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Fe1+yTe with y ≲ 0.05 exhibits a first-order phase transition on cooling to a state with a lowered structural symmetry, bicollinear antiferromagnetic order, and metallic conductivity, dp/dT > 0. Here, we study samples with y = 0.09(1), where the frustration effects of the interstitial Fe decouple different orders, leading to a sequence of transitions. While the lattice distortion is closely followed by incommensurate magnetic order, the development of bicollinear order and metallic electronic coherence is uniquely associated with a separate hysteretic first-order transition, at a markedly lower temperature, to a phase with dramatically enhanced bond-order wave (BOW) order. The BOW state suggests ferro-orbital ordering, where electronic delocalization in ferromagnetic zigzag chains decreases local spin and results in metallic transport. [ABSTRACT FROM AUTHOR]

Here we report small-angle neutron scattering (SANS) measurements and theoretical modeling of U3Al2Ge3. Analysis of the SANS data reveals a phase transition to sinusoidally modulated magnetic order at TN=63 K to be second order and a first-order phase transition to ferromagnetic order at Tc=48 K. Within the sinusoidally modulated magnetic phase (Tc

Investigation of the inelastic neutron scattering spectra in Fe1+yTe1-xSex near a signature wave vector Q=(1,0,0) for the bond-order wave (BOW) formation of parent compound Fe1+yTe [D. Fobes et al., Phys. Rev. Lett. 112, 187202 (2014)] reveals an acoustic-phonon-like dispersion present in all structural phases. While a structural Bragg peak accompanies the mode in the low-temperature phase of Fe1+yTe, it is absent in the high-temperature tetragonal phase, where Bragg scattering at this Q is forbidden by symmetry. Notably, this mode is also observed in superconducting FeTe0.55Se0.45, where structural and magnetic transitions are suppressed, and no BOW has been observed. The presence of this "forbidden" phonon indicates that the lattice symmetry is dynamically or locally broken by magneto-orbital BOW fluctuations, which are strongly coupled to lattice in these materials. [ABSTRACT FROM AUTHOR]

The (001) surfaces of cleaved Sr3RuO4 and Sr2RuO4 samples were investigated using low-temperature scanning tunneling microscopy and density functional theory calculations. Intrinsic defects are not created during cleaving. This experimental observation is consistent with calculations, where the formation energy for a Sr and O vacancy, 4.19 eV and 3.81 eV, respectively, is significantly larger than that required to cleave the crystal, 1.11 eV/(1 × 1) unit cell. Surface oxygen vacancies can be created through electron bombardment, however, and their appearance is shown to vary strongly with the imaging conditions. Point defects observed on as-cleaved surfaces result from bulk impurities and adsorption from the residual gas. [ABSTRACT FROM AUTHOR]

Adsorption of CO at the Sr3Ru2O7(001) surface was studied with low-temperature scanning tunneling microscopy (STM) and density functional theory. In situ cleaved single crystals terminate in an almost perfect SrO surface. At 78 K, CO first populates impurities and then adsorbs above the apical surface O with a binding energy Eads = -0.7 eV. Above 100 K, this physisorbed CO replaces the surface O, forming a bent CO2 with the C end bound to the Ru underneath. The resulting metal carboxylate (Ru-COO) can be desorbed by STM manipulation. A low activation (0.2 eV) and high binding (-2.2 eV) energy confirm a strong reaction between CO and regular surface sites of Sr3Ru2O7; likely, this reaction causes the "UHV aging effect" reported for this and other perovskite oxides. [ABSTRACT FROM AUTHOR]

We report the electronic and magnetic phase diagram of Ca3(Ru1-xTix)2O7. With Ti doping, the system evolves from a quasi-two-dimensional metal with ferromagnetic (FM) bilayers coupled antiferromagnetically along the c axis (AFM-b) for x = 0, to a weakly localized state for 0 < x < 0.05, and finally to a Mott insulator with G-type antiferromagnetic (G-AFM) order for x ... 0.05. The magnetic state switching from the AFM-b to the G-AFM occurs in the weakly localized state near x = 0.03. We show that such a magnetic transition is controlled by the charge carrier itinerancy and can be understood in light of competing interactions between FM double exchange and AFM superexchange. An incommensurate component is also observed due to competing magnetic interactions. [ABSTRACT FROM AUTHOR]

(Sr1-xCax)3Ru2O7 is characterized by complex magnetic states, spanning from a long-range antiferromagnet-ically ordered state over an unusual heavy-mass nearly ferromagnetic (NFM) state to an itinerant metamagnetic (IMM) state. The NFM state, which occurs in the 0.4 > x > 0.08 composition range, freezes into a cluster spin glass (CSG) phase at low temperatures [Z. Qu et al., Phys. Rev. B 78, 180407(R) (2008)]. In this article, we present the scaling analyses of magnetization and the specific heat for (Sr1-xCax)3Ru2O7 in the 0.4 > x > 0.08 composition range. We find that in a temperature region immediately above the spin freezing temperature Tf, the isothermal magnetization M(H) and the temperature dependence of electronic specific heat Ce(T) exhibit anomalous power-law singularities; both quantities are controlled by a single exponent. The temperature dependence of magnetization M(T) also displays a power-law behavior, but its exponent differs remarkably from that derived from M(H) and Ce(T). Our analyses further reveal that the magnetization data M(H,T) obey a phenomenological scaling law of M(H,T) ∝ H&agr;f(H/T&dgr;) in a temperature region between the spin freezing temperature Tf and the scaling temperature Tscaiing. Tscaiing systematically decreases with the decease of Ca content. This scaling law breaks down near the critical concentration x = 0.1 where a CSG-to-IMM phase transition occurs. We discussed these behaviors in term of the effect of disorder on the quantum phase transition. [ABSTRACT FROM AUTHOR]

Adsorption of CO at the Sr(3)Ru(2)O(7)(001) surface was studied with low-temperature scanning tunneling microscopy (STM) and density functional theory. In situ cleaved single crystals terminate in an almost perfect SrO surface. At 78 K, CO first populates impurities and then adsorbs above the apical surface O with a binding energy E(ads)=-0.7  eV. Above 100 K, this physisorbed CO replaces the surface O, forming a bent CO(2) with the C end bound to the Ru underneath. The resulting metal carboxylate (Ru-COO) can be desorbed by STM manipulation. A low activation (0.2 eV) and high binding (-2.2  eV) energy confirm a strong reaction between CO and regular surface sites of Sr(3)Ru(2)O(7); likely, this reaction causes the "UHV aging effect" reported for this and other perovskite oxides.

Fe(1+y)Te with y≲0.05 exhibits a first-order phase transition on cooling to a state with a lowered structural symmetry, bicollinear antiferromagnetic order, and metallic conductivity, dρ/dT>0. Here, we study samples with y=0.09(1), where the frustration effects of the interstitial Fe decouple different orders, leading to a sequence of transitions. While the lattice distortion is closely followed by incommensurate magnetic order, the development of bicollinear order and metallic electronic coherence is uniquely associated with a separate hysteretic first-order transition, at a markedly lower temperature, to a phase with dramatically enhanced bond-order wave (BOW) order. The BOW state suggests ferro-orbital ordering, where electronic delocalization in ferromagnetic zigzag chains decreases local spin and results in metallic transport.