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Spatial, momentum and energy separation of electronic spins in condensed-matter systems guides the development of new devices in which spin-polarized current is generated and manipulated 1 – 3 . Recent attention on a set of previously overlooked symmetry operations in magnetic materials 4 leads to the emergence of a new type of spin splitting, enabling giant and momentum-dependent spin polarization of energy bands on selected antiferromagnets 5 – 10 . Despite the ever-growing theoretical predictions, the direct spectroscopic proof of such spin splitting is still lacking. Here we provide solid spectroscopic and computational evidence for the existence of such materials. In the noncoplanar antiferromagnet manganese ditelluride (MnTe 2 ), the in-plane components of spin are found to be antisymmetric about the high-symmetry planes of the Brillouin zone, comprising a plaid-like spin texture in the antiferromagnetic (AFM) ground state. Such an unconventional spin pattern, further found to diminish at the high-temperature paramagnetic state, originates from the intrinsic AFM order instead of spin–orbit coupling (SOC). Our finding demonstrates a new type of quadratic spin texture induced by time-reversal breaking, placing AFM spintronics on a firm basis and paving the way for studying exotic quantum phenomena in related materials. Examining the in-plane spin components of the noncoplanar antiferromagnet manganese ditelluride provides spectroscopic and computational evidence of materials with a new type of plaid-like spin splitting in the antiferromagnetic ground state.

A detailed phenomenology of low energy excitations is a crucial starting point for microscopic understanding of complex materials, such as the cuprate high-temperature superconductors. Because of its unique momentum-space discrimination, angle-resolved photoemission spectroscopy (ARPES) is ideally suited for this task in the cuprates, where emergent phases, particularly superconductivity and the pseudogap, have anisotropic gap structure in momentum space. We present a comprehensive doping- and temperature-dependence ARPES study of spectral gaps in Bi ₂Sr ₂CaCu ₂O ₈₊δ, covering much of the superconducting portion of the phase diagram. In the ground state, abrupt changes in near-nodal gap phenomenology give spectroscopic evidence for two potential quantum critical points, p = 0.19 for the pseudogap phase and p = 0.076 for another competing phase. Temperature dependence reveals that the pseudogap is not static below T c and exists p > 0.19 at higher temperatures. Our data imply a revised phase diagram that reconciles conflicting reports about the endpoint of the pseudogap in the literature, incorporates phase competition between the superconducting gap and pseudogap, and highlights distinct physics at the edge of the superconducting dome.

Photocathodes—materials that convert photons into electrons through a phenomenon known as the photoelectric effect—are important for many modern technologies that rely on light detection or electron-beam generation 1 – 3 . However, current photocathodes are based on conventional metals and semiconductors that were mostly discovered six decades ago with sound theoretical underpinnings 4 , 5 . Progress in this field has been limited to refinements in photocathode performance based on sophisticated materials engineering 1 , 6 . Here we report unusual photoemission properties of the reconstructed surface of single crystals of the perovskite oxide SrTiO 3 (100), which were prepared by simple vacuum annealing. These properties are different from the existing theoretical descriptions 4 , 7 – 10 . In contrast to other photocathodes with a positive electron affinity, our SrTiO 3 surface produces, at room temperature, discrete secondary photoemission spectra, which are characteristic of efficient photocathode materials with a negative electron affinity 11 , 12 . At low temperatures, the photoemission peak intensity is enhanced substantially and the electron beam obtained from non-threshold excitations shows longitudinal and transverse coherence that differs from previous results by at least an order of magnitude 6 , 13 , 14 . The observed emergence of coherence in secondary photoemission points to the development of a previously undescribed underlying process in addition to those of the current theoretical photoemission framework. SrTiO 3 is an example of a fundamentally new class of photocathode quantum materials that could be used for applications that require intense coherent electron beams, without the need for monochromatic excitations. The reconstructed surface of single crystals of SrTiO 3 (100), prepared by simple vacuum annealing, produces discrete secondary photoemission spectra at room temperature and has increased peak intensities at low temperatures.

Liver fibrosis is a major cause of liver failure, but treatment remains ineffective. In the present study, we investigated the mechanisms and anti-hepatofibrotic activities of asiatic acid (AA) in a rat model of liver fibrosis induced by carbon tetrachloride (CCl(4)) and in vitro in TGF-beta1-stimulated rat hepatic stellate cell line (HSC-T6). Treatment with AA significantly attenuated CCl(4)-induced liver fibrosis and functional impairment in a dosage-dependent manner, including blockade of the activation of HSC as determined by inhibiting de novo alpha smooth muscle actin (a-SMA) and collagen matrix expression, and an increase in ALT and AST (all p<0.01). The hepatoprotective effects of AA on fibrosis were associated with upregulation of hepatic Smad7, an inhibitor of TGF-beta signaling, thereby blocking upregulation of TGF-beta1 and CTGF and the activation of TGF-beta/Smad signaling. The anti-fibrosis activity and mechanisms of AA were further detected in vitro in HSC-T6. Addition of AA significantly induced Smad7 expression by HSC-T6 cells, thereby inhibiting TGF-beta1-induced Smad2/3 activation, myofibroblast transformation, and collagen matrix expression in a dosage-dependent manner. In contrast, knockdown of Smad7 in HSC-T6 cells prevented AA-induced inhibition of HSC-T6 cell activation and fibrosis in response to TGF-beta1, revealing an essential role for Smad7 in AA-induced anti-fibrotic activities during liver fibrosis in vivo and in vitro. In conclusion, AA may be a novel therapeutic agent for liver fibrosis. Induction of Smad7-dependent inhibition of TGF-beta/Smad-mediated fibrogenesis may be a central mechanism by which AA protects liver from injury.

The nature of the pseudogap phase of cuprate high-temperature superconductors is a major unsolved problem in condensed matter physics. We studied the commencement of the pseudogap state at temperature T* using three different techniques (angle-resolved photoemission spectroscopy, polar Kerr effect, and time-resolved reflectivity) on the same optimally doped Bi2201 crystals. We observed the coincident, abrupt onset at T* of a particle-hole asymmetric antinodal gap in the electronic spectrum, a Kerr rotation in the reflected light polarization, and a change in the ultrafast relaxational dynamics, consistent with a phase transition. Upon further cooling, spectroscopic signatures of superconductivity begin to grow close to the superconducting transition temperature (T c ), entangled in an energy-momentum—dependent manner with the preexisting pseudogap features, ushering in a ground state with coexisting orders.

Perovskite iridates are a promising material platform for hosting unconventional superconductivity. Transport measurements of Sr2IrO4 thin-film field-effect transistors are expected to provide irrefutable evidence for the existence of superconductivity. However, these experiments have revealed a remarkably robust insulating state over wide electron and hole doping ranges; this finding is in contrast to the case of the bulk material, in which metallicity appears upon moderate electron doping by substituting cations in place of Sr. The nature of this robust insulating state and whether any metallic state can be realized in the Sr2IrO4 thin film are two remaining challenges that preclude further progress in the search for superconductivity in this system. Here, we show that this insulating state is enhanced in Sr2IrO4 thin films by thermal annealing under vacuum conditions, while it can be destroyed upon annealing in an oxygen atmosphere within restricted ranges of oxygen pressure, annealing temperature and ion substitution levels. The resulting films exhibit metallic transport behavior near room temperature and a metal–insulator crossover at ~200 K. Our results point to the potentially important roles of the oxygen vacancies at different atomic sites in the formation of the robust insulating state and the new metallic state and to their interplay in the Sr2IrO4 thin film. This finding opens new possibilities in the search for unconventional superconductivity by further tailoring the as-found metallic state in properly oxygen-annealed Sr2IrO4 thin films.Despite enormous efforts by many research groups, Sr2IrO4 was found to stay remarkably insulating in thin film form. Now, a high-pressure oxygen annealing treatment on the Sr2IrO4 thin film realized the long-sought metallicity for the first time. An emerging transport phase diagram was deduced from the experiment that features an interplay between two states: the robust insulating state, which is likely dominated by the defect scattering effect of planar oxygen vacancies O(2), and the new metallic state, which likely reflects an intrinsic bulk-like property of the IrO2 planes with effective electron doping due to apical oxygen vacancies O(1).

Recent developments in high-temperature superconductivity highlight a generic tendency of the cuprates to develop competing electronic (charge) supermodulations. While coupled with the lattice and showing different characteristics in different materials, these supermodulations themselves are generally conceived to be quasi-two-dimensional, residing mainly in individual CuO 2 planes, and poorly correlated along the c axis. Here we observed with resonant elastic X-ray scattering a distinct type of electronic supermodulation in YBa 2 Cu 3 O 7− x (YBCO) thin films grown epitaxially on La 0.7 Ca 0.3 MnO 3 (LCMO). This supermodulation has a periodicity nearly commensurate with four lattice constants in-plane, eight out of plane, with long correlation lengths in three dimensions. It sets in far above the superconducting transition temperature and competes with superconductivity below this temperature for electronic states predominantly in the CuO 2 plane. Our finding sheds light on the nature of charge ordering in cuprates as well as a reported long-range proximity effect between superconductivity and ferromagnetism in YBCO/LCMO heterostructures. Understanding the nature of competing phases is a key to understanding the superconducting mechanism of unconventional superconductors. Here, the authors demonstrate a three-dimensional charge ordering state which competes with superconductivity in epitaxial YBa 2 Cu 3 O 7-x thin films grown on La 0.7 Ca 0.3 MnO 3 substrates.

The spectral energy gap is an important signature that defines states of quantum matter: insulators, density waves and superconductors have very different gap structures. The momentum-resolved nature of angle-resolved photoemission spectroscopy (ARPES) makes it a powerful tool to characterize spectral gaps. ARPES has been instrumental in establishing the anisotropic d -wave structure of the superconducting gap in high-transition-temperature ( T c ) cuprates, which is different from the conventional isotropic s -wave superconducting gap. Shortly afterwards, ARPES demonstrated that an anomalous gap above T c , often termed the pseudogap, follows a similar anisotropy. The nature of this poorly understood pseudogap and its relationship with superconductivity has since become the focal point of research in the field. To address this issue, the momentum, temperature, doping and materials dependence of spectral gaps have been extensively examined with significantly improved instrumentation and carefully matched experiments in recent years. This article overviews the current understanding and unresolved issues of the basic phenomenology of gap hierarchy. We show how ARPES has been sensitive to phase transitions, has distinguished between orders having distinct broken electronic symmetries, and has uncovered rich momentum- and temperature-dependent fingerprints reflecting an intertwined and competing relationship between the ordered states and superconductivity that results in multiple phenomenologically distinct ground states inside the superconducting dome. These results provide us with microscopic insights into the cuprate phase diagram. The superconducting energy gap is perhaps the best-known of the spectral gaps in a superconductor, but there are many other types, including density waves and the mysterious pseudogap. This Review Article surveys what angle-resolved photoemission spectroscopy has revealed about the various gaps.

Photoelectron spectroscopy measurements uncover a singularity over a wide doping range in the cuprate superconductor Bi 2 Sr 2 CaCu 2 O 8+ δ , suggesting a competition between the charge-ordering and the superconducting phases. In the high-temperature ( T c ) cuprate superconductors, a growing body of evidence suggests that the pseudogap phase 1 , existing below the pseudogap temperature T ∗ , is characterized by some broken electronic symmetries distinct from those associated with superconductivity 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 , 21 . In particular, recent scattering experiments have suggested that charge ordering competes with superconductivity 18 , 19 , 20 , 21 . However, no direct link of an interplay between the two phases has been identified from the important low-energy excitations. Here, we report an antagonistic singularity at T c in the spectral weight of Bi 2 Sr 2 CaCu 2 O 8+ δ as compelling evidence for phase competition, which persists up to a high hole concentration p ~ 0.22. Comparison with theoretical calculations confirms that the singularity is a signature of competition between the order parameters for the pseudogap and superconductivity. The observation of the spectroscopic singularity at finite temperatures over a wide doping range provides new insights into the nature of the competitive interplay between the two orders and the complex phase diagram near the pseudogap critical point.

Photoemission studies in the pseudogap state of a cuprate superconductor show differences depending on whether a particle is added or removed, revealing broken translational symmetry. Moreover, this particle–hole asymmetry coincides with the opening of the pseudogap. In conventional superconductors, a gap exists in the energy absorption spectrum only below the transition temperature ( T c ), corresponding to the price to pay in energy for breaking a Cooper pair of electrons and creating two excited states. In high- T c cuprate superconductors above T c but below a temperature T * , an energy gap called the pseudogap 1 exists, and is controversially attributed either to pre-formed superconducting pairs, which would show particle–hole symmetry, or to competing phases that would typically break it. Scanning tunnelling microscopy (STM) studies suggest that the pseudogap stems from lattice translational symmetry breaking 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 and is associated with a different characteristic spectrum for adding or removing electrons (particle–hole asymmetry; refs  2 , 3 ). However, no signature of either energy or spatial symmetry breaking of the pseudogap has previously been observed by angle-resolved photoemission spectroscopy 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 (ARPES). Here we report ARPES data from Bi2201, which reveal both particle–hole symmetry breaking and pronounced spectral broadening—indicative of spatial symmetry breaking without long-range order at the opening of the pseudogap. Our finding supports the STM proposal that the pseudogap state is a broken-symmetry state that is distinct from homogeneous superconductivity.