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In Co$_x$NbSe$_2$, crystal symmetry, and cobalt site occupation drive the formation of two distinct magnetic phases. At $x = 1/4$, the centrosymmetric structure ($P$6$_3$/$mmc$) promotes Co-Co interactions leading to the formation of an $A$-type antiferromagnetic structure phase with a transition temperature of $T_N^A$ = 169 K. At $x = 1/3$, the non-centrosymmetric structure ($P$6$_3$22) induces a lower-temperature magnetic phase with $T_N^S$ = 28 K. We report the coexistence of both substructures within a superlattice, with a nuclear propagation vector of (1/3, 1/3, 0) relative to the host lattice. Single crystals of Co$_{0.28}$NbSe$_2$ exhibit both magnetic transitions, with $T_N^A$ corresponding to the $x \sim 1/4$ phase and $T_N^S$ corresponding to the $x \sim 1/3$ phase. Magnetic susceptibility and specific heat measurements confirm these transitions, although only the high-temperature $T_N^A$ phase significantly affects resistivity. We successfully isolate each phase in powder samples, while single crystals with an intercalation ratio of $x = 0.28$ display the coexistence of both phases in a single sample. Using single-crystal neutron diffraction, we solved the magnetic structure of the high-temperature centrosymmetric phase ($T_N^A$), and neutron powder diffraction revealed the double-$q$ magnetic structure of the low-temperature noncentrosymmetric phase ($T_N^S$)

We present the magnetic behavior and resulting transport properties of TaSe$_2$ when intercalated by magnetically active cobalt cations. Acting as the layered host, TaSe$_2$ is a transition metal dichalcogenide (TMD) that adopts the 2H-polytype. We find through our single crystal and powder diffraction studies that we can prepare the stoichiometry Co$_{0.25}$TaSe$_2$, which crystallizes in the centrosymmetric space group $P6_3/mmc$. From magnetic susceptibility and x-ray photoelectron spectroscopy measurements, we find a transition consistent with antiferromagnetic order below the temperature $T_N$ = 173 K and Co$^{2+}$ in the high-spin state. Neutron powder diffraction and specific heat measurements, however, point to a much smaller than anticipated ordered moment in this sample. From the neutron results, the magnetic structure can be described as an A-type antiferromagnet with an ordered moment size of 1.35(11) $\mu_B$ per Co cation. The direction of the moments are all long the c-axis, which is consistent with the magnetization and susceptibility studies showing this direction to be the easy axis. Interestingly, we find that a weak and subtle ferromagnetic component appears to exist along the $ab$-plane of the Co$_{0.25}$TaSe$_2$ crystals. We place the results of this work in the context of other magnetic-ion intercalated TMDs, especially those of Ta and Nb., Comment: 13 pages, 8 figures

FeSi is a curious example of a $d$-electron system that manifests many of the same phenomena associated with $f$-electron Kondo insulators, including conducting surface states with potentially non-trivial topology. Here we investigate the magnetization and magnetotransport of these surface states and how a 2D ferromagnetic state at the surface of FeSi influences the surface conductivity. We confirm the 2D ferromagnetism via a systematic study of magnetization on groups of filtered fragments with increasing surface area-to-volume ratios, identifying characteristic temperatures and magnetic fields associated with the ordered state. The paramagnetic to ferromagnetic transition appears broadened, suggesting disorder, which allows spin fluctuations to manifest up to at least 9 T at 2 K. This highlights the need to understand the relation between the disorder of the 2D ferromagnetism and the surface conductivity in FeSi., Comment: 8 pages, 5 figures

BaNi$_2$As$_2$ is a structural analog of the pnictide superconductor BaFe$_2$As$_2$, which, like the iron-based superconductors, hosts a variety of ordered phases including charge density waves (CDWs), electronic nematicity, and superconductivity. Upon isovalent Sr substitution on the Ba site, the charge and nematic orders are suppressed, followed by a sixfold enhancement of the superconducting transition temperature ($T_c$). To understand the mechanisms responsible for enhancement of $T_c$, we present high-resolution angle-resolved photoemission spectroscopy (ARPES) measurements of the Ba$_{1-x}$Sr$_{x}$Ni$_2$As$_2$ series, which agree well with our density functional theory (DFT) calculations throughout the substitution range. Analysis of our ARPES-validated DFT results indicates a Lifshitz transition and reasonably nested electron and hole Fermi pockets near optimal substitution where $T_c$ is maximum. These nested pockets host Ni $d_{xz}$/$d_{yz}$ orbital compositions, which we associate with the enhancement of nematic fluctuations, revealing unexpected connections to the iron-pnictide superconductors. This gives credence to a scenario in which nematic fluctuations drive an enhanced $T_c$., Comment: 19 pages, 12 figures, 1 table

CeAlGe, a proposed type-II Weyl semimetal, orders antiferromagnetically below 5 K. At 2 K, spin-flop and a spin-flip transitions to less than 1 $\mu_B$/Ce are observed in the $M(H)$ data below 30 kOe, ($\bf{H}\|\bf{a}$ and $\bf{b}$, and 4.3 kOe, $\bf{H}\|\langle110\rangle$, respectively, indicating a four-fold symmetry of the $M(H)$ data along the principal directions in the tetragonal $\it{ab}$ plane with $\langle110\rangle$ set of easy directions. However, anomalously robust and complex twofold symmetry is observed in the angular dependence of resistivity and magnetic torque data in the magnetically ordered state once the field is swept in the $\it{ab}$ plane. This twofold symmetry is independent of temperature and field hystereses and suggests a magnetic phase transition that separates two different magnetic structures in the $\it{ab}$ plane. The boundary of this magnetic phase transition and possibly the type of low-field magnetic structure can be tuned by an Al deficiency.

We present a method for producing high quality KCo2As2 crystals, stable in air and suitable for a variety of measurements. X-ray diffraction, magnetic susceptibility, electrical transport and heat capacity measurements confirm the high quality and an absence of long range magnetic order down to at least 2 K. Residual resistivity values approaching 0.25 $\mu\Omega$~cm are representative of the high quality and low impurity content, and a Sommerfeld coefficient $\gamma$ = 7.3 mJ/mol K$^2$ signifies weaker correlations than the Fe-based counterparts. Together with Hall effect measurements, angle-resolved photoemission experiments reveal a Fermi surface consisting of electron pockets at the center and corner of the Brillouin zone, in line with theoretical predictions and in contrast to the mixed carrier types of other pnictides with the ThCr2Si2 structure. A large, linear magnetoresistance of 200\% at 14~T, together with an observed linear and hyperbolic, rather than parabolic, band dispersions are unusual characteristics of this metallic compound and may indicate more complex underlying behavior., Comment: 9 pages, 4 figures

Structural phase transitions serve as the basis for many functional applications including shape memory alloys (SMAs), switches based on metal-insulator transitions (MITs), etc. In such materials, lattice incompatibility between phases often results in a thermal hysteresis, which is intimately tied to degradation of reversibility of the transformation. The non-linear theory of martensite suggests that the hysteresis of a martensitic phase transformation is solely determined by the lattice constants, and the conditions proposed for geometrical compatibility have been successfully applied to minimizing the hysteresis in SMAs. In this work, we apply the non-linear theory to a strongly correlated oxide system (W doped VO2), and show that the hysteresis of the MIT in the system can be directly tuned by adjusting the lattice constants of the phases. The results underscore the profound influence structural compatibility has on intrinsic electronic properties, and indicate that the theory provides a universal guidance for optimizing phase transforming materials., Comment: 5 figures plus supplementary materials

The electronic nematic phase, wherein electronic degrees of freedom lower the crystal rotational symmetry, is a common motif across a number of high-temperature superconductors. However, understanding the role and influence of nematicity and nematic fluctuations in Cooper pairing is often complicated by the coexistence of other orders, particularly long-range magnetic order. Here we report the enhancement of superconductivity in a model electronic nematic system absent of magnetism, and show that the enhancement is directly born out of strong nematic fluctuations emanating from a tuned quantum phase transition. We use elastoresistance measurements of the Ba$_{1-x}$Sr$_{x}$Ni$_2$As$_2$ substitution series to show that strontium substitution promotes an electronically driven $B_{1g}$ nematic order in this system, and that the complete suppression of that order to absolute zero temperature evokes a dramatic enhancement of the pairing strength, as evidenced by a sixfold increase in the superconducting transition temperature. The direct relation between enhanced pairing and nematic fluctuations in this model system, as well as the interplay with a unidirectional charge density wave order comparable to that found in the cuprates, offers a means to elucidate the role of nematicity in boosting superconductivity., Comment: 16 pages, 15 figures

Quantum-mechanical fluctuations between competing phases at $T=0$ induce exotic finite-temperature collective excitations that are not described by the standard Landau Fermi liquid framework. These excitations exhibit anomalous temperature dependences, or non-Fermi liquid behavior, in the transport and thermodynamic properties in the vicinity of a quantum critical point, and are often intimately linked to the appearance of unconventional Cooper pairing as observed in strongly correlated systems including the high-$T_c$ cuprate and iron pnictide superconductors. The presence of superconductivity, however, precludes direct access to the quantum critical point, and makes it difficult to assess the role of quantum-critical fluctuations in shaping anomalous finite-temperature physical properties. Here we report temperature-field scale invariance of non-Fermi liquid thermodynamic, transport, and Hall quantities in a non-superconducting iron-pnictide, Ba(Fe$_{1/3}$Co$_{1/3}$Ni$_{1/3}$)$_{2}$As$_{2}$, indicative of quantum criticality at zero temperature and zero applied magnetic field. Beyond a linear in temperature resistivity, the hallmark signature of strong quasiparticle scattering, we find the scattering rate that obeys a universal scaling relation between temperature and applied magnetic fields down to the lowest energy scales. Together with the dominance of hole-like carriers close to the zero-temperature and zero-field limits, the scale invariance, isotropic field response, and lack of applied pressure sensitivity suggests a unique quantum critical system that does not drive a pairing instability., Comment: 37 pages, 15 figures

The transition metal dichalcogenide TiSe$_2$ has received significant research attention over the past four decades. Different studies have presented ways to suppress the 200~K charge density wave transition, vary low temperature resistivity by several orders of magnitude, and stabilize magnetism or superconductivity. Here we give the results of a new synthesis technique whereby samples were grown in a high pressure environment with up to 180~bar of argon gas. Above 100~K, properties are nearly unchanged from previous reports, but a hysteretic resistance region that begins around 80~K, accompanied by insulating low temperature behavior, is distinct from anything previously observed. An accompanying decrease in carrier concentration is seen in Hall effect measurements, and photoemission data show a removal of an electron pocket from the Fermi surface in an insulating sample. We conclude that high inert gas pressure synthesis accesses an underlying nonmetallic ground state in a material long speculated to be an excitonic insulator., Comment: 11 pages, 7 figures

We present details of materials synthesis, crystal structure, and anisotropic magnetic properties of single crystals of CeAlGe, a proposed type-II Weyl semimetal. Single-crystal x-ray diffraction confirms that CeAlGe forms in noncentrosymmetric I4$_1$md space group, in line with predictions of non-trivial topology. Magnetization, specific heat and electrical transport measurements were used to confirm antiferromagnetic order below 5 K, with an estimated magnon excitation gap of $\Delta$ = 9.11 K from heat capacity and hole-like carrier density of 1.44 $\times$ 10$^{20}$ cm$^{-3}$ from Hall effect measurements. The easy magnetic axis is along the [100] crystallographic direction, indicating that the moment lies in the tetragonal $\it{ab}$-plane below 7 K. A spin-flop transition to less than 1 $\mu_B$/Ce is observed to occur below 30 kOe at 1.8 K in the $M(H)$ ($\bf{H}\|\bf{a}$) data. Small magnetic fields of 3 kOe and 30 kOe are sufficient to suppress magnetic order when applied along the $\it{a}$- and $\it{c}$-axes, respectively, resulting in a complex $\it{T-H}$ phase diagram for $\bf{H}\|\bf{a}$ and a simpler one for $\bf{H}\|\bf{c}$.

The effects of Co-substitution on Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ ($0\leq x\leq 0.251$) single crystals grown out of Pb flux are investigated via transport, magnetic, and thermodynamic measurements. BaNi$_2$As$_2$ exhibits a first order tetragonal to triclinic structural phase transition at $T_s=137 K$ upon cooling, and enters a superconducting phase below $T_c=0.7 K$. The structural phase transition is sensitive to cobalt content and is suppressed completely by $x\geq0.133$. The superconducting critical temperature, $T_c$, increases continuously with $x$, reaching a maximum of $T_c=2.3 K$ at the structural critical point $x=0.083$ and then decreases monotonically until superconductivity is no longer observable well into the tetragonal phase. In contrast to similar BaNi$_2$As$_2$ substitutional studies, which show an abrupt change in $T_c$ at the triclinic-tetragonal boundary that extends far into the tetragonal phase, Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ exhibits a dome-like phase diagram centered around the first-order critical point. Together with an anomalously large heat capacity jump $\Delta C_e/\gamma T\sim 2.2$ at optimal doping, the smooth evolution of $T_c$ in the Ba(Ni$_{1-x}$Co$_x$)$_2$As$_2$ system suggests a mechanism for pairing enhancement other than phonon softening., Comment: 7 pages, 8 figures

Single crystals of A2Cr3As3 (A = K, Rb) were successfully grown using a self-flux method and studied via structural, transport and thermodynamic measurement techniques. The superconducting state properties between the two species are similar, with critical temperatures of 6.1 K and 4.8 K in K2Cr3As3 and Rb2Cr3As3, respectively. However, the emergence of a strong normal state electronic anisotropy in Rb2Cr3As3 suggests a unique electronic tuning parameter is coupled to the inter-chain spacing in the A2Cr3As3 structure, which increases with alkali metal ionic size while the one-dimensional [(Cr3As3)^{2-}]_{\infty} chain structure itself remains essentially unchanged. Together with dramatic enhancements in both conductivity and magnetoresistance (MR), the appearance of a strong anisotropy in the MR of Rb2Cr3As3 is consistent with the proposed quasi-one-dimensional character of band structure and its evolution with alkali metal species in this new family of superconductors., Comment: 6 pages, 8 figures; to appear in Phys. Rev. B

The structure and properties of the ferromagnet Tb_{1-x}Dy_xFe_2 (Terfenol-D) are explored through the morphotropic phase boundary (MPB) separating ferroic phases of differing symmetry. Our synchrotron data support a first order structural transition, with a broadening MPB width at higher temperatures. The optimal point for magnetomechanical applications is not centered on the MPB but lies on the rhombohedral side, where the high striction of the rhombohedral majority phase combines with the softened anisotropy of the MPB. We compare our findings with single ion crystal field theory and with ferroelectric MPBs, where the controlling energies are different., Comment: 5 pages, 4 figures

The effects of aliovalent rare earth substitution on the physical properties of Sr0.3Ca0.7Fe2As2 solid solutions are explored. Electrical transport, magnetic susceptibility and structural characterization data as a function of La substitution into (Sr_1-y_Ca_y)_1-x_La_x_Fe2As2 single crystals confirm the ability to suppress the antiferromagnetic ordering temperature from 200 K in the undoped compound down to 100 K approaching the solubility limit of La. Despite up to ~30% La substitution, the persistence of magnetic order and lack of any signature of superconductivity above 10 K present a contrasting phase diagram to that of Ca1-xLaxFe2As2, indicating that the suppression of magnetic order is necessary to induce the high-temperature superconducting phase observed in Ca1-xLaxFe2As2., Comment: Submitted to Superconductor Science and Technology focus issue: "Fe-based superconductors: materials, theory, experiment and applications (Iron 12)"

Resistivity, magnetic susceptibility, neutron scattering and x-ray crystallography measurements were used to study the evolution of magnetic order and crystallographic structure in single-crystal samples of the Ba1-xSrxFe2As2 and Sr1-yCayFe2As2 series. A non-monotonic dependence of the magnetic ordering temperature T0 on chemical pressure is compared to the progression of the antiferromagnetic staggered moment, characteristics of the ordering transition and structural parameters to reveal a distinct relationship between the magnetic energy scale and the tetrahedral bond angle, even far above T0. In Sr1-yCayFe2As2, an abrupt drop in T0 precisely at the Ca concentration where the tetrahedral structure approaches the ideal geometry indicates a strong coupling between the orbital bonding structure and the stabilization of magnetic order, providing strong constraints on the nature of magnetism in the iron-arsenide superconducting parent compounds., Comment: 4 pages, 4 figures. To appear in Phys. Rev. B Rapid Communications

Aliovalent rare earth substitution into the alkaline earth site of CaFe2As2 single-crystals is used to fine-tune structural, magnetic and electronic properties of this iron-based superconducting system. Neutron and single crystal x-ray scattering experiments indicate that an isostructural collapse of the tetragonal unit cell can be controllably induced at ambient pressures by choice of substituent ion size. This instability is driven by the interlayer As-As anion separation, resulting in an unprecedented thermal expansion coefficient of $180\times 10^{-6}$ K$^{-1}$. Electrical transport and magnetic susceptibility measurements reveal abrupt changes in the physical properties through the collapse as a function of temperature, including a reconstruction of the electronic structure. Superconductivity with onset transition temperatures as high as 47 K is stabilized by the suppression of antiferromagnetic order via chemical pressure, electron doping or a combination of both. Extensive investigations are performed to understand the observations of partial volume-fraction diamagnetic screening, ruling out extrinsic sources such as strain mechanisms, surface states or foreign phases as the cause of this superconducting phase that appears to be stable in both collapsed and uncollapsed structures., Comment: 15 pages, 18 figures

hrough neutron diffraction experiments, including spin-polarized measurements, we find a collinear incommensurate spin-density wave with propagation vector $ \mathbf k = $ ($0.4481(4) \, \,0 \, \, \frac1 2$) at base temperature in the superconducting parent compound Fe$_{1+x}$Te. This critical concentration of interstitial iron corresponds to $x \approx 12%$ and leads crystallographic phase separation at base temperature. The spin-density wave is short-range ordered with a correlation length of 22(3) \AA, and as the ordering temperature is approached its propagation vector decreases linearly in the H-direction and becomes long-range ordered. Upon further populating the interstitial iron site, the spin-density wave gives way to an incommensurate helical ordering with propagation vector $ \mathbf k =$ ($0.3855(2) \, \,0 \, \, \frac1 2$) at base temperature. For a sample with $x \approx 9(1) %$, we also find an incommensurate spin-density wave that competes with the bicollinear commensurate ordering close to the N\'eel point. The shifting of spectral weight between competing magnetic orderings observed in several samples is supporting evidence for the phase separation being electronic in nature, and hence leads to crystallographic phase separation around the critical interstitial iron concentration of 12%. With results from both powder and single crystal samples, we construct a magnetic-crystallographic phase diagram of Fe$_{1+x}$Te for $ 5% < x <17%$

Using neutron inelastic scattering, we investigate the low-energy spin fluctuations in Fe1+xTe as a function of both temperature and interstitial iron concentration. For Fe1.057(7)Te the magnetic structure is defined by a commensurate wavevector of (1/2,0,1/2). The spin fluctuations are gapped with a sharp onset at 7 meV and are three dimensional in momentum transfer, becoming two dimensional at higher energy transfers. On doping with interstitial iron, we find in Fe1.141(5)Te the ordering wavevector is located at the (0.38, 0, 1/2) position and the fluctuations are gapless with the intensity peaked at an energy transfer of 4 meV. These results show that the spin fluctuations in the Fe1+xTe system a can be tuned not only through selenium doping, but also with interstitial iron. We also compare these results with superconducting concentrations and in particular the resonance mode in the Fe_1+xTe_1-ySe_y system., Comment: 6 pages, 5 figures, (submitted February 8, 2011)

With single crystal X-ray diffraction studies, we compare the structures of three sample showing optimal superconductivity, K0.774(4)Fe1.613(2)Se2, K0.738(6)Fe1.631(3)Se2 and Cs0.748(2)Fe1.626(1)Se2. All have an almost identical ordered vacancy structure with a ({\sqrt}5 x {\sqrt}5 x 1) super cell. The tetragonal unit cell, space group I4/m, possesses lattice parameters at 250K of a = b = 8.729(2) {\AA} and c = 14.120(3) {\AA}, a = b = 8.7186(12) {\AA} and c = 14.0853(19) {\AA} and at 295 K, a = b = 8.8617(16) {\AA} and c = 15.304(3) {\AA} for the three crystals, respectively. The structure contains two iron sites; one is almost completely empty, whilst the other is fully occupied. There are similarly two alkali metal sites that are occupied in the range of 72.2(2) % to 85.3(3) %. The inclusion of alkali metals and the presence of vacancies within the structure allows for considerable relaxation of the FeSe4 tetrahedron, compared with members of the Fe(Te, Se, S) series, and the resulting shift of the Se - F - Se bond angles to less distorted geometry could be important in understanding the associated increase in the superconducting transition temperature. The structure of these superconductors distinguishes themselves from the structure of the non-superconducting phases by an almost complete absence of Fe on the (0 0.5 0.25) site as well as lower alkali metal occupancy that ensures an exact Fe2+ oxidation state, which are clearly critical parameters in the promotion of superconductivity.

The effect of alkaline earth substitution on structural parameters was studied in high-quality single crystals of Ba(1-x)Sr(x)Fe2As2 and Sr(1-x)Ca(x)Fe2As2 grown by the self-flux method. The results of single-crystal and powder x-ray diffraction measurements suggest a continuous monotonic decrease of both a- and c-axis lattice parameters, the c/a tetragonal ratio, and the unit cell volume with decreasing alkaline earth atomic radius as expected by Vegard's law. As a result, the system experiences a continuously increasing chemical pressure effect in traversing the phase diagram from x=0 in Ba(1-x)Sr(x)Fe2As2 to x=1 in Sr(1-x)Ca(x)Fe2As2., Comment: 4 pages, 4 figures, and 1 table, submitted to SCES2010 proceedings

We report superconductivity in single crystals of the new iron-pnictide system BaFe1.9Pt0.1As2 grown by a self-flux solution method and characterized via x-ray, transport, magnetic and thermodynamic measurements. The magnetic ordering associated with a structural transition at 140 K present in BaFe2As2 is completely suppressed by substitution of 5% Fe with Pt and superconductivity is induced at a critical temperature Tc=23 K. Full diamagnetic screening in the magnetic susceptibility and a jump in the specific heat at Tc confirm the bulk nature of the superconducting phase. All properties of the superconducting state including transition temperature Tc, the lower critical field Hc1=200 mT, upper critical field Hc2~65 T, and the slope dHc2/dT are comparable in value to the those found in other transition-metal-substituted BaFe2As2 series, indicating the robust nature of superconductivity induced by substitution of Group VIII elements., Comment: 6 pgs, 4 figs, and 1 tbl, slightly revised, updated references