Recent high-pressure studies found that superconductivity can be achieved under very low pressure in the parent iron arsenide compound CaFe2As2, although details of the sharpness and temperature of transitions vary between liquid medium and gas medium measurements. To better understand this issue, we performed high-pressure susceptibility and transport studies on CaFe2As2, using helium as the pressure medium. The signatures of the transitions to the low-temperature orthorhombic and collapsed tetragonal phase remained exceptionally sharp and no signature of bulk superconductivity was found under our hydrostatic conditions. Our results suggest that phase separation and superconductivity in CaFe2As2 are induced by non-hydrostatic conditions associated with the frozen liquid media. PACS numbers: 74.20.Mn, 74.25.Fy, 74.25.Dw, 74.62.Fj, 64.70.Tg The recent discovery of superconductivity in doped iron arsenide compounds[1, 2, 3] and the later improvement of the superconducting transition temperature T c in both the pnictide oxides such as ROFeAs (R111) [4,5,6,7] and the ThCr 2 Si 2 -structure compounds such as (Ba,K)Fe 2 As 2 (R122) [3] have caused extensive experimental and theoretical studies in this new class of materials with layered FeAs planes. Similar to the high-T c cuprates, the parent compounds exhibit structural transitions from a high-temperature tetragonal phase to a lowtemperature orthorhombic phase, and the orthorhombic phase is usually antiferromagnetically (AF) ordered [8]. Upon doping, both the orthorhombic structure and the AF phase are suppressed and superconductivity is induced.Several unique properties have been found in the iron arsenide superconductors. For example, these materials are semimetals and therefore metallic even without doping, in contrast to the cuprates. In BaFe 2 As 2 , doping Co into the FeAs-plane also induces superconductivity [9], which differs from the suppression of superconductivity and formation of local moments by any doping into the cuprate CuO-planes. Superconductivity has been reported under hydrostatic pressure in the parent compounds CaFe 2 As 2 [10, 11, 12], SrFe 2 As 2 [13, 14, 15], and BaFe 2 As 2 [14]. In particular, for CaFe 2 As 2 , T c as high as 10K has been found in a moderate 0.4GPa pressure [10,11,12], while for SrFe 2 As 2 and BaFe 2 As 2 , superconductivity is achieved at about 28K at P=3.2 GPa and 4.5 GPa respectively [14].In CaFe 2 As 2 in ambient pressure, a structural phase transition (from tetragonal to orthorhombic) is seen at T S1 = 170 K[16], accompanied by the appearance of magnetic order [17]; this transition is seen as a sharp upwards anomaly in resistivity. Hydrostatic pressure causes a reduction of T S1 . The signature in resistivity becomes a broad upturn, rather than the sharp discontinuous change seen in ambient pressure [10,12]. Above 0.5GPa, a collapsed tetragonal structure is identified below a separate structural transition temperature (T S2 ) [10,18]. The collapsed tetragonal phase has the same crystal symmetry as the high-temperat...

a b s t r a c tAt ambient pressure CaFe 2 As 2 has been found to undergo a first order phase transition from a high temperature, tetragonal phase to a low-temperature orthorhombic/antiferromagnetic phase upon cooling through T $ 170 K. With the application of pressure this phase transition is rapidly suppressed and by $0.35 GPa it is replaced by a first order phase transition to a low-temperature collapsed tetragonal, non-magnetic phase. Further application of pressure leads to an increase of the tetragonal to collapsed tetragonal phase transition temperature, with it crossing room temperature by $1.7 GPa. Given the exceptionally large and anisotropic change in unit cell dimensions associated with the collapsed tetragonal phase, the state of the pressure medium (liquid or solid) at the transition temperature has profound effects on the low-temperature state of the sample. For He-gas cells the pressure is as close to hydrostatic as possible and the transitions are sharp and the sample appears to be single phase at low temperatures. For liquid media cells at temperatures below media freezing, the CaFe 2 As 2 transforms when it is encased by a frozen media and enters into a low-temperature multi-crystallographic-phase state, leading to what appears to be a strain stabilized superconducting state at low temperatures.

α-RuCl_{3} is a leading candidate material for the observation of physics related to the Kitaev quantum spin liquid (QSL). By combined susceptibility, specific-heat, and nuclear-magnetic-resonance measurements, we demonstrate that α-RuCl_{3} undergoes a quantum phase transition to a QSL in a magnetic field of 7.5 T applied in the ab plane. We show further that this high-field QSL phase has gapless spin excitations over a field range up to 16 T. This highly unconventional result, unknown in either Heisenberg or Kitaev magnets, offers insight essential to establishing the physics of α-RuCl_{3}.

(TMTTF)2AsF6 undergoes two phase transitions upon cooling from 300 K. At TCO=103 K a charge-ordering (CO) occurs, and at TSP (B=9 T)=11 K the material undergoes a spin-Peierls (SP) transition. Within the intermediate, CO phase, the charge disproportionation ratio is found to be at least 3:1 from 13 C NMR T −1 1 measurements on spin-labeled samples. Above TSP up to about 3TSP T −1 1 is independent of temperature, indicative of low-dimensional magnetic correlations. With the application of about 0.15 GPa pressure, TSP increases substantially, while TCO is rapidly suppressed, demonstrating that the two orders are competing. The experiments are compared to results obtained from calculations on the 1D extended Peierls-Hubbard model. PACS numbers: 71.20.Rv, 71.30.+h, 71.45.Lr, Inhomogenous charge and spin structures are a consequence of competing interactions and therefore of general interest in correlated electron systems. Examples include the high-T c cuprates 1 and manganites 2 as well as the quasi-2D organic conductors 3 . The quasi-1D salts made from TMTTF or TMTSF molecules are also susceptible to charge-ordered states. Independent of that, they are well-known for the sequence of ground states accessible by applying pressure or selecting different counterions. For example, the material (TMTTF) 2 PF 6 undergoes transitions from spin-Peierls, antiferromagnetic (AF), spin-density wave (SDW), and finally to superconducting (SC) ground states as the pressure is increased to 4-5 GPa 4,5 . For a long time, it was known that another phase transition occurs in a number of TMTTF salts with both centrosymmetric (e.g., AsF 6 , SbF 6 ) and non-centrosymmetric (e.g., ReO 4 ) counterions. Only recently 6,7 was the broken symmetry associated with this transition identified as a charge disproportionation.In TMTTF salts the characteristic temperature of the onset of the charge-ordered (CO) phase is high, on the order of 100 K. It indicates that the interactions driving the CO are relatively strong, and therefore potentially impact the electronic and magnetic properties of the disordered phase. Issues associated with CO correlations in these systems take particular relevance when considering that the nature of the metallic phase of TMTSF salts remains controversial 8,9,10 . Below we report the results of a number of NMR measurements on 13 C spin-labeled samples of (TMTTF) 2 AsF 6 in the CO phase. Our principle result is a mapping of the temperature/pressure phase diagram of the SP and CO phases that includes a tetracritical point with a region of coexistence of the two forms of order. There is good agreement between the experiments and the results of calculations on the 1D extended Hubbard 11 and Peierls-Hubbard models 12,13 .A review of the characteristics of the CO phase and the phase transition is in order. With counterions PF 6 , AsF 6 , and SbF 6 , the ordering temperature is 62 K, 103 K, and 154 K, respectively. Upon cooling, the salts made with the first two are already well into a region of thermally activated resistivitie...

In condensed matter physics, there is a novel phase termed "quantum spin liquid", in which strong quantum fluctuations prevent the long-range magnetic order from being established, and so the electron spins do not form an ordered pattern but remain "liquid" like even at absolute zero temperature. Such a phase is not involved with any spontaneous symmetry breaking and local order parameter, and to understand it is beyond the conventional phase transition theory. Due to the rich physics and exotic properties of quantum spin liquids, such as the long-range entanglement and fractional quantum excitations, which are believed to hold great potentials in quantum communication and computation, they have been intensively studied since the concept was proposed in 1973 by P. W. Anderson. Currently, experimental identifications of a quantum spin liquid still remain as a great challenge. Here, we highlight some interesting experimental progress that has been made recently. We also discuss some outstanding issues and raise questions that we consider to be important for future research. I. THE ROAD TO QUANTUM SPIN LIQUIDSarXiv:1904.04435v1 [cond-mat.str-el]

Titanium oxide nanotube layers by anodization have received considerable attention in biomedical application. Previous studies have demonstrated increased osteoblast (bone-forming cell) adhesion and function on nanotube layers compared with unanodized counterparts. More recently, one study showed amorphous TiO(2) nanotube diameter determined cell fate. The anatase phase is known to be much more beneficial for bone growth than amorphous phase, so there is increasing demand to explore the response of osteoblast on anatase phase TiO(2) nanotube layers. For this reason, we evaluated MC3T3-E1 preosteoblast behavior on different diameter nanotube layers with anatase phase. The results showed that the diameter of 20-70 nm provided an effective length scale for cell adhesion, alkaline phosphatase activity, and mineralization. However, cell adhesion, alkaline phosphatase activity, and mineralization were severely impaired on nanotube layers with 100-120 nm. Interestingly, the filopodia seemed not spread into the nanotubular and like extending anatase nanotube walls, where there may be higher numbers of atoms at the surface compared to the nanotubular architecture. To our surprise, the proliferation rates of cells cultured on anatase nanotube layers increased with increasing tube diameter from 20 to 120 nm, which may be attributed to different length and nanometer-scale roughness of the nanotube layers.

TMTTF)2SbF6 is known to undergo a charge ordering (CO) phase transition at TCO ≈ 156K and another transition to an antiferromagnetic (AF) state at TN ≈ 8K. Applied pressure P causes a decrease in both TCO and TN . When P > 0.5GP a, the CO is largely supressed, and there is no remaining signature of AF order. Instead, the ground state is a singlet. In addition to establishing an expanded, general phase diagram for the physics of TMTTF salts, we establish the role of electron-lattice coupling in determining how the system evolves with pressure.

We report on the structural, electrical, and optical properties of 5% niobium doped TiO2 thin films grown on various substrates by pulsed laser deposition. The epitaxial anatase Nb:TiO2 film on LaAlO3 is shown to be an intrinsic transparent metal and its metallic property arises from Nb substitution into Ti site as evidenced by the Rutherford backscattering channeling result. In contrast, the rutile Nb:TiO2 thin films show insulating behaviors with 2–3 orders higher room temperature electrical resistivity and ∼30 times lower mobility. A blueshift in the optical absorption edge is observed in both phases, though of differing magnitude.