In-plane Resistivity Measurements Research Articles

A -type antiferromagnetic order in the Zintl-phase insulator EuZn2P2

Zintl phases, containing strongly covalently bonded frameworks with separate ionically bonded ions, have emerged as a critical materials family in which to couple magnetism and strong spin-orbit coupling to drive diverse topological phases of matter. Here we report the single-crystal synthesis, magnetic, thermodynamic, transport, and theoretical properties of the Zintl compound EuZn2P2 that crystallizes in the anti-La2O3 P-3m1 structure, containing triangular layers of Eu2+ ions. In-plane resistivity measurements reveal insulating behavior with an estimated bandgap of Eg=0.11eV. Comparing Eu magnetic ordering temperatures across trigonal EuM2X2 (M=divalent metal, X=pnictide) shows that EuZn2P2 exhibits the highest ordering temperature, with variations in TN correlating with changes in expected dipolar interaction strengths within and between layers and independent of the magnitude of electrical conductivity. These results provide experimental validation of the cytochemical intuition that the cation Eu2+ layers and the anionic (M2X2)2- framework can be treated as electronically distinct subunits, enabling further predictive materials design.

Tuning Jahn–Teller distortion and electron localization of LaMnO3 epitaxial films via substrate temperature

High-quality epitaxial LaMnO3 films have been grown on (001)-oriented LaAlO3 substrates at different substrate temperatures by pulsed laser deposition. The layer-by-layer growth is indicated by oscillations of reflection high-energy electron diffraction. Raman spectra together with in-plane resistivity measurements reveal that the degree of Jahn–Teller (JT) distortion can be well controlled by the substrate temperature during film deposition. The JT distortion-related/induced electron localization is studied by fitting temperature-dependent resistivity with a three-dimensional variable range hopping model. It is found that the larger JT distortion leads to a stronger localization of electrons. This study might pave the way for further study of JT interaction and highly correlated electronic states in perovskites.

Different behavior of upper critical field in Fe1−xSe single crystals**Project supported by the National Natural Science Foundation of China (Grant Nos. 11888101 and 11834016), the National Key Research and Development Program of China (Grant Nos. 2017YFA0303003 and 2016YFA0300300), and the Strategic Priority Research Program and Key Research Program of Frontier Sciences of the Chinese Academy of Sciences (Grant Nos. QYZDY-SSW-SLH001

The temperature dependences of upper critical field (Hc2) for a series of iron-deficient Fe1−xSe single crystals are obtained from the measurements of in-plane resistivity in magnetic fields up to 9 T and perpendicular to the ab plane. For the samples with lower superconducting transition temperature Tc (<7.2 K), the temperature dependence of Hc2 is appropriately described by an effective two-band model. For the samples with higher Tc (≳ 7.2 K), the temperature dependence can also be fitted by a single-band Werthamer–Helfand–Hohenberg formula, besides the two-band model. Such a Tc-dependent change in Hc2(T) behavior is discussed in connection with recent related experimental results, showing an inherent link between the changes of intrinsic superconducting and normal state properties in the FeSe system.

Doping evolution of the anisotropic upper critical fields in the iron-based superconductor Ba1−xKxFe2As2

In-plane resistivity measurements as a function of temperature and magnetic field up to 35 T with precise orientation within the crystallographic $ac$ plane were used to study the upper critical field ${H}_{c2}$ of the hole-doped iron-based superconductor ${\mathrm{Ba}}_{1\ensuremath{-}x}{\mathrm{K}}_{x}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$. Compositions of the samples studied spanned from under- doped $x=0.17$ $({T}_{c}=12$ K) and $x=0.22$ $({T}_{c}=20$ K), both in the coexistence range of stripe magnetism and superconductivity, through optimal doping $x=0.39$ $({T}_{c}=38.4$ K) and $x=0.47$ $({T}_{c}=37.2$ K), to overdoped $x=0.65$ $({T}_{c}=22$ K) and $x=0.83$ $({T}_{c}=10$ K). We find notable doping asymmetry of the shapes of the anisotropic ${H}_{c2}(T)$, suggesting the important role of paramagnetic limiting effects in the $H\ensuremath{\parallel}a$ configuration in overdoped compositions and multiband effects in underdoped compositions.

Magnetization and transport properties of single crystalline RPd2P2 (R=Y, La–Nd, Sm–Ho, Yb)

Single crystals of RPd2P2 (R=Y, La–Nd, Sm–Ho, Yb) were grown out of a high temperature solution rich in Pd and P and characterized by room-temperature powder X-ray diffraction, anisotropic temperature- and field-dependent magnetization and temperature-dependent in-plane resistivity measurements. In this series, YPd2P2 and LaPd2P2 YbPd2P2 (with Yb2+) are non-local-moment bearing. Furthermore, YPd2P2 and LaPd2P2 are found to be superconducting with Tc≃0.75 and 0.96K respectively. CePd2P2 and PrPd2P2 magnetically order at low temperature with a ferromagnetic component along the crystallographic c-axis. The rest of the series manifest low temperature antiferromagnetic ordering. EuPd2P2 has Eu2+ ions and both EuPd2P2 and GdPd2P2 have isotropic paramagnetic susceptibilities consistent with L=0 and J=S=72 and exhibit multiple magnetic transitions. For R=Eu–Dy, there are multiple, T>1.8K transitions in zero applied magnetic field and for R=Nd, Eu, Gd, Tb, and Dy there are clear metamagnetic transitions at T=2.0K for H<55kOe. Strong anisotropies arising mostly from crystal electric field (CEF) effects were observed for most magnetic rare earths with L≠0. The experimentally estimated CEF parameters B20 were calculated from the anisotropic paramagnetic θab and θc values and compared to theoretical trends across the rare earth series. The ordering temperatures as well as the polycrystalline averaged paramagnetic Curie–Weiss temperature, θave, were extracted from magnetization and resistivity measurements, and compared to the de-Gennes factor.

Tilted vortex lattice in irradiate Bi2Sr2CaCu2O8+δ single crystals

In order to enlighten the structure of vortex matter in irradiated layered Bi2Sr2CaCu2O8+δ single crystals, the interaction of Josephson vortices and pancake vortices in was investigated by means of the local ac-magnetic permeability measurements by using the miniature local coils, while vortex matter in pristine crystals was studied by in-plane resistivity measurements. The transition anomaly, separating the strong pinning phase and the weak pinning vortex phase was found by both techniques deep in the vortex solid phase solid near ab-plane, indicating crossover from the vortex chains + lattice phase to tilted vortex chains phase. While the columnar defects affect strongly the first-order vortex-lattice melting transition, the magnetic permeability anomaly, associated with the crossover from vortex chains + lattice phase to tilted lattice, is surprisingly still clear, deep in the vortex solid phase. However, the stronger columnar defects eventually affect the crossover anomaly that it disappears too.

Resistivity and magnetoresistance of FeSe single crystals under helium-gas pressure

We present temperature-dependent in-plane resistivity measurements on FeSe single crystals under He-gas pressure up to 800 MPa and magnetic fields $B \leq$ 10 T. A sharp phase transition anomaly is revealed at the tetragonal-to-orthorhombic transition at $T_s$ slightly below 90 K. $T_s$ becomes reduced with increasing pressure in a linear fashion at a rate d$T_{s}$/d$P$ $\simeq$ -31 K/GPa. This is accompanied by a $P$-linear increase of the superconducting transition temperature at $T_c \sim$ 8.6 K with d$T_{c}$/d$P$ $\simeq$ +5.8 K/GPa. Pressure studies of the normal-state resistivity highlight two distinctly different regimes: for $T > T_s$, i.e., in the tetragonal phase, the in-plane resistivity changes strongly with pressure. This contrasts with the state deep in the orthorhombic phase at $T \ll T_s$, preceding the superconducting transition. Here a $T$-linear resistivity is observed the slope of which does not change with pressure. Resistivity studies in varying magnetic fields both at ambient and finite pressure reveal clear changes of the magnetoresistance, $\Delta \rho \propto B^{2}$, upon cooling through $T_s$. Our data are consistent with a reconstruction of the Fermi surface accompanying the structural transition.

Simultaneous vanishing of nematic electronic state and structural orthorhombicity inNaFe1−xCoxAssingle crystals

We have carried out in-plane resistivity measurements under a uniaxial pressure in NaFe$_{1-x}$Co$_x$As single crystals. A clear distinction of the in-plane resistivity $\rho_a$ and $\rho_b$ with the uniaxial pressure along $b$-axis was discovered in the parent and underdoped regime with the doping level up to about x=0.025$\pm$0.002. From the deviating point of $\rho_a$ and $\rho_b$, and the unique kinky structure of resistivity together with the published data we determined the temperatures for the nematic, structural and antiferromagnetic transitions. It is clearly shown that the nematic electronic state vanishes simultaneously with the structural transition. The antiferromagnetic state disappears however at a lower doping level. Our results, in combination with the data in BaFe$_{2-x}$Co$_x$As$_2$, indicate a close relationship between nematicity and superconductivity.

Single-crystal Growth of Underdoped Bi-2223

To investigate the origin of the enhanced Tc (≈ 110K) of the trilayer cuprate superconductor Bi2Sr2Ca2Cu3O10+δ (Bi-2223), its underdoped single crystals are a critical requirement. Here, we demonstrate the first successful in-plane resistivity measurements of heavily underdoped Bi-2223 (zero-resistivity temperatures ≈ 20∼35K). Detailed crystal growth methods, the annealing process, as well as X-ray diffraction (XRD) and magnetic susceptibility measurement results are also reported.

In-Plane Magnetoresistance Obeys Kohler’s Rule in the Pseudogap Phase of Cuprate Superconductors

We report in-plane resistivity (ρ) and transverse magnetoresistance (MR) measurements for underdoped HgBa(2)CuO(4+δ) (Hg1201). Contrary to the long-standing view that Kohler's rule is strongly violated in underdoped cuprates, we find that it is in fact satisfied in the pseudogap phase of Hg1201. The transverse MR shows a quadratic field dependence, δρ/ρ(0)=aH(2), with a(T)∝T(-4). In combination with the observed ρ∝T(2) dependence, this is consistent with a single Fermi-liquid quasiparticle scattering rate. We show that this behavior is typically masked in cuprates with lower structural symmetry or strong disorder effects.

Strong and nonmonotonic temperature dependence of Hall coefficient in superconductingKxFe2−ySe2single crystals

In-plane resistivity, magnetoresistance, and Hall-effect measurements have been conducted on quenched ${\text{K}}_{x}{\text{Fe}}_{2\ensuremath{-}y}{\text{Se}}_{2}$ single crystals in order to analyze the normal-state transport properties. It is found that Kohler's rule is well obeyed below about 80 K, but clearly violated above 80 K. Measurements of the Hall coefficient reveal a strong but nonmonotonic temperature dependence with a maximum at about 80 K, in contrast to any other FeAs-based superconductors. With the two-band model analysis on the Hall coefficient, we conclude that a gap may open below 65 K. The data above 65 K are interpreted as a temperature-induced crossover from a metallic state at a low temperature to an orbital-selective Mott phase at a high temperature. This is consistent with the recent data of angle-resolved photoemission spectroscopy. These results call for a refined theoretical understanding, especially when the hole pockets are absent or become trivial in ${\text{K}}_{x}{\text{Fe}}_{2\ensuremath{-}y}{\text{Se}}_{2}$ superconductors.

Crossover from bad to good metal in BaFe2(As1−xPx)2induced by isovalent P substitution

Bad metallic behavior or incoherent charge dynamics is characteristic of the normal state of high-transition-temperature copper-oxide superconductors. A parent compound of iron-based superconductors, BaFe${}_{2}$As${}_{2}$, is also a bad metal in the high-temperature phase, whereas its phosphorous counterpart, BaFe${}_{2}$P${}_{2}$, is a good metal. Combined measurements of in-plane resistivity and optical conductivity for BaFe${}_{2}$(As${}_{1\ensuremath{-}x}$P${}_{x}$)${}_{2}$ make it possible to disentangle the coherent channel from the incoherent one in the charge transport. The isovalent P substitution promotes coherent motion of carriers without changing the balance of the electron and hole Fermi-surface volume and hence transforms a bad metal into a good metal.

Upper critical field of high-quality single crystals of KFe2As2

Measurements of temperature-dependent in-plane resistivity, $\rho(T)$, were used to determine the upper critical field and its anisotropy in high quality single crystals of stoichiometric iron arsenide superconductor KFe$_2$As$_2$. The crystals were characterized by residual resistivity ratio, $\rho(300K)/\rho(0)$ up to 3000 and resistive transition midpoint temperature, $T_c$=3.8 K, significantly higher than in previous studies on the same material. We find increased $H_{c2}(T)$ for both directions of the magnetic field, which scale with the increased $T_c$. This unusual linear $H_{c2}(T_c)$ scaling is not expected for orbital limiting mechanism of the upper critical field in clean materials.

Electrical properties of thermoelectric cobalt Ca3Co4O9 epitaxial heterostructures

Heterostructures fabricated from layered cobalt oxides offer substantial advantages for thermoelectric applications. C-axis-oriented Ca3Co4O9 (CCO) thin films on SrTiO3 substrates and Ca3Co4O9/SrTi0.993Nb0.007O3 p-n heterojunctions were fabricated by pulsed laser deposition. The measurements of in-plane resistivity, thermopower, and magnetic properties performed on the Ca3Co4O9 thin films were found to be comparable to ab-plane those of the single crystals due to good orientation of the films. The temperature dependence of the electrical transport properties of Ca3Co4O9/SrTi0.993Nb0.007O3 p-n heterojunction was also investigated. The junction shows two distinctive transport mechanisms at different temperature regimes under forward bias: tunneling across the Schottky barrier in the temperature range of 100-380 K, and tunneling mechanism at low bias and thermal emission mechanism at high bias between 10 and 100 K. However, for the case of low reverse bias, the trap assisted tunneling process should be considered for the leakage current. Negative magnetoresistance effect is observed at low temperatures, related to the electron spin-dependent scattering and the interface resistance of the heterostructures.

Angular-dependent upper critical field of overdoped Ba(Fe1−xNix)2As2

In-plane resistivity measurements as a function of temperature, magnetic field and its orientation with respect to the crystallographic $ab-$plane were used to study the upper critical field, $H_{c2}$, of two overdoped compositions of the iron-based superconductor Ba(Fe$_{1-x}$Ni$_x$)$_2$As$_2$, $x=$0.054 and $x$=0.072. Measurements were performed using precise alignment (with accuracy less than 0.1$^o$) of magnetic field with respect to the Fe-As-plane. The dependence of the $H_{c2}$ on angle $\theta$ between the field and the $ab$-plane was measured in isothermal conditions in a broad temperature range. We found that the shape of $H_{c2}(\theta)$, clearly deviates from Ginzburg-Landau functional form.

Structural and physical properties of SrMn2As2

SrMn2As2 single crystals were grown by the Sn flux method. Structural features of these crystals were characterized by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The XRD results show that the single crystal has a rhombohedral structure and grows along the c-axis direction. The microstructure and layered structural features of this material have been examined by SEM and high-resolution TEM observations. The measurements of in-plane resistivity as a function of temperature demonstrate that SrMn2As2 undergoes a phase transition of semiconductor–insulator at a low temperature; the active energies are estimated to be Δ=0.64 and 0.29eV for two distinct regions. Magnetic measurements show a clear antiferromagnetic (AFM) transition at about TN=125K. Therefore, the SrMn2As2 material is an AFM insulator at low temperatures and could be a potential parent compound for superconductors.

Vortex glass scaling in Pb-doped Bi-2223 single crystal

We report on study of the vortex liquid in Pb-doped Bi-2223 single crystal using the in-plane resistivity measurements as a function of temperature and magnetic field up to 6T applied perpendicular to CuO planes. Below Tc at the upper part of superconducting transition we found Arrhenius-like resistivity behavior. With further temperature decrease close to resistivity onset resistivity shows power law dependence on temperature signaling approaching vortex-glass transition. The critical exponents nu(z-1)=4.6 plus-minus 0.5 are found to be field independent within experimental errors. We also present magnetic phase diagram defining region of nonzero critical current for Pb-doped Bi-2223 single crystal.

Charge ordering and elastic anomalies in NaxCoO2

Abstract We report in-plane resistivity and elastic constant C33 measurements on the Na0.8CoO2 and Na0.5CoO2 systems. An ordering transition is found at T0=280 K for Na0.8CoO2. The temperature dependence of the elastic constant C33 propagating perpendicular to the CoO2 layers is interpreted, in a phenomenological approach, as being due to the anharmonicity of atomic vibrations. The effective Gruneisen parameter γeff deduced directly from the temperature dependence of C33 exhibits important changes at the charge ordering, magnetic and metal-insulator phase transitions.

Possible microscopic phase separation in the overdoped regime of La2−xSrxCuO4 studied by the magnetic susceptibility and electrical resistivity

Precise measurements of the magnetic susceptibility and the in-plane electrical resistivity in the overdoped regime of La2−xSrxCuO4 have been carried out using single crystals of good quality, in order to investigate the inhomogeneity of the superconductivity in the overdoped regime of high-Tc cuprates. From the magnetic-susceptibility measurements, it has been found that the Meissner volume fraction decreases with increasing x, indicating that phase separation into superconducting (SC) and normal-state regions takes place in the overdoped regime. From the in-plane resistivity measurements, it has been found that the SC transition width becomes broad with increasing x, which is possibly due to the increase of the volume fraction of the proximity region between SC and normal-state regions in the phase-separated CuO2 plane. It appears that microscopic phase separation takes place in the overdoped regime and that the proximity region becomes dominant with increase x.

Unconventional Hall effect in orientedCa3Co4O9thin films

Transport properties of the good thermoelectric misfit oxide Ca$_3$Co$_4$O$_9$ are examined. In-plane resistivity and Hall resistance measurements were made on epitaxial thin films which were grown on {\it c}-cut sapphire substrates using the pulsed laser deposition technique. Interpretation of the in-plane transport experiments relates the substrate-induced strain in the resulting film to single crystals under very high pressure ($\sim$ 5.5 GPa) consistent with a key role of strong electronic correlation. They are confirmed by the measured high temperature maxima in both resistivity and Hall resistance. While hole-like charge carriers are inferred from the Hall effect measurements over the whole investigated temperature range, the Hall resistance reveals a non monotonic behavior at low temperatures that could be interpreted with an anomalous contribution. The resulting unconventional temperature dependence of the Hall resistance seems thus to combine high temperature strongly correlated features above 340 K and anomalous Hall effect at low temperature, below 100 K.