J. P. Sun

Dome-shaped magnetic order competing with high-temperature superconductivity at high pressures in FeSe

The coexistence and competition between superconductivity and electronic orders, such as spin or charge density waves, have been a central issue in high transition-temperature (Tc) superconductors. Unlike other iron-based superconductors, FeSe exhibits nematic ordering without magnetism whose relationship with its superconductivity remains unclear. Moreover, a pressure-induced fourfold increase of Tc has been reported, which poses a profound mystery. Here we report high-pressure magnetotransport measurements in FeSe up to ∼15 GPa, which uncover the dome shape of magnetic phase superseding the nematic order. Above ∼6 GPa the sudden enhancement of superconductivity (Tc≤38.3 K) accompanies a suppression of magnetic order, demonstrating their competing nature with very similar energy scales. Above the magnetic dome, we find anomalous transport properties suggesting a possible pseudogap formation, whereas linear-in-temperature resistivity is observed in the normal states of the high-Tc phase above 6 GPa. The obtained phase diagram highlights unique features of FeSe among iron-based superconductors, but bears some resemblance to that of high-Tc cuprates.

Pressure Induced Superconductivity on the border of Magnetic Order in MnP

We report the discovery of superconductivity on the border of long-range magnetic order in the itinerant-electron helimagnet MnP via the application of high pressure. Superconductivity with T(sc)≈1  K emerges and exists merely near the critical pressure P(c)≈8  GPa, where the long-range magnetic order just vanishes. The present finding makes MnP the first Mn-based superconductor. The close proximity of superconductivity to a magnetic instability suggests an unconventional pairing mechanism. Moreover, the detailed analysis of the normal-state transport properties evidenced non-Fermi-liquid behavior and the dramatic enhancement of the quasiparticle effective mass near P(c) associated with the magnetic quantum fluctuations.

High- Tc Superconductivity in FeSe at High Pressure: Dominant Hole Carriers and Enhanced Spin Fluctuations

The importance of electron-hole interband interactions is widely acknowledged for iron-pnictide superconductors with high transition temperatures (T_{c}). However, the absence of hole pockets near the Fermi level of the iron-selenide (FeSe) derived high-T_{c} superconductors raises a fundamental question of whether iron pnictides and chalcogenides have different pairing mechanisms. Here, we study the properties of electronic structure in the high-T_{c} phase induced by pressure in bulk FeSe from magnetotransport measurements and first-principles calculations. With increasing pressure, the low-T_{c} superconducting phase transforms into the high-T_{c} phase, where we find the normal-state Hall resistivity changes sign from negative to positive, demonstrating dominant hole carriers in contrast to other FeSe-derived high-T_{c} systems. Moreover, the Hall coefficient is enlarged and the magnetoresistance exhibits anomalous scaling behaviors, evidencing strongly enhanced interband spin fluctuations in the high-T_{c} phase. These results in FeSe highlight similarities with high-T_{c} phases of iron pnictides, constituting a step toward a unified understanding of iron-based superconductivity.

Evidence for pressure-induced node-pair annihilation in C d 3 A s 2

As an intermediate state in the topological phase diagram, Dirac semimetals are of particular interest as a platform for studying topological phase transitions under external modulations. Despite a growing theoretical interest in this topic, it remains a substantial challenge to experimentally tune the system across topological phase transitions. Here, we investigate the Fermi surface evolution of Cd3As2 under high pressure through magnetotransport. A sudden change in Berry phase occurs at 1.3 GPa along with the unanticipated shrinkage of the Fermi surface, which occurs well below the structure transition point (~2.5 GPa). High pressure X-ray diffraction also reveals an anisotropic compression of the Cd3As2 lattice around a similar pressure. Corroborated by the first-principles calculations we show that an axial compression will shift the Dirac nodes towards the Brillouin zone center and eventually introduces a finite energy gap. The ability to tune the node position, a vital parameter of Dirac semimetals, can have dramatic impacts on the corresponding topological properties such as the Fermi arc surface states and the chiral anomaly. Our study demonstrates axial compression as an efficient approach for manipulating the band topology and exploring the critical phenomena near the topological phase transition in Cd3As2.

Pressure-induced phase transitions and superconductivity in a black phosphorus single crystal

Significance A high-pressure study of a black phosphorus crystal establishes a rich phase diagram, including Weyl semimetal and superconducting states, Lifshitz-type semiconductor–semimetal transitions, and two structural phase transitions. Transport properties and quantum oscillations under high pressure provide critically valuable information to understand the physics of these new phases. The pressure dependence of physical properties has been reliably measured under hydrostatic pressure and applied magnetic fields using a large-volume apparatus. Superconductivity in the A7 phase has been found to exhibit the largest magnetoresistance effect observed in its normal state so far. The Bardeen–Cooper–Schrieffer superconductivity in the A7 phase identified by the experiment can be accounted for by the phonon mechanism based on a first-principles calculation. We report a thorough study of the transport properties of the normal and superconducting states of black phosphorus (BP) under magnetic field and high pressure with a large-volume apparatus that provides hydrostatic pressure to induce transitions from the layered A17 phase to the layered A7 phase and to the cubic phase of BP. Quantum oscillations can be observed at P ≥ 1 GPa in both resistivity and Hall voltage, and their evolutions with pressure in the A17 phase imply a continuous enlargement of Fermi surface. A significantly large magnetoresistance (MR) at low temperatures is observed in the A7 phase that becomes superconducting below a superconducting transition temperature Tc ∼ 6–13 K. Tc increases continuously with pressure on crossing the A7 to the cubic phase boundary. The strong MR effect can be fit by a modified Kohler’s rule. A correlation between Tc and fitting parameters suggests that phonon-mediated interactions play dominant roles in driving the Cooper pairing, which is further supported by our density functional theory (DFT) calculations. The change of effective carrier mobility in the A17 phase under pressure derived from the MR effect is consistent with that obtained from the temperature dependence of the quantum oscillations. In situ single-crystal diffraction under high pressure indicates a total structural reconstruction instead of simple stretching of the A17 phase layers in the A17-to-A7-phase transition. This finding helps us to interpret transport properties on crossing the phase transition under high pressure.

Cubic anvil cell apparatus for high-pressure and low-temperature physical property measurements

We will build a cubic anvil cell (CAC) apparatus for high-pressure and low-temperature physical property measurements in the synergic extreme condition user facility (SECUF). In this article, we first introduce the operating principle, the development history, and the current status of the CAC apparatus, and subsequently describe the design plan and technical targets for the CAC in SECUF. We will demonstrate the unique advantages of CAC, i.e., excellent pressure homogeneity and large hydrostatic pressure capacity, by summarizing our recent research progresses using CAC. Finally, we conclude by providing some perspectives on the applications of CAC in the related research fields.

Competition of superconductivity with the structural transition in Mo3Sb7

Prior to the superconducting transition at T c ≈ 2.3 K, Mo 3Sb 7 undergoes a symmetry-lowering, cubic-to-tetragonal structural transition at T s = 53 K. In this paper, we have monitored the pressure dependence of these two transitions by measuring the resistivity of Mo 3Sb 7 single crystals under various hydrostatic pressures up to 15 GPa. The application of external pressure enhances T c but suppresses T s until P c ≈ 10 GPa, above which a pressure-induced first order structural transition takes place and is manifested by the phase coexistence in the pressure range 8 ≤ P ≤ 12 GPa. The cubic phase above 12 GPa is also found to be superconducting with a higher T c ≈ 6 K that decreases slightly with further increasing pressure. The variations with pressure of T c and T s satisfy the Bilbro-McMillan equation, i.e. T c nT s 1-n = constant, thus suggesting the competition of superconductivity with the structural transition that has been proposed to be accompanied with a spin-gap formation at T s. Finally, this scenario is supported by our first-principles calculations which imply the plausible importance of magnetism that competes with the superconductivity in Mo 3Sb 7.