arXiv subjects

N. Ni

Publications and source records attributed to N. Ni.

At least 37 records · Page 2Linked to original sources

arXiv2011

Systematics of the temperature-dependent inter-plane resistivity in Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ with T= Rh, Ni, and Pd

Temperature-dependent inter-plane resistivity, $ρ_c(T)$, was measured systematically as a function of transition metal substitution in the iron-arsenide superconductors Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$, $T$= Ni, Pd, Rh. The data are compared with the behavior found in Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$, revealing resistive signatures of pseudogap. In all compounds we find resistivity crossover at a characteristic pseudogap temperature $T^*$ from non-metallic to metallic temperature dependence on cooling. Suppression of $T^*$ proceeds very similar in cases of Ni and Pd doping and much faster than in similar cases of Co and Rh doping. In cases of Co and Rh doping an additional minimum in the temperature-dependent $ρ_c$ emerges for high dopings, when superconductivity is completely suppressed. These features are consistent with the existence of a charge gap covering part of the Fermi surface. The part of the Fermi surface affected by this gap is notably larger for Ni and Pd doped compositions than in Co and Rh doped compounds.

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arXiv2010

In-plane anisotropy of electrical resistivity in the strain-detwinned SrFe2As2

Intrinsic, in-plane anisotropy of electrical resistivity was studied on mechanically detwinned single crystals of SrFe$_2$As$_2$ above and below the temperature of the coupled structural/magnetic transition, $T_{\textrm{TO}}$. Resistivity is smaller for electrical current flow along the orthorhombic $a_o$ direction (direction of antiferromagnetically alternating magnetic moments) and is larger for transport along the $b_o$ direction (direction of ferromagnetic chains), which is similar to CaFe$_2$As$_2$ and BaFe$_2$As$_2$ compounds. A strongly first order structural transition in SrFe$_2$As$_2$ was confirmed by high-energy x-ray measurements, with the transition temperature, and character unaffected by moderate strain. For small strain levels, which are just sufficient to detwin the sample, we find a negligible effect on the resistivity above $T_{\textrm{TO}}$. With the increase of strain, the resistivity anisotropy starts to develop above $T_{\textrm{TO}}$, clearly showing the relation of anisotropy to an anomalously strong response to strain. Our study suggests that electronic nematicity cannot be observed in the FeAs based compounds in which the structural transition is strongly first order.

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arXiv2010

Superfluid Density and Field-Induced Magnetism in Ba(Fe1-xCox)2As2 and Sr(Fe1-xCox)2As2 Measured with Muon Spin Relaxation

We report muon spin rotation ($μ$SR) measurements of single crystal Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ and Sr(Fe$_{1-x}$Co$_x$)$_2$As$_2$. From measurements of the magnetic field penetration depth $λ$ we find that for optimally- and over-doped samples, $1/λ(T\to 0)^2$ varies monotonically with the superconducting transition temperature T$_{\rm C}$. Within the superconducting state we observe a positive shift in the muon precession signal, likely indicating that the applied field induces an internal magnetic field. The size of the induced field decreases with increasing doping but is present for all Co concentrations studied.

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arXiv2010

Local Magnetic Inhomogeneities in Lightly Doped BaFe$_2$As$_2$

We report $^{75}$As NMR measurements in BaFe$_2$As$_2$ doped with Ni. Like Co, Ni doping suppresses the antiferromagnetic and structural phase transitions and gives rise to superconductivity for sufficiently large Ni doping. The spin lattice relaxation rate diverges at $T_N$, with a critical exponent consistent with 3D ordering of local moments. In the ordered state the spectra quickly broaden inhomogeneously with doping. We extract the average size of the ordered moment as a function of doping, and show that a model in which the order remains commensurate but with local amplitude variations in the vicinity of the dopant fully explains our observations.

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arXiv2010

Physical and magnetic properties of Ba(Fe$_{1-x}$Ru$_x$)$_2$As$_2$ single crystals

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 $x\sim 0.29$. 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.

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arXiv2010

Paramagnetic Spin Correlations in CaFe2As2 Single Crystals

Magnetic correlations in the paramagnetic phase of CaFe2As2 (T_N=172 K) have been examined by means of inelastic neutron scattering from 180 K (~ 1.05 T_N) up to 300 K (~1.8 T_N). Despite the first-order nature of the magnetic ordering, strong but short-range antiferromagnetic (AFM) correlations are clearly observed. These correlations, which consist of quasi-elastic scattering centered at the wavevector Q_{AFM} of the low-temperature AFM structure, are observed up to the highest measured temperature of 300 K and at high energy transfer (E> 60 meV). The L dependence of the scattering implies rather weak interlayer coupling in the tetragonal c-direction corresponding to nearly two-dimensional fluctuations in the (ab) plane. The spin correlation lengths within the Fe layer are found to be anisotropic, consistent with underlying fluctuations of the AFM stripe structure. Similar to the cobalt doped superconducting BaFe2As2 compounds, these experimental features can be adequately reproduced by a scattering model that describes short-range anisotropic spin correlations with overdamped spin dynamics.

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arXiv2010

Temperature-doping phase diagrams for Ba(Fe1-xTMx)2As2(TM=Ni, Cu, Cu / Co) single crystals

Microscopic, structural, transport and thermodynamic measurements of single crystalline Ba(Fe1-xTMx)2As2 (TM = Ni and Cu) series, as well as two mixed TM = Cu / Co series, are reported. All the transport and thermodynamic measurements indicate that the structural and magnetic phase transitions at 134 K in pure BaFe2As2 are monotonically suppressed and increasingly separated in a similar manner by these dopants. In the Ba(Fe1-xNix)2As2 (x =< 0.072), superconductivity, with Tc up to 19 K, is stabilized for 0.024 =< x =< 0.072. In the Ba(Fe1-xCux)2As2 (x =< 0.356) series, although the structural and magnetic transitions are suppressed, there is only a very limited region of superconductivity: a sharp drop of the resistivity to zero near 2.1 K is found only for the x = 0.044 samples. In the Ba(Fe1-x-yCoxCuy)2As2 series, superconductivity, with Tc values up to 12 K (x ~ 0.022 series) and 20 K (x ~ 0.047 series), is stabilized. Quantitative analysis of the detailed temperature-dopant concentration (T-x) and temperature-extra electrons (T-e) phase diagrams of these series shows that there exists a limited range of the number of extra electrons added, inside which the superconductivity can be stabilized if the structural and magnetic phase transitions are suppressed enough. Moreover, comparison with pressure-temperature phase diagram data, for samples spanning the whole doping range, further reenforces the conclusion that suppression of the structural / magnetic phase transition temperature enhances Tc on the underdoped side, but for the overdoped side Tcmax is determined by e. Therefore, by choosing the combination of dopants that are used, we can adjust the relative positions of the upper phase lines (structural and magnetic phase transitions) and the superconducting dome to control the occurrence and disappearance of the superconductivity in transition metal, electron-doped BaFe2As2.

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arXiv2010

Evidence from neutron diffraction for superconductivity in the stabilized tetragonal phase of CaFe2As2 under uniaxial pressure

CaFe2As2 single crystals under uniaxial pressure applied along the c axis exhibit the coexistence of several structural phases at low temperatures. We show that the room-temperature tetragonal phase is stabilized at low temperatures for pressures above 0.06 GPa, and its weight fraction attains a maximum in the region where superconductivity is observed under applied uniaxial pressure. Simultaneous resistivity measurements strongly suggest that this phase is responsible for the superconductivity in CaFe2As2 found below 10 K in samples subjected to nonhydrostatic pressure conditions.

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arXiv2010

Nodes in the gap structure of the iron-arsenide superconductor Ba(Fe_{1-x}Co_x)_2As_2 from c-axis heat transport measurements

The thermal conductivity k of the iron-arsenide superconductor Ba(Fe_{1-x}Co_x)_2As_2 was measured down to 50 mK for a heat current parallel (k_c) and perpendicular (k_a) to the tetragonal c axis, for seven Co concentrations from underdoped to overdoped regions of the phase diagram (0.038 < x < 0.127). A residual linear term k_c0/T is observed in the T = 0 limit when the current is along the c axis, revealing the presence of nodes in the gap. Because the nodes appear as x moves away from the concentration of maximal T_c, they must be accidental, not imposed by symmetry, and are therefore compatible with an s_{+/-} state, for example. The fact that the in-plane residual linear term k_a0/T is negligible at all x implies that the nodes are located in regions of the Fermi surface that contribute strongly to c-axis conduction and very little to in-plane conduction. Application of a moderate magnetic field (e.g. H_c2/4) excites quasiparticles that conduct heat along the a axis just as well as the nodal quasiparticles conduct along the c axis. This shows that the gap must be very small (but non-zero) in regions of the Fermi surface which contribute significantly to in-plane conduction. These findings can be understood in terms of a strong k dependence of the gap Delta(k) which produces nodes on a Fermi surface sheet with pronounced c-axis dispersion and deep minima on the remaining, quasi-two-dimensional sheets.

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arXiv2010

London penetration depth in Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ (T=Co, Ni) superconductors irradiated with heavy ions

Irradiation with Pb ions was used to study the effect of disorder on the in-plane London penetration depth, $λ(T)$, in single crystals of Ba(Fe$_{1-x}$T$_x$)$_2$As$_2$ (T=Co, Ni). An increase of the irradiation dose results in a monotonic decrease of the superconducting transition temperature, $T_c$, without affecting much the transition width. In both Co and Ni doped systems we find a power-law behavior, $Δλ(T) \propto T^n$, with the exponent $n$ systematically decreasing with the increase of disorder. This observation, supported by the theoretical analysis, conclusively points to a nodeless $s^\pm$ state with pairbreaking impurity scattering (interband) with strength being intermediate between Born and unitary limits.

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arXiv2010

Unconventional pairing in the iron arsenide superconductors

We use magnetic long range order as a tool to probe the Cooper pair wave function in the iron arsenide superconductors. We show theoretically that antiferromagnetism and superconductivity can coexist in these materials only if Cooper pairs form an unconventional, sign-changing state. The observation of coexistence in Ba(Fe$_{1-x}$Co$_{x}$)$_{2}$As$_{2}$ then demonstrates unconventional pairing in this material. The detailed agreement between theory and neutron diffraction experiments, in particular for the unusual behavior of the magnetic order below $T_{c}$, demonstrates the robustness of our conclusions. Our findings strongly suggest that superconductivity is unconventional in all members of the iron arsenide family.

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arXiv2010

Neutron scattering and scaling behavior in URu2Zn20 and YbFe2Zn20

The dynamic susceptibility chi"(deltaE), measured by inelastic neutron scattering measurements, shows a broad peak centered at Emax = 16.5 meV for the cubic actinide compound URu2Zn20 and 7 meV at the (1/2, 1/2, 1/2) zone boundary for the rare earth counterpart compound YbFe2Zn20. For URu2Zn20, the low temperature susceptibility and magnetic specific heat coefficient gamma = Cmag/T take the values chi = 0.011 emu/mole and gamma = 190 mJ/mole-K2 at T = 2 K. These values are roughly three times smaller, and Emax is three times larger, than recently reported for the related compound UCo2Zn20, so that chi and gamma scale inversely with the characteristic energy for spin fluctuations, Tsf = Emax/kB. While chi(T), Cmag(T), and Emax of the 4f compound YbFe2Zn20 are very well described by the Kondo impurity model, we show that the model works poorly for URu2Zn20 and UCo2Zn20, suggesting that the scaling behavior of the actinide compounds arises from spin fluctuations of itinerant 5f electrons.

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arXiv2010

Uniaxial strain detwinning of CaFe2As2 and BaFe2As2: optical and transport study

TThe parent compounds of iron-arsenide superconductors, $A$Fe$_{2}$As$_{2}$ ($A$=Ca, Sr, Ba), undergo a tetragonal to orthorhombic structural transition at a temperature $T_{\mathrm{TO}}$ in the range 135 to 205K depending on the alkaline earth element. Below $T_{\mathrm{TO}}$ the free standing crystals split into equally populated structural domains, which mask intrinsic, in-plane, anisotropic properties of the materials. Here we demonstrate a way of mechanically detwinning CaFe$_{2}$As$_{2}$ and BaFe$_{2}$As$_{2}$. The detwinning is nearly complete, as demonstrated by polarized light imaging and synchrotron $X$-ray measurements, and reversible, with twin pattern restored after strain release. Electrical resistivity measurements in the twinned and detwinned states show that resistivity, $ρ$, decreases along the orthorhombic $a_{o}$-axis but increases along the orthorhombic $b_{o}$-axis in both compounds. Immediately below $T_{\mathrm{TO}}$ the ratio $ρ_{bo}/ ρ_{ao}$ = 1.2 and 1.5 for Ca and Ba compounds, respectively. Contrary to CaFe$_{2}$As$_{2}$, BaFe$_{2}$As$_{2}$ reveals an anisotropy in the nominally tetragonal phase, suggesting that either fluctuations play a larger role above $T_{\mathrm{TO}}$ in BaFe$_{2}$As$_{2}$ than in CaFe$_{2}$As$_{2}$, or that there is a higher temperature crossover or phase transition.

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arXiv2010

Suppression of antiferromagnetic order and orthorhombic distortion in superconducting Ba(Fe0.961Rh0.039)2As2

Neutron diffraction and high-resolution x-ray diffraction studies find that, similar to the closely related underdoped Ba(Fe[1-x]Cox)2As2 superconducting compounds, Ba(Fe0.961Rh0.039)2As2 shows strong evidence of competition and coexistence between superconductivity and antiferromagnetic order below the superconducting transition, Tc = 14 K. The transition temperatures for both the magnetic order and orthorhombic distortion are in excellent agreement with those inferred from resistivity measurements, and both order parameters manifest a distinct decrease in magnitude below Tc. These data suggest that the strong interaction between magnetism and superconductivity is a general feature of electron-doped Ba(Fe[1-x]TMx)2As2 superconductors (TM = Transition Metal).

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arXiv2010

Dispersion of the superconducting spin resonance in underdoped and antiferromagnetic Ba(Fe1-xCox)2As2

Inelastic neutron scattering measurements have been performed on underdoped Ba(Fe1-xCox)2As2 (x = 4.7%) where superconductivity and long-range antiferromagnetic (AFM) order coexist. The broad magnetic spectrum found in the normal state develops into a magnetic resonance feature below TC that has appreciable dispersion along c-axis with a bandwidth of 3-4 meV. This is in contrast to the optimally doped x = 8.0% composition, with no long-range AFM order, where the resonance exhibits a much weaker dispersion [see Lumsden et al. Phys. Rev. Lett. 102, 107005 (2009)]. The results suggest that the resonance dispersion arises from interlayer spin correlations present in the AFM ordered state.

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arXiv2010

Anomalous suppression of the orthorhombic distortion in superconducting Ba[Fe(1-x)Cox]2As2

High-resolution x-ray diffraction measurements reveal an unusually strong response of the lattice to superconductivity in Ba[Fe(1-x)Cox]2As2. The orthorhombic distortion of the lattice is suppressed and, for Co-doping near x = 0.063, the orthorhombic structure evolves smoothly back to a tetragonal structure. We propose that the coupling between orthorhombicity and superconductivity is indirect and arises due to the magneto-elastic coupling, in the form of emergent nematic order, and the strong competition between magnetism and superconductivity.

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arXiv2010

Doping dependence of heat transport in the iron-arsenide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$: from isotropic to strongly $k$-dependent gap structure

The temperature and magnetic field dependence of the in-plane thermal conductivity $κ$ of the iron-arsenide superconductor Ba(Fe$_{1-x}$Co$_x$)$_2$As$_2$ was measured down to $T \simeq 50$ mK and up to $H = 15$ T as a function of Co concentration $x$ in the range 0.048 $ \leq x \leq $ 0.114. In zero magnetic field, a negligible residual linear term in $κ/T$ as $T \to 0$ at all $x$ shows that there are no zero-energy quasiparticles and hence the superconducting gap has no nodes in the $ab$-plane anywhere in the phase diagram. However, the field dependence of $κ$ reveals a systematic evolution of the superconducting gap with doping $x$, from large everywhere on the Fermi surface in the underdoped regime, as evidenced by a flat $κ(H)$ at $T \to 0$, to strongly $k$-dependent in the overdoped regime, where a small magnetic field can induce a large residual linear term, indicative of a deep minimum in the gap magnitude somewhere on the Fermi surface. This shows that the superconducting gap structure has a strongly $k$-dependent amplitude around the Fermi surface only outside the antiferromagnetic/orthorhombic phase.

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arXiv2010

The magnetic form factor of iron in SrFe2As2

The iron magnetic form factor in SrFe2As2 has been determined by neutron diffraction and by density functional theory (DFT). As noted previously, the magnitude of the calculated moment using DFT is sensitive to the Fe-As distance. However, the shape of the calculated form factor is practically insensitive to the Fe-As distance, and further we show that the form factor closely resembles that of bcc iron, and agrees well with experiment. The spin density exhibits some anisotropy due to geometry and As hybridization.

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