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Register a new userPhysical properties in hole-doped SrFe$_{2-x}$Cu$_x$As$_2$ single crystals
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We report the structural, magnetic and electronic transport properties of SrFe$_{2-x}$Cu$_x$As$_2$ single crystals grown by self-flux technique. SrCu$_2$As$_2$ and SrFe$_2$As$_2$ both crystallize in ThCr$_2$Si$_2$-type (122-type) structure at room temperature, but exhibit distinct magnetic and electronic transport properties. The x-ray photoelectron spectroscopy(XPS) Cu-2p core line position, resistivity, susceptibility and positive Hall coefficient indicate that SrCu$_2$As$_2$ is an sp-band metal with Cu in the 3d$^{10}$ electronic configuration corresponding to the valence state Cu$^{1+}$. The almost unchanged Cu-2p core line position in SrFe$_{2-x}$Cu$_x$As$_2$ compared with SrCu$_2$As$_2$ indicates that partial Cu substitutions for Fe in SrFe$_2$As$_2$ may result in hole doping rather than the expected electron doping. No superconductivity is induced by Cu substitution on Fe sites, even though the structural/spin density wave(SDW) transition is gradually suppressed with increasing Cu doping.
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We report the synthesis and basic physical properties of single crystals of CaFe2As2, an isostructural compound to BaFe2As2 which has been recently doped to produce superconductivity. CaFe2As2 crystalizes in the ThCr2Si2 structure with lattice parameters a = 3.907(4) A and c = 11.69(2) A. Magnetic susceptibility, resistivity, and heat capacity all show a first order phase transition at T_0 171 K. The magnetic susceptibility is nearly isotropic from 2 K to 350 K. The heat capacity data gives a Sommerfeld coefficient of 8.2 +- 0.3 mJ/molK2, and does not reveal any evidence for the presence of high frequency (> 300 K) optical phonon modes. The Hall coefficient is negative below the transition indicating dominant n-type carriers.
A series of high quality NaFe$_{1-x}$Cu$_x$As single crystals has been grown by a self-flux technique, which were systematically characterized via structural, transport, thermodynamic, and high pressure measurements. Both the structural and magnetic transitions are suppressed by Cu doping, and bulk superconductivity is induced by Cu doping. Superconducting transition temperature ($T_c$) is initially enhanced from 9.6 to 11.5 K by Cu doping, and then suppressed with further doping. A phase diagram similar to NaFe$_{1-x}$Co$_x$As is obtained except that insulating instead of metallic behavior is observed in extremely overdoped samples. $T_c$s of underdoped, optimally doped, and overdoped samples are all notably enhanced by applying pressure. Although a universal maximum transition temperature ($T_c^{max}$) of about 31 K under external pressure is observed in underdoped and optimally doped NaFe$_{1-x}$Co$_x$As, $T_c^{max}$ of NaFe$_{1-x}$Cu$_x$As is monotonously suppressed by Cu doping, suggesting that impurity potential of Cu is stronger than Co in NaFeAs. The comparison between Cu and Co doping effect in NaFeAs indicates that Cu serves as an effective electron dopant with strong impurity potential, but part of the doped electrons are localized and do not fill the energy bands as predicted by the rigid-band model.
We report both experimental and theoretical investigations of the physical properties of Ba$_mathrm{2}$Mn$_mathrm{2}$Sb$_mathrm{2}$O single crystals. This material exhibits a hexagonal structure with lattice constants: a = 4.7029(15) AA{} and c = 19.9401(27) AA{}, as obtained from powder X-ray diffraction measurements, and in agreement with structural optimization through density functional theory (DFT) calculations. The magnetic susceptibility and specific heat show anomalies at T$_mathrm{N}$ = 60 K, consistent with antiferromagnetic ordering. However, the magnitude of T$_mathrm{N}$ is significantly smaller than the Curie-Weiss temperature ($mid$$mathrm{Theta_{CW}}$$mid$ $approx$ 560 K), suggesting a magnetic system of reduced dimensionality. The temperature dependence of both the in-plane and out-of-plane resistivity changes from an activated at $T$ $>$ T$_mathrm{x}$ $sim$ 200 K to a logarithmic at $T$ $<$ T$_mathrm{x}$. Correspondingly, the magnetic susceptibility displays a bump at T$_mathrm{x}$. DFT calculations at the DFT + U level support the experimental observation of an antiferromagnetic ground state.
Single crystals of Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$, $x<0.37$, have been grown and characterized by structural, magnetic and transport measurements. These measurements show that the structural/magnetic phase transition found in pure BaFe$_2$As$_2$ 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 dome like, with maximum T$_c$ ($sim16.5$ K) found around $xsim 0.29$. 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$_x$)$_2$As$_2$ (TM=Co, Ni, Rh, Pd) series as well as to the temperature-pressure phase diagram for pure BaFe$_2$As$_2$. 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$_2$As$_2$ tuned with pressure; by plotting $T_S/T_m$ and $T_c$ as a function of changes in unit cell dimensions, we find that changed in the $c/a$ ratio, rather than changes in $c$, $a$ or V, unify the $T(p)$ and $T(x)$ phase diagrams for BaFe$_2$As$_2$ and Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ respectively.
The detailed optical properties have been determined for the iron-based materials $A$Fe$_2$As$_2$, where $A=,$Ca, Sr, and Ba, for light polarized in the iron-arsenic ($a-b$) planes over a wide frequency range, above and below the magnetic and structural transitions at $T_N =$ 172, 195, and 138 K, respectively. The real and imaginary parts of the complex conductivity are fit simultaneously using two Drude terms in combination with a series of oscillators. Above $T_N$, the free-carrier response consists of a weak, narrow Drude term, and a strong, broad Drude term, both of which show only a weak temperature dependence. Below $T_N$ there is a slight decrease of the plasma frequency but a dramatic drop in the scattering rate for the narrow Drude term, and for the broad Drude term there is a significant decrease in the plasma frequency, while the decrease in the scattering rate, albeit significant, is not as severe. The small values observed for the scattering rates for the narrow Drude term for $Tll{T_N}$ may be related to the Dirac cone-like dispersion of the electronic bands. Below $T_N$ new features emerge in the optical conductivity that are associated with the reconstruction Fermi surface and the gapping of bands at $Delta_1 simeq$ 45 $-$ 80 meV, and $Delta_2 simeq$ 110 $-$ 210 meV. The reduction in the spectral weight associated with the free carriers is captured by the gap structure, specifically, the spectral weight from the narrow Drude term appears to be transferred into the low-energy gap feature, while the missing weight from the broad term shifts to the high-energy gap.
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