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Register a new userStructural phase transition in Ba(Fe0.973Cr0.027)2As2 single crystals
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We present thermodynamic, structural and transport measurements on Ba(Fe0.973Cr0.027)2As2 single crystals. All measurements reveal sharp anomalies at ~ 112 K. Single crystal x-ray diffraction identifies the structural transition as a first order, from the high-temperature tetragonal I4/mmm to the low-temperature orthorhombic Fmmm structure, in contrast to an earlier report.
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Single crystalline Ba(Fe(1-x)TMx)2As2 (TM = Rh, Pd) series have been grown and characterized by structural, thermodynamic and transport measurements. These measurements show that the structural/magnetic phase transitions, found in pure BaFe2As2 at 134 K, are suppressed monotonically by the doping and that superconductivity can be stablized over a dome-like region. Temperature-composition (T-x) phase diagrams based on electrical transport and magnetization measurements are constructed and compared to those of the Ba(Fe(1-x)TMx)2As2 (TM = Co, Ni) series. Despite the generic difference between 3d and 4d shells and the specific, conspicuous differences in the changes to the unit cell parameters, the effects of Rh doping are exceptionally similar to the effects of Co doping and the effects of Pd doping are exceptionally similar to the effects of Ni doping. These data show that whereas the structural / antiferromagnetic phase transition temperatures can be parameterized by x and the superconducting transition temperature can be parameterized by some combination of x and e, the number of extra electrons associated with the TM doping, the transition temperatures of 3d- and 4d- doped BaFe2As2 can not be simply parameterized by the changes in the unit cell dimensions or their ratios.
The London penetration depth, $lambda(T)$, has been measured in several single crystals of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$. Thermodynamic, electromagnetic, and structural characterization measurements confirm that these crystals are of excellent quality. The observed low temperature variation of $lambda(T)$ follows a power-law, $Delta lambda (T) sim T^n$ with $n=2.4 pm 0.1$, indicating the existence of normal quasiparticles down to at least $0.02T_c$. This is in contrast to recent penetration depth measurements on single crystals of NdFeAsO$_{1-x}$F$_x$ and SmFeAsO$_{1-x}$F$_x$, which indicate an anisotropic but nodeless gap. We propose that a more three-dimensional character in the electronic structure of Ba(Fe$_{0.93}$Co$_{0.07}$)$_2$As$_2$ may lead to an anisotropic $s-$wave gap with point nodes that would explain the observed $lambda(T)$.
We performed high-field magnetotransport and magnetization measurements on a single crystal of the 122-phase iron pnictide Ba(Fe1-xCox)2As2. Unlike the HTS cuprates and 1111-phase oxypnictides, Ba(Fe1-xCox)2As2 showed practically no broadening of the resistive transitions under magnetic fields up to 45 T. The mass anisotropy gamma = Hc2ab/Hc2c deduced from the slopes of the upper critical field dHc2ab/dT = 4.9T/K and dHc2c/dT = 2.5T/K decreases from ~2 near Tc, to ~1.5 at lower temperatures. We observed the irreversibility field close to Hc2, and a rather unusual symmetric volume pinning force curve Fp(H) suggestive of strong pinning nano-structure.
The vibrational properties of $mathrm{CrI_3}$ single crystals were investigated using Raman spectroscopy and were analyzed with respect to the changes of the crystal structure. All but one mode are observed for both the low-temperature $Rbar{3}$ and the high-temperature C2/$m$ phase. For all observed modes the energies and symmetries are in good agreement with DFT calculations. The symmetry of a single-layer was identified as $pbar{3}1/m$. In contrast to previous studies we observe the transition from the $Rbar{3}$ to the $mathrm{C2}/m$ phase at 180,K and find no evidence for coexistence of both phases over a wide temperature range.
We study the effects of bismuth doping on the crystal structure and phase transitions in single crystals of the perovskite semiconductor methylammonium lead tribromide, MAPbBr3. By measuring temperature-dependent specific heat capacity (Cp) we find that, as Bi doping increases, the phase transition assigned to the cubic to tetragonal phase boundary decreases in temperature. Furthermore, after doping we observe one phase transition between 135 and 155 K, in contrast to two transitions observed in the undoped single crystal. These results appear strikingly similar to previously reported effects of mechanical pressure on perovskite crystal structure. Using X-ray diffraction, we show that the lattice constant decreases as Bi is incorporated into the crystal, as predicted by density functional theory (DFT). We propose that bismuth substitutional doping on the lead site is dominant, resulting in BiPb+ centers which induce compressive chemical strain that alters the crystalline phase transitions.
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