Electron-phonon coupling enhancement and displacive magnetostructural transition in SrCr2 As2 under magneto-Raman spectroscopy.

A Raman spectroscopy study on high quality single crystals of SrCr2 As2 (SCA) in the temperature T range 4 K < T < 300 K and high applied magnetic fields up to H = 9 T is presented. The chromium B 1g phonon analysis reveals two anomalous shifts in the frequency, the first below T = 250 K at H = 0 T in the saturated AFM G-type order likely due to an enhanced electron-phonon coupling by the magnetic order, whereas the second anomaly occurs above H = 4 T at T = 4 K likely as a consequence of a magnetostructural displacive transition. Renormalization of the electronic Raman spectra in both studies reveals a decrease in the electronic density of states with decreasing T and increasing H, respectively, with consequent changes in the Fermi surface, which are intrinsically related to the observed anomalies.

Magnetic crystalline-symmetry-protected axion electrodynamics and field-tunable unpinned Dirac cones in EuIn2As2.

Knowledge of magnetic symmetry is vital for exploiting nontrivial surface states of magnetic topological materials. EuIn2As2 is an excellent example, as it is predicted to have collinear antiferromagnetic order where the magnetic moment direction determines either a topological-crystalline-insulator phase supporting axion electrodynamics or a higher-order-topological-insulator phase with chiral hinge states. Here, we use neutron diffraction, symmetry analysis, and density functional theory results to demonstrate that EuIn2As2 actually exhibits low-symmetry helical antiferromagnetic order which makes it a stoichiometric magnetic topological-crystalline axion insulator protected by the combination of a 180∘ rotation and time-reversal symmetries: [Formula: see text]. Surfaces protected by [Formula: see text] are expected to have an exotic gapless Dirac cone which is unpinned to specific crystal momenta. All other surfaces have gapped Dirac cones and exhibit half-integer quantum anomalous Hall conductivity. We predict that the direction of a modest applied magnetic field of µ0H ≈ 1 to 2 T can tune between gapless and gapped surface states.

Short-range ferromagnetic order due to Ir substitutions in single-crystalline Ba(Co1- x Ir x )2As2 (0 ⩽ x ⩽ 0.25).

The ternary-arsenide compound BaCo2As2 was previously proposed to be in proximity to a quantum-critical point where long-range ferromagnetic (FM) order is suppressed by quantum fluctuations. Here we report the effect of Ir substitutions for Co on the magnetic and thermal properties of Ba[Formula: see text] (0 ⩽ x ⩽ 0.25) single crystals. These compositions all crystallize in an uncollapsed body-centered-tetragonal ThCr2Si2 structure with space group I4/mmm. Magnetic susceptibility measurements reveal clear signatures of short-range FM ordering for x ⩾ 0.11 below a nearly composition-independent characteristic temperature T cl ≈ 13 K. The small variation of T cl with x, thermomagnetic irreversibility between zero-field-cooled and field-cooled magnetic susceptibility versus T, the occurrence of hysteresis in magnetization versus field isotherms at low field and temperature, and very small spontaneous and remanent magnetizations <0.01 µ B/f.u. together indicate that the FM response arises from short-range FM ordering of FM spin clusters as previously inferred to occur in Ca(Co1-x Ir x )2-y As2. Heat-capacity C p(T) data do not exhibit any clear feature around T cl, consistent with the very small moments of the FM clusters. The C p(T) in the paramagnetic temperature regime 25-300 K is well described by the sum of a Sommerfeld electronic contribution and Debye and Einstein lattice contributions where the latter lattice contribution suggests the presence of low-frequency optic modes associated with the heavy Ba atoms in the crystals.

Effective One-Dimensional Coupling in the Highly Frustrated Square-Lattice Itinerant Magnet CaCo_{2-y}As_{2}.

Inelastic neutron scattering measurements on the itinerant antiferromagnet CaCo_{2-y}As_{2} at a temperature of 8 K reveal two orthogonal planes of scattering perpendicular to the Co square lattice in reciprocal space, demonstrating the presence of effective one-dimensional spin interactions. These results are shown to arise from near-perfect bond frustration within the J_{1}-J_{2} Heisenberg model on a square lattice with ferromagnetic J_{1} and hence indicate that the extensive previous experimental and theoretical study of the J_{1}-J_{2} Heisenberg model on local-moment square spin lattices should be expanded to include itinerant spin systems.

Multiple crossovers between positive and negative magnetoresistance versus field due to fragile spin structure in metallic GdPd3.

Studies on the phenomenon of magnetoresistance (MR) have produced intriguing and application-oriented outcomes for decades-colossal MR, giant MR and recently discovered extremely large MR of millions of percents in semimetals can be taken as examples. We report here the discovery of novel multiple sign changes versus applied magnetic field of the MR in the cubic intermetallic compound GdPd3. Our study shows that a very strong correlation between magnetic, electrical and magnetotransport properties is present in this compound. The magnetic structure in GdPd3 is highly fragile since applied magnetic fields of moderate strength significantly alter the spin arrangement within the system-a behavior that manifests itself in the oscillating MR. Intriguing magnetotransport characteristics of GdPd3 are appealing for field-sensitive device applications, especially if the MR oscillation could materialize at higher temperature by manipulating the magnetic interaction through perturbations caused by chemical substitutions.

Anomalous Composition-Induced Crossover in the Magnetic Properties of the Itinerant-Electron Antiferromagnet Ca_{1-x}Sr_{x}Co_{2-y}As_{2}.

The inference of Ying et al. [Europhys. Lett. 104, 67005 (2013)EULEEJ0295-507510.1209/0295-5075/104/67005] of a composition-induced change from c-axis ordered-moment alignment in a collinear A-type antiferromagnetic (AFM) structure at small x to ab-plane alignment in an unknown AFM structure at larger x in Ca_{1-x}Sr_{x}Co_{2-y}As_{2} with the body-centered tetragonal ThCr_{2}Si_{2} structure is confirmed. Our major finding is an anomalous magnetic behavior in the crossover region 0.2≲x≲0.3 between these two phases. In this region the magnetic susceptibility vs temperature χ_{ab}(T) measured with magnetic fields H applied in the ab plane exhibit typical AFM behaviors with cusps at the Néel temperatures of ∼ 65 K, whereas χ_{c}(T) and the low-temperature isothermal magnetization M_{c}(H) with H aligned along the c axis exhibit extremely soft ferromagneticlike behaviors.

Collinear antiferromagnetism in trigonal SrMn2As2 revealed by single-crystal neutron diffraction.

Iron pnictides and related materials have been a topic of intense research for understanding the complex interplay between magnetism and superconductivity. Here we report on the magnetic structure of SrMn2As2 that crystallizes in a trigonal structure ([Formula: see text]) and undergoes an antiferromagnetic (AFM) transition at [Formula: see text] K. The magnetic susceptibility remains nearly constant at temperatures [Formula: see text] with [Formula: see text] whereas it decreases significantly with [Formula: see text]. This shows that the ordered Mn moments lie in the [Formula: see text] plane instead of aligning along the [Formula: see text]-axis as in tetragonal BaMn2As2. Single-crystal neutron diffraction measurements on SrMn2As2 demonstrate that the Mn moments are ordered in a collinear Néel AFM phase with [Formula: see text] AFM alignment between a moment and all nearest neighbor moments in the basal plane and also perpendicular to it. Moreover, quasi-two-dimensional AFM order is manifested in SrMn2As2 as evident from the temperature dependence of the order parameter.

Competing Magnetic Fluctuations in Iron Pnictide Superconductors: Role of Ferromagnetic Spin Correlations Revealed by NMR.

In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic and in-plane ferromagnetic (FM) wave vectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using ^{75}As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole- and electron-doped BaFe_{2}As_{2} families of iron-pnictide superconductors. These FM fluctuations appear to compete with superconductivity and are thus a crucial ingredient to understanding the variability of T_{c} and the shape of the superconducting dome in these and other iron-pnictide families.

Itinerant ferromagnetism in the As 4p conduction band of Ba_{0.6}K_{0.4}Mn_{2}As_{2} identified by X-ray magnetic circular dichroism.

X-ray magnetic circular dichroism (XMCD) measurements on single-crystal and powder samples of Ba_{0.6}K_{0.4}Mn_{2}As_{2} show that the ferromagnetism below T_{C}≈100 K arises in the As 4p conduction band. No XMCD signal is observed at the Mn x-ray absorption edges. Below T_{C}, however, a clear XMCD signal is found at the As K edge which increases with decreasing temperature. The XMCD signal is absent in data taken with the beam directed parallel to the crystallographic c axis indicating that the orbital magnetic moment lies in the basal plane of the tetragonal lattice. These results show that the previously reported itinerant ferromagnetism is associated with the As 4p conduction band and that distinct local-moment antiferromagnetism and itinerant ferromagnetism with perpendicular easy axes coexist in this compound at low temperature.

The magnetic structure of EuCu2Sb2.

Antiferromagnetic ordering of EuCu2Sb2 which forms in the tetragonal CaBe2Ge2-type structure (space group P4/nmm #129) has been studied using neutron powder diffraction and (151)Eu Mössbauer spectroscopy. The room temperature (151)Eu isomer shift of -12.8(1) mm s(-1) shows the Eu to be divalent, while the (151)Eu hyperfine magnetic field (B(hf)) reaches 28.7(2) T at 2.1 K, indicating a full Eu(2+) magnetic moment. B(hf)(T) follows a smooth S = 7/2 Brillouin function and yields an ordering temperature of 5.1(1) K. Refinement of the neutron diffraction data reveals a collinear A-type antiferromagnetic arrangement with the Eu moments perpendicular to the tetragonal c-axis. The refined Eu magnetic moment at 0.4 K is 7.08(15) µ(B) which is the full free-ion moment expected for the Eu(2+) ion with S = 7/2 and a spectroscopic splitting factor of g = 2.

Thermodynamics of the noninteracting Bose gas in a two-dimensional box.

Bose-Einstein condensation (BEC) of a noninteracting Bose gas of N particles in a two-dimensional box with Dirichlet boundary conditions is studied. Confirming previous work, we find that BEC occurs at finite N at low temperatures T without the occurrence of a phase transition. The conventionally-defined transition temperature T(E) for an infinite three-dimensional (3D) system is shown to correspond in a 2D system with finite N to a crossover temperature between a slow and rapid increase in the fractional boson occupation N(0)/N of the ground state with decreasing T. We further show that T(E)∼1/logN at fixed area per boson, so in the thermodynamic limit there is no significant BEC in 2D at finite T. Thus, paradoxically, BEC only occurs in 2D at finite N with no phase transition associated with it. Calculations of thermodynamic properties versus T and area A are presented, including Helmholtz free energy, entropy S, pressure p, ratio of p to the energy density U/A, heat capacity at constant volume (area) C(V) and at constant pressure C(p), isothermal compressibility κ(T) and thermal expansion coefficient α(p), obtained using both the grand-canonical ensemble (GCE) and canonical ensemble (CE) formalisms. The GCE formalism gives acceptable predictions for S, p, p/(U/A), κ(T) and α(p) at large N, T and A but fails for smaller values of these three parameters for which BEC becomes significant, whereas the CE formalism gives accurate results for all thermodynamic properties of finite systems even at low T and/or A where BEC occurs.

Superconductivity and physical properties of CaPd2Ge2 single crystals.

We present the superconducting and normal state properties of CaPd(2)Ge(2) single crystals investigated by magnetic susceptibility χ, isothermal magnetization M, heat capacity Cp, in-plane electrical resistivity ρ and London penetration depth λ versus temperature T and magnetic field H measurements. Bulk superconductivity is inferred from the ρ(T) and Cp(T) data. The ρ(T) data exhibit metallic behavior and a superconducting transition with T(c onset) = 1.98 K and zero resistivity at T(c 0) = 1.67 K. The χ(T) reveals the onset of superconductivity at 2.0 K. For T > 2.0 K, the χ(T) and M(H) are weakly anisotropic paramagnetic with χ(ab) > χ(c). The Cp(T) data confirm the bulk superconductivity below T(c) = 1.69(3) K. The superconducting state electronic heat capacity is analyzed within the framework of a single-band α-model of BCS superconductivity and various normal and superconducting state parameters are estimated. Within the α-model, the Cp(T) data and the ab plane λ(T) data consistently indicate a moderately anisotropic s-wave gap with Δ(0)/k(B)T(c) ≈ 1.6, somewhat smaller than the BCS value of 1.764. The relationship of the heat capacity jump at Tc and the penetration depth measurement to the anisotropy in the s-wave gap is discussed.

Physical properties of EuPd2As2 single crystals.

The physical properties of self-flux grown EuPd2As2 single crystals have been investigated by means of magnetization M, magnetic susceptibility χ, specific heat Cp, and electrical resistivity ρ measurements versus temperature T and magnetic field H. The crystal structure was determined using powder x-ray diffraction measurements, which confirmed the ThCr2Si2-type body-centered tetragonal structure (space group I4/mmm) reported previously. The ρ(T) data indicate that EuPd2As2 is metallic. The χ(T) data indicate that the Eu(+2) moments have spin S = 7/2 with g = 2. Long-range antiferromagnetic (AFM) ordering is apparent from the χ(T), Cp(T), and ρ(T) measurements. For H∥c the χ(T) indicates two transitions at TN1 = 11.0 K and TN2 = 5.5 K, whereas for H⊥c only one transition is observed at TN1 = 11.0 K. Between TN1 and TN2 the anisotropic χ(T) data suggest a planar noncollinear AFM structure, whereas at T < TN2 the χ(T) and M(H, T) data suggest a spin reorientation transition in which equal numbers of spins cant in opposite directions out of the ab plane. We estimate the critical field at 2 K at which all Eu moments become aligned with the field to be about 22 T. An upturn in ρ at T < TN1 suggests superzone energy gap formation below TN1. This behavior of ρ(T < TN1) is not sensitive to applied magnetic fields up to H = 12 T.

Stripe antiferromagnetic spin fluctuations in SrCo2As2.

Inelastic neutron scattering measurements of paramagnetic SrCo2As2 at T=5 K reveal antiferromagnetic (AFM) spin fluctuations that are peaked at a wave vector of Q(AFM)=(1/2,1/2,1) and possess a large energy scale. These stripe spin fluctuations are similar to those found in AFe2As2 compounds, where spin-density wave AFM is driven by Fermi surface nesting between electron and hole pockets separated by Q(AFM). SrCo2As2 has a more complex Fermi surface and band-structure calculations indicate a potential instability toward either a ferromagnetic or stripe AFM ground state. The results suggest that stripe AFM magnetism is a general feature of both iron and cobalt-based arsenides and the search for spin fluctuation-induced unconventional superconductivity should be expanded to include cobalt-based compounds.

Coexistence of half-metallic itinerant ferromagnetism with local-moment antiferromagnetism in Ba0.60K0.40Mn2As2.

Magnetization, nuclear magnetic resonance, high-resolution x-ray diffraction, and magnetic field-dependent neutron diffraction measurements reveal a novel magnetic ground state of Ba0.60K0.40Mn2As2 in which itinerant ferromagnetism (FM) below a Curie temperature TC≈100 K arising from the doped conduction holes coexists with collinear antiferromagnetism (AFM) of the Mn local moments that order below a Néel temperature TN=480 K. The FM ordered moments are aligned in the tetragonal ab plane and are orthogonal to the AFM ordered Mn moments that are aligned along the c axis. The magnitude and nature of the low-T FM ordered moment correspond to complete polarization of the doped-hole spins (half-metallic itinerant FM) as deduced from magnetization and ab-plane electrical resistivity measurements.

Magnetic susceptibility of collinear and noncollinear heisenberg antiferromagnets.

Predictions of the anisotropic magnetic susceptibility χ below the antiferromagnetic (AFM) ordering temperatures T(N) of local moment Heisenberg AFMs have been made previously using molecular field theory (MFT) but are very limited in their applicability. Here a MFT calculation of χ(T ≤ T(N)) is presented for a wide variety of collinear and noncollinear Heisenberg AFMs containing identical crystallographically equivalent spins without recourse to magnetic sublattices. The results are expressed in terms of directly measurable experimental parameters and are fitted with no adjustable parameters to experimental χ(T ≤ T(N)) data from the literature for several collinear and noncollinear AFMs. The influence of spin correlations and fluctuations beyond MFT is quantified by the deviation of the theory from the data. The origin of the universal χ(T ≤ T(N)) observed for triangular lattice AFMs exhibiting coplanar noncollinear 120° AFM ordering is clarified.

Ba(1-x)K(x)Mn2As2: an antiferromagnetic local-moment metal.

The compound BaMn2As2 with the tetragonal ThCr2Si2 structure is a local-moment antiferromagnetic insulator with a Néel temperature T(N)=625 K and a large ordered moment µ=3.9µ(B)/Mn. We demonstrate that this compound can be driven metallic by partial substitution of Ba by K while retaining the same crystal and antiferromagnetic structures together with nearly the same high T(N) and large µ. Ba(1-x)K(x)Mn2As2 is thus the first metallic ThCr2Si2-type MAs-based system containing local 3d transition metal M magnetic moments, with consequences for the ongoing debate about the local-moment versus itinerant pictures of the FeAs-based superconductors and parent compounds. The Ba(1-x)K(x)Mn2As2 class of compounds also forms a bridge between the layered iron pnictides and cuprates and may be useful to test theories of high T(c) superconductivity.

Role of strong electronic correlations in the metal-to-insulator transition in disordered LiAlyTi2-yO4.

The compound LiAlyTi2-yO4 undergoes a metal-to-insulator transition for yc approximately 0.33. It is known that disorder alone is insufficient to explain this transition; e.g., a quantum site percolation model predicts yc approximately 0.8. We have included (Hubbard) electronic interactions into a model of this compound, using a real-space Hartree-Fock approach that achieves self-consistency at every site, and have found that for a Hubbard energy equal to 1.5 times the non-interacting bandwidth one obtains yc approximately 0.3. Further, with increasing Hubbard energy we find an Altshuler-Aronov suppression of the density of states, deltaN(epsilon) approximately square root /epsilon-epsilonF/, that reduces the density of states at the Fermi energy to zero at the critical Hubbard interaction. Using this ratio of correlation to hopping energy one is led to a prediction of the near-neighbor superexchange (J/t approximately 1/3) which is similar to that for the cuprate superconductors.

Dynamics of magnetic defects in heavy fermion LiV2O4 from stretched exponential 7Li NMR relaxation.

7Li NMR measurements on LiV2O4 from 0.5 to 4.2 K are reported. A small concentration of magnetic defects within the structure drastically changes the nuclear magnetization relaxation versus time from a pure exponential as in pure LiV2O4 to a stretched exponential, indicating glassy behavior of the magnetic defects. The stretched exponential function is described as arising from a distribution of 7Li nuclear spin-lattice relaxation rates and we present a model for the distribution in terms of the dynamics of the magnetic defects. Our results explain the origin of recent puzzling 7Li NMR literature data on LiV2O4 and our model is likely applicable to other glassy systems.

Low rate of complications of cerebral angiography in routine clinical practice.

OBJECTIVE: The use of cerebral angiography (ANGIO) to select patients for carotid endarterectomy (CEA) has been limited by concern about the risk of complications of the procedure. The authors sought to determine the rate of neurologic complications at both an academic medical center (AMC) and a community hospital (CH). METHODS: The authors reviewed the records of 569 patients undergoing ANGIO. Any documentation of stroke, transient neurologic event, myocardial infarction, or death occurring in the 24 hours after the procedure was recorded. The rate of neurologic complications at the AMC and CH were compared to published studies. RESULTS: The overall complication rate was 0.5% for stroke and 0.4% for TIA. There was no difference between the AMC and CH. One of the strokes was of moderate severity and four of the five patients with complications subsequently underwent endarterectomy. CONCLUSIONS: The rate of major neurologic complications due to ANGIO may be lower than expected, even when performed in a non-academic setting. Awareness of local ANGIO complication rates is important when selecting patients for CEA.