摘要 :
Anisotropies of electrical resistivity, upper critical field, London penetration depth, and critical currents have been measured in single crystals of the optimally doped iron pnictide superconductor Ba(Fe(1-x)Co_x)_2As_2 (x=0.074...
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Anisotropies of electrical resistivity, upper critical field, London penetration depth, and critical currents have been measured in single crystals of the optimally doped iron pnictide superconductor Ba(Fe(1-x)Co_x)_2As_2 (x=0.074 and T_c ～23 K). The normal-state resistivity anisotropy was obtained by employing both the Montgomery technique and direct measurements on samples cut along principal crystallographic directions. The ratio γ_ρ=ρ_c/ρ_a is about 4 ± 1 just above T_c and becomes half of that at room temperature. The anisotropy of the upper critical field, γ_H=H_(c2,ab)/H_(c2,c), as determined from specific-heat measurements close to T_c is in the range of 2.1-2.6, depending on the criterion used. A comparable low anisotropy of the London penetration depth, γ_λ=λ_c/λ_(ab), was recorded from tunnel diode resonator measurements and found to persist deep into the superconducting state. An anisotropy of comparable magnitude was also found in the critical currents, γ_j=j_(c,ab)/j_(c,c), as determined from both direct transport measurements (～1.5) and from the analysis of the magnetization data (～3). Overall, our results show that iron pnictide superconductors manifest anisotropies consistent with essentially three-dimensional intermetallic compounds and bear little resemblance to cuprates.
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摘要 :
Irreversible magnetic properties of Ba(Fe_(0.93)Co_(0.07))_2As_2 single crystals were studied by using several techniques including transport, magnetization, and magneto-optics. The mixed-state response was found to be very simila...
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Irreversible magnetic properties of Ba(Fe_(0.93)Co_(0.07))_2As_2 single crystals were studied by using several techniques including transport, magnetization, and magneto-optics. The mixed-state response was found to be very similar to high-T_c cuprates, such as Y-Ba-Cu-O and Nd-Ce-Cu-O. In particular, we find a nonmonotonic "fishtail" shape of the magnetization loops M(T=const,H) and a corresponding feature in M{T,H=const) scans, as well as a very large magnetic relaxation rate. The supercurrent density, j, in the critical state was evaluated from magnetization and from direct transport measurements close to T_c. At 5 K we estimate a moderate j ≈ 2.6±0.2× 10~5 A/cm~2. Analysis of the temperature and field dependencies of the magnetic relaxation is consistent with the collective pinning and creep (weak pinning and fast creep) model and suggests a crossover from the collective to the plastic creep regime in fields exceeding the field at which the fishtail magnetization is maximum. While the zero-field anisotropy of the magnetic properties in the superconducting state of Ba(Fe_(0.93)Co_(0.07))_2As_2 is small, the vortex behavior changes for different orientations of magnetic field with respect to the crystal axes, implying field-dependent anisotropy of pinning properties.
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摘要 :
Single crystals of Ba(Fe_(1-x)Ru_x)_2As_2, x<0.37, have been grown and characterized by structural, magnetic, and transport measurements. These measurements show that the structural/magnetic phase transition found in pure BaFe_2As_2 at 134 K is suppressed monotonically by Ru doping but, unlike doping with TM=Co, Ni, Cu, Rh, or Pd, the coupled transition seen in the parent compound does not detectably split into two separate ones. Superconductivity is stabilized at low temperatures for x> 0.2 and continues through the highest doping levels we report. The superconducting region is domelike, with maximum T_c(~ 16.5 K) found around x～0.29. A phase diagram of temperature ve...
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Single crystals of Ba(Fe_(1-x)Ru_x)_2As_2, x<0.37, have been grown and characterized by structural, magnetic, and transport measurements. These measurements show that the structural/magnetic phase transition found in pure BaFe_2As_2 at 134 K is suppressed monotonically by Ru doping but, unlike doping with TM=Co, Ni, Cu, Rh, or Pd, the coupled transition seen in the parent compound does not detectably split into two separate ones. Superconductivity is stabilized at low temperatures for x> 0.2 and continues through the highest doping levels we report. The superconducting region is domelike, with maximum T_c(~ 16.5 K) found around x～0.29. A phase diagram of temperature versus doping, based on electrical transport and magnetization measurements, has been constructed and compared to those of the Ba(Fe_(1-x)TM_x)_2As_2 (TM=Co, Ni, Rh, and Pd) series as well as to the temperature-pressure phase diagram for pure BaFe_2As_2. Suppression of the structural/magnetic phase transition as well as the appearance of superconductivity is much more gradual in Ru doping, as compared to Co, Ni, Rh, and Pd doping, and appears to have more in common with BaFe_2As_2 tuned with pressure; by plotting T_S/T_m and T_c as a function of changes in unit-cell dimensions, we find that changes in the c/a ratio, rather than changes in c, a, or V, unify the T(p) and T(x) phase diagrams for BaFe_2As_2 and Ba(Fe_(1-x)Ru_x)_2As_2, respectively.
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摘要 :
The effect of heat resistance on the performance of an air heat pump is analyzed in this article. This work extends the recent flurry of publications in heat pump performance by incorporating nonisentropic compression and expansio...
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The effect of heat resistance on the performance of an air heat pump is analyzed in this article. This work extends the recent flurry of publications in heat pump performance by incorporating nonisentropic compression and expansion. Relationships between heating load and pressure ratio and better COP and pressure ratio for the air heat pump are derived. They include the irreversibilities of heat resistance losses in hot-and cold-side heat exchangers and nonisentropic losses in the compression and expansion processes. Calculations show that there exists a maximum value of COP and That the heating load versus COP curve behaves like aparaboal, Unlike the monotonically decreasing behaviour for an endoeversible Heat pump.
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摘要 :
Heat capacity and anisotropic thermal expansion were measured for Ba(Fe_(1-x)Co_x)_2As_2 (x=0,0.038,0.074) single crystals. Thermal expansion is anisotropic and, in tetragonal phase, is significantly higher along the c axis. Previ...
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Heat capacity and anisotropic thermal expansion were measured for Ba(Fe_(1-x)Co_x)_2As_2 (x=0,0.038,0.074) single crystals. Thermal expansion is anisotropic and, in tetragonal phase, is significantly higher along the c axis. Previously reported phase transitions, including possibly split structural and magnetic for x=0.038, are clearly seen in both measurements. Uniaxial pressure derivatives of the superconducting transition temperature inferred from the Ehrenfest relation have opposite signs for in-plane and c-axis pressures for both Ba(Fe_(0.962)Co_(0.038))_2As_2 and Ba(Fe_(0.926)Co_(0.074))_2As_2, with the opposite sign of this anisotropy.
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摘要 :
Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe_2As_2 undergoes a pressure-induced t...
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Recent investigations of the superconducting iron-arsenide families have highlighted the role of pressure, be it chemical or mechanical, in fostering superconductivity. Here we report that CaFe_2As_2 undergoes a pressure-induced transition to a nonmagnetic volume "collapsed" tetragonal phase, which becomes superconducting at lower temperature. Spin-polarized total-energy calculations on the collapsed structure reveal that the magnetic Fe moment itself collapses, consistent with the absence of magnetic order in neutron diffraction.
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The effects of pressure up to -20 kbar on the structural phase transition of SrFe_2As_2 and lightly Sn-doped BaFe_2As_2 as well as on the superconducting transition temperature and upper critical field of (Ba_(0.55)K_(0.45))Fe_2As...
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The effects of pressure up to -20 kbar on the structural phase transition of SrFe_2As_2 and lightly Sn-doped BaFe_2As_2 as well as on the superconducting transition temperature and upper critical field of (Ba_(0.55)K_(0.45))Fe_2As_2 single crystals have been studied. All the transition temperatures decrease with pressure in an almost linear fashion. Under pressure, the upper critical-field curve H_(c2)(T) for (Ba_(0.55)K_(0.45))Fe_2As_2 shifts down in temperature to follow the zero-field T_c with very little change in slope. Composite P-T phase diagrams for three parent compounds AFe_2As_2 (A=Ba,Sr,Ca) are constructed and appear to be remarkably similar: (i) having a structural (antiferromagnetic) phase-transition line with a negative slope and (ii) showing signs of the emerging superconducting state at intermediate pressures.
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Zero-field (ZF) muon-spin-relaxation (μSR) measurements have revealed static commensurate magnetic order of Fe moments in NdFeAsO below T_N～ 135 K, with the ordered moment size nearly equal to that in LaFeAsO, and confirmed simi...
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Zero-field (ZF) muon-spin-relaxation (μSR) measurements have revealed static commensurate magnetic order of Fe moments in NdFeAsO below T_N～ 135 K, with the ordered moment size nearly equal to that in LaFeAsO, and confirmed similar behavior in BaFe_2As_2, In single crystals of superconducting Ba_(0.55)K_(0.45)Fe_2As_2, μSR spectra indicate static magnetism with incommensurate or short-ranged spin structure in ～70% of volume below T_N～80 K, coexisting with the remaining volume which exhibits a superfluid response below T_c～30 K.
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摘要 :
Inelastic neutron-scattering measurements of CaFe_2As_2 under applied hydrostatic pressure show that the antiferromagnetic spin fluctuations observed in the ambient pressure, paramagnetic, anil tetragonal (T) phase are strongly su...
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Inelastic neutron-scattering measurements of CaFe_2As_2 under applied hydrostatic pressure show that the antiferromagnetic spin fluctuations observed in the ambient pressure, paramagnetic, anil tetragonal (T) phase are strongly suppressed, if not absent, in the collapsed tetragonal (cT) phase. These results are consistent with a quenched Fe moment in the cT phase and the strong decrease in resistivity observed upon crossing the boundary from the T to cT phases. The suppression or absence of static antiferromagnetic order and dynamic spin fluctuations in the nonsuperconducting cT phase supports the notion of a coupling between spin fluctuations and superconductivity in the iron arsenides.
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Neutron diffraction measurements of a high-quality single crystal of CaFe_2As_2 are reported. A sharp transition was observed between the high-temperature tetragonal and low-temperature orthorhombic structures at T_s= 172.5 K (on ...
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Neutron diffraction measurements of a high-quality single crystal of CaFe_2As_2 are reported. A sharp transition was observed between the high-temperature tetragonal and low-temperature orthorhombic structures at T_s= 172.5 K (on cooling) and 173.5 K (on warming). Concomitant with the structural transition, the Fe moments order in a commensurate antiferromagnetic structure with a saturated moment of 0.80(5)μB/Fe directed along the orthorhombic a axis. The hysteresis of both the structural and magnetic transitions is 1 K between cooling and warming, consistent with previous thermodynamic, transport, and single-crystal x-ray studies and provides clear evidence of the coupling between the structural and magnetic transitions in this material.
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