in superconductivity under high pressure, and considering that record critical temperatures (Tcs) across various systems have been achieved under high pressure, the primary challenge for higher Tc should no longer solely be to increase Tc 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 Bi0.5Sb1.5Te3 (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 Tc 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 Tc, possibly due to microstructures formed during these processes, offering an added avenue to raise Tc. These findings are supported by our density-functional theory calculations.

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.

Vanadium is a nontoxic, earth-abundant metal promising for sustainable biomass upgrading. But vanadium catalysts remain uneconomical because of limited productivity. Redox cooperativity between vanadium centers has been proposed to boost catalytic activity, but this hypothesis remains challenging to study. We report discrete divanadium complexes of the binucleating ligand 1,8-naphthyridine-2,7-dicarboxylic acid (H2ndc), with the composition V2O2(OiPr)2(DMSO)2(ndc)•DMSO (1) and V2O2(OiPr)2(pyridine)2(ndc), and a tetravanadium complex [V2O3(DMF)2(ndc)]2O•DMF. XRD, modelling, and spin magnetometry substantiate two ferromagnetically coupled, alkoxide-bridged vanadium(IV) centers in 1. Complex 1 shows significantly greater activity in the aerobic cleavage of diols and a lignin model compound than its monometallic analogs. Mechanistic experiments including characterization of a substrate-bound catalytic intermediate, provide insight into cooperativity in vanadium redox catalysis. These results establish binucleating ligands as a strategy toward sustainable aerobic oxidations with earth-abundant metals.

We report discrete divanadium complexes of 1,8-naphthyridine-2,7-dicarboxylate, characterized by SCXRD, DFT modelling, and magnetometry. One complex shows significantly greater activity in the aerobic cleavage of diols and a lignin model compound than its monometallic analogs. Mechanistic experiments and a substrate-bound complex provide insight into cooperativity in vanadium redox catalysis.

Fine-tuning the superparamagnetic (SPM) properties of iron oxide nanoparticles (NPs) through precise control over size, shape, and assembly into superclusters is essential for advanced biomedical and electronic applications. We first analyzed the size-dependent magnetic properties of FeO@Fe3O4 core/shell NPs in both spherical and cubic shapes prepared via the thermal decomposition of iron(III) oleate. The detailed analyses of structure, composition, and crystallinity confirmed the presence of both FeO and Fe3O4 phases and the formation of the core/shell structure, with an increasing FeO/Fe3O4 phase ratio correlated with larger particle size. Overall, the SPM properties of these core/shell NPs were maintained, although saturation magnetization and varied with size, shape, and FeO/Fe3O4 ratio. Notably, iron oxide nanocubes exhibited enhanced saturation magnetization compared to their spherical counterparts. Next, we introduced a unique strategy to enhance and fine-tune the SPM properties of FeO@Fe3O4 NPs by assembling them into supercluster particles to promote interparticle interaction. By controlling the size and shape of the primary nanocrystals, we demonstrated the creation of SPM superclusters of consistent sizes, including the 150 and 240 nm superclusters reported here, which exhibit different SPM behaviors. Our research presents a synthetic strategy for optimizing the SPM properties of iron oxide NPs and their superclusters across a wide range of magnetically driven applications, especially useful for biomedical technologies.

The research on supercurrent diodes has surged rapidly due to their potential applications in electronic circuits at cryogenic temperatures. To unlock this functionality, it is essential to find supercurrent diodes that can work consistently at zero magnetic field and under ubiquitous stray fields generated in electronic circuits. However, a supercurrent diode with robust field tolerance is currently lacking. Here, we demonstrate a field-resilient supercurrent diode by incorporating a 2D multiferroic material into a Josephson junction, and observed a pronounced supercurrent diode effect at zero magnetic field. More importantly, the supercurrent rectification persists over a wide and bipolar magnetic field range beyond industrial standards for field tolerance. By theoretically modeling a multiferroic Josephson junction, we unveil that the interplay between spin-orbit coupling and multiferroicity underlies the unusual field resilience of the observed diode effect. This work introduces multiferroic Josephson junctions as a new field-resilient superconducting device for cryogenic electronics.

This article will first briefly review the impressive advancements made in high-temperature superconductivity (HTS) before the arrival of room-temperature superconductivity (RTS). Accompanying the advancements made in superconductivity science and technology over the last century, a solid experimental framework concerning the search, development, and even authentication of new discoveries has been established. All these can serve as valuable references in the infancy of RTS research. In this spirit, we will comment on the current status of rare-earth hydride RTS and present our preliminary negative results on Lu-N-H and LK-99, the two most studied materials in the search for RTS in the last few months, although several more reports of negation than affirmation have appeared.

Emergent phases often appear when the electronic kinetic energy is comparable to the Coulomb interactions. One approach to seek material systems as hosts of such emergent phases is to realize localization of electronic wavefunctions due to the geometric frustration inherent in the crystal structure, resulting in flat electronic bands. Recently, such efforts have found a wide range of exotic phases in the two-dimensional kagome lattice, including magnetic order, time-reversal symmetry breaking charge order, nematicity, and superconductivity. However, the interlayer coupling of the kagome layers disrupts the destructive interference needed to completely quench the kinetic energy. Here we demonstrate that an interwoven kagome network-a pyrochlore lattice-can host a three dimensional (3D) localization of electron wavefunctions. Meanwhile, the nonsymmorphic symmetry of the pyrochlore lattice guarantees all band crossings at the Brillouin zone X point to be 3D gapless Dirac points, which was predicted theoretically but never yet observed experimentally. Through a combination of angle-resolved photoemission spectroscopy, fundamental lattice model and density functional theory calculations, we investigate the novel electronic structure of a Laves phase superconductor with a pyrochlore sublattice, CeRu$_2$. We observe flat bands originating from both the Ce 4$f$ orbitals as well as from the 3D destructive interference of the Ru 4$d$ orbitals. We further observe the nonsymmorphic symmetry-protected 3D gapless Dirac cones at the X point. Our work establishes the pyrochlore structure as a promising lattice platform to realize and tune novel emergent phases intertwining topology and many-body interactions.

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.

The cubic Laves phase compound CeRu2 with a Kagome substructure of Ru has been investigated to understand myriad fascinating phenomena resulting from competition among its various physical and geometric features. Such phenomena include flat bands, van Hove singularities, Dirac cones, reentrant superconductivity, magnetism, the Fulde-Ferrell-Larkin-Ovchinnikov state, valence fluctuations, time-irreversible anisotropic s-state superconductivity, etc. Extensive studies have thus been carried out since 1958 when the highly unusual coexistence of superconductivity and ferromagnetism was proposed for the mixed compounds (Ce,Gd)Ru2. Activity has accelerated in recent years due to increasing interest in topological states in superconductors. However, there has been little investigation of the mutual influence of these fascinating states. Therefore, we systematically investigated the superconductivity and possible Fermi surface topological change in CeRu2 via magnetic, resistivity, and structural measurements under pressure up to -168 GPa. An unusual phase diagram that suggests an intriguing interplay between the compound's superconducting order and Fermi surface topological order has been constructed. A resurgence in its superconducting transition temperature was observed above 28 GPa. Our experiments have identified a structural transition above 76 GPa and a few tantalizing phase transitions driven by high pressure. Our high-pressure results further suggest that superconductivity and Fermi surface topology in CeRu2 are strongly intertwined.

High-pressure studies on elements play an essential role in superconductivity research, with implications for both fundamental science and applications. Here we report the experimental discovery of surprisingly low pressure driving a novel germanium allotrope into a superconducting state in comparison to that for alpha-Ge. Raman measurements revealed structural phase transitions and possible electronic topological transitions under pressure up to 58 GPa. Based on pressure-dependent resistivity measurements, superconductivity was induced above 2 GPa and the maximum Tc of 6.8 K was observed under 4.6 GPa. Interestingly, a superconductivity enhancement was discovered during decompression, indicating the possibility of maintaining pressure-induced superconductivity at ambient pressure with better superconducting performance. Density functional theory analysis further suggested that the electronic structure of Ge (oP32) is sensitive to its detailed geometry and revealed that disorder in the beta-tin structure leads to a higher Tc in comparison to the perfect beta-tin Ge.

Contrary to the similar thermoelectric performance among both AZn2Sb2 2 Sb 2 and AMg2Bi2 2 Bi 2 compounds, their isostructural counterparts, AMg2Sb2, 2 Sb 2 , can exhibit thermoelectric figure of merit values that vary by orders of magnitude with different A elements. Here, we reveal physical origins accounting for the significantly differing performance among AMg2Sb2-based 2 Sb 2-based compounds (A = Ca, Sr, Sm, Yb, and Mg) through comprehensive analyses, where it is shown that the disparities in performance at the macroscale essentially originate from the widely varying activation energies that equal amounts of dopant can induce. Meanwhile, a few unusual transport behaviors regarding electrical conductivity, carrier concentration, or lattice thermal conductivity among these compounds have been identified, and we also present their rationales in depth. This mechanism-focused study can not only promote further understanding of the complex transport behaviors in condensed matter but be instrumental in rationally tuning the physical properties of materials as well.

We have studied LK-99 [Pb$_{10-x}$Cu$_x$(PO$_4$)$_6$O], 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 Cu$_2$S impurity in their sample and not with superconductivity.

Little is so far known about the magnetism of the $\rm A_2B_2O_7$ monoclinic layered perovskites that replace the spin-ice supporting pyrochlore structure for $r_A/r_B>1.78$. We show that high quality monoclinic Pr$_2$Ti$_2$O$_7$ single crystals with a three-dimensional network of non-Kramers Pr$^{3+}$ ions that interact through edge-sharing super-exchange interactions, form a singlet ground state quantum paramagnet that does not undergo any magnetic phase transitions down to at least 1.8 K. The chemical phase stability, structure, and magnetic properties of the layered perovskite Pr$_2$Ti$_2$O$_7$ were investigated using x-ray diffraction, transmission electron microscopy, and magnetization measurements. Synthesis of polycrystalline samples with the nominal compositions of Pr$_2$Ti$_{2+x}$O$_7$ ($-0.16 \leq x \leq 0.16$) showed that deviations from the Pr$_2$Ti$_2$O$_7$ stoichiometry lead to secondary phases of related, structures including the perovskite phase Pr$_{2/3}$TiO$_3$ and the orthorhombic phases Pr$_4$Ti$_9$O$_{24}$ and Pr$_2$TiO$_5$. No indications of site disordering (stuffing and anti-stuffing) or vacancy defects were observed in the Pr$_2$Ti$_2$O$_7$ majority phase. A procedure for growth of high-structural-quality, stoichiometric single crystals of Pr$_2$Ti$_2$O$_7$ by the traveling solvent floating zone (TSFZ) method is reported. Thermo-magnetic measurements of single-crystalline Pr$_2$Ti$_2$O$_7$ reveal an isolated singlet ground state that we associate with the low symmetry crystal electric field environments that split the $2J+1=9$-fold degenerate spin-orbital multiplets of the four differently coordinated Pr$^{3+}$ ions into 36 isolated singlets resulting in an anisotropic temperature independent van-Vleck susceptibility at low $T$. A small isotropic Curie term is associated with 0.96(2)\% non-interacting Pr$^{4+}$ impurities.

We present the results from torque magnetometry studies of the kagome superconductor RbV3Sb5 under applied fields up to 45 T and temperatures down to liquid 3He temperature (0.32 K). The torque signal shows clear de Haas-van Alphen (dHvA) oscillations with eight distinct frequencies ranging from approximate to 150 to 3000 T. Among these, five are above 500 T. Angle-dependent measurement of dHvA oscillations shows that all frequencies follow 1/cos theta dependence, where theta is the tilt angle with respect to the applied field direction, and the oscillations disappear above theta = 60 degrees, which confirms that the Fermi surfaces corresponding to these frequencies are two dimensional. The Berry phase (phi), calculated by constructing a Landau level fan diagram, is found to be approximate to it, which strongly supports the nontrivial topology of RbV3Sb5. Using the Lifshitz-Kosevich formula, we estimate the effective mass (m*) of charge carriers in RbV3Sb5, and it is found to be heavier (approximate to 0.7mo, where mo is the free electron mass) than that for other topological insulators. The findings of high frequencies up to 3000 Tin RbV3Sb5 have not been reported previously, and the results regarding the Fermi surface of RbV3Sb5 are crucial for understanding the charge density wave order, superconductivity, and nontrivial topology in AV3Sb5 (A = K, Rb, and Cs), as well as the interplay among them.

Here, we present the Fermi surface properties of the kagome superconductor KV3Sb5 using torque magnetom-etry at applied fields up to 45 T and temperatures down to that of liquid 3He (0.32 K). The torque signal shows clear de Haas-van Alphen (dHvA) oscillations with 14 major frequencies ranging from similar to 33 to 2149 T, nine of which are higher frequencies (above 500 T) that have never been reported in KV3Sb5. Angular dependence measurements of the dHvA oscillations were carried out to investigate the dimensionality of the Fermi surface. Based on our analysis, several frequencies follow the 1/cos 0 dependence, where 0 is the tilt angle with respect to the applied field direction and oscillations disappear above 0 = 60 degrees, which suggest that Fermi surfaces corresponding to these frequencies are quasitwo dimensional. The Berry phase (GB), determined by constructing a Landau level fan diagram, was found to be (1.B similar to it, which strongly suggests the nontrivial topology of KV3Sb5. To explain the experimental results, we carried out band-structure and Fermi-surface calculations using density functional theory (DFT) for both pristine and charge-density wave (CDW) phases. We found that the Fermi surface undergoes severe reconstruction in the CDW phase and, more importantly, our calculation results are in reasonable agreement with the experimentally measured Fermi-surface frequencies. The observation in this paper of very high quantum oscillation frequencies in KV3Sb5 and the determination of their detailed Fermi-surface properties, along with the analyses of corresponding DFT calculation results, are crucial for understanding CDW order, unconventional superconductivity, and nontrivial topology in AV3Sb5 (A = K, Rb, and Cs), as well as the interplay among them.

The two-dimensional (2D) self-intercalated van der Waalsmagnets,chromium tellurides (Cr1+delta Te2), with roomtemperature ferromagnetism and exotic topological spin textures, haveemerged as an attractive platform for the development of ultrathinspintronic devices. While many prior studies claim Cr1+delta Te2 are air-stable, which is crucial for practical applications,we demonstrate that within only minutes of exposure to air dramaticchanges occur in the Raman and X-ray photoelectron spectroscopy spectraof nanoplates at room temperature. Time-dependent magnetization measurementsand transmission electron microscopy studies suggested the rapid oxidationis not self-limited and has distinct processes on the surfaces andedges. Density functional theory calculations confirmed the spontaneousoxygen adsorption on the surface and the thermodynamically favorableoxidation process. These findings demonstrate unambiguously that thesurface of Cr1+delta Te2 is extremely air-sensitive,highlighting the necessity of surface protection for fundamental studiesof intrinsic 2D magnetism and practical applications of advanced spintronicdevices.

Silver chalcogenide systems have recently attracted significant attention due to their promising topological insulating properties. Here we conducted systematic low-temperature local scanning tunneling microscopy/spectroscopy and first-principles studies on the surface states of monoclinic Ag2Se thin films grown using molecular beam epitaxy. Through the use of quasi-particle interference patterns, we have observed evidence for topological surface states on the selenium-terminated surfaces with different types of defect densities. The results of ab initio calculations confirm the existence of nontrivial topological surface states in the monoclinic Ag2Se structure, for which such properties have not been previously reported. The energy dispersion determined using voltage-dependent standing wave patterns suggests that these topological states have an anisotropic Dirac cone structure. This discovery may lead to new applications for monoclinic Ag2Se in the rapidly growing fields of nanoelectronics and spintronics.