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Thermoelectric materials operating at cryogenic temperatures are in high demand for efficient cooling and power generation in applications ranging from superconductors to quantum computing. The narrow band-gap semiconductor FeSb2, known for its colossal Seebeck coefficient, holds promise for such applications, provided its thermal conductivity value can be reduced. This study investigates the impact of isoelectronic substitution (Bi) and hole doping (Pb) at the Sb site on the transport properties of FeSb2, with a particular focus on thermal conductivity (\k{appa}). Polycrystalline FeSb2 powder, along with Bi- and Pb-doped samples, were synthesized using a simple co-precipitation approach, followed by thermal treatment in an H2 atmosphere. XRD and SEM analysis confirms the formation of the desired phase pre- and post-consolidation using spark plasma sintering (SPS). The consolidation process resulted in a high compaction density and the formation of submicrometer-sized grains, as substantiated by electron backscattered diffraction (EBSD) analysis. Substituting 1% of Bi and Pb at the Sb site successfully suppressed the thermal conductivity (\k{appa}) from ~15 W/m-K in pure FeSb2 to ~10 and ~8.7 W/m-K, respectively. Importantly, resistivity measurements revealed a metal-to-insulator transition at around 6.5 K in undoped FeSb2 and isoelectronically Bi-substituted FeSb2, suggesting the existence of metallic surface states and provides valuable evidence for the perplexing topological behavior exhibited by FeSb2., Comment: 19 pages, 5 figures

We explore the electronic and topological properties of single crystal SrAl$_2$Si$_2$ using magnetotransport experiments in conjunction with first-principle calculations. We find that the temperature-dependent resistivity shows a pronounced peak near 50 K. We observe several remarkable features at low temperatures, such as large non-saturating magnetoresistance, Shubnikov-de Haas oscillations and cusp-like magneto-conductivity. The maximum value of magnetoresistance turns out to be 459\% at 2 K and 12 T. The analysis of the cusp-like feature in magneto-conductivity indicates a clear signature of weak anti-localization. Our Hall resistivity measurements confirm the presence of two types of charge carriers in SrAl$_2$Si$_2$, with low carrier density.

Combining experimental investigations and first-principles DFT calculations, we report physical and magnetic properties of Gd-substituted Y$_2$CoMnO$_6$ double perovskite, which are strongly influenced by antisite-disorder-driven spin configurations. On Gd doping, Co and Mn ions are present in mixed-valence (Co$^{3+}$, Co$^{2+}$, Mn$^{3+}$ and Mn$^{4+}$) states. Multiple magnetic transitions have been observed: i) paramagnetic to ferromagnetic transition is found to occur at \textit{T}$_C$=95.5 K, ii) antiferromagnetic transition at \textit{T}$_N$=47 K is driven by $3d-4f$ polarisation and antisite disorder present in the sample, iii) change in magnetization below \textit{T}$\leq$20 K, primarily originating from Gd ordering, as revealed from our DFT calculations. AC susceptibility measurement confirms the absence of any spin-glass or cluster-glass phases in this material. A significantly large exchange bias effect (\textit{H}$_{EB}$=1.07 kOe) is found to occur below 47 K due to interfaces of FM and AFM clusters created by antisite-disorder., Comment: 19 pages, 12 figures

The mixed rare-earth orthochromite Nd$_{0.5}$Dy$_{0.5}$CrO$_{3}$ has a N\'eel temperature ($T_\mathrm{{N}}$) of ${\sim}$ 175\,K, resulting in the G-type antiferromagnetic ordering of Cr$^{3+}$ spins. The inverse susceptibility shows a deviation from Curie-Weiss law at 230\,K, with a large effective paramagnetic moment of 8.8\,${\mu}_{\mathrm{B}}$. The ZFC-FC magnetization bifurcate just above $T_\mathrm{{N}}$ and show a distinct signature of spin reorientation near 60\,K. Neutron diffraction show that below $T_\mathrm{{N}}$, the Cr$^{3+}$ spins align in ${\Gamma}_{2}$ representation as ($F_{x}$, $G_{z}$). Below 60\,K, due to spin reorientation, the magnetic structure is in ${\Gamma}_{1}$ ($G_{y}$) configuration. The neutron diffraction does not show any signature of rare-earth ordering even at 1.5\,K. First principles density functional theory calculations within GGA+U and GGA+U+SO approximations reveal that the G-type antiferromagnetic order is the ground state magnetic structure of Cr sublattice and the spin-reorientation of Cr$^{3+}$ spins can happen in the absence of 3d-4f interactions unlike in the case of orthoferrites. The specific heat shows a `${\lambda}$' anomaly at $T_\mathrm{{N}}$, while at low temperature two distinct Schottky anomalies are observed; a Schottky peak at 2\,K and an additional step-like feature above 10\,K. Above $T_\mathrm{{N}}$, the magnetic transition is preceded by structural anomalies as seen in our x-ray diffraction and Raman measurements. The deviation of structural parameters near N\'eel temperature is smaller. The phonon frequencies show deviation from the standard anharmonic behaviour: first near 250\,K, due to magneto-volume effects while the second deviation occurs near 200\,K due to spin-phonon coupling., Comment: 12 pages, 13 figures

The complex magnetic structures, spin-reorientation and associated exchange interactions have been investigate in Er$_{0.5}$Dy$_{0.5}$FeO$_3$ using bulk magnetization, neutron diffraction, specific heat measurements and density functional theory calculations. The Fe$^{3+}$ spins order as G-type antiferromagnet structure depicted by ${\Gamma}_{4}$($G_{x}$,$A_{y}$,$F_{z}$) irreducible representation below 700K, similar to its end compounds. The bulk magnetization data indicate occurrence of the spin-reorientation and rare-earth magnetic ordering below $\sim$75 K and 10 K, respectively. The neutron diffraction studies confirm an "incomplete" ${\Gamma}_{4}$${\rightarrow}$ ${\Gamma}_{2}$($F_{x}$,$C_{y}$,$G_{z}$) spin-reorientation initiated $\leq$75 K. Although, the relative volume fraction of the two magnetic structures varies with decreasing temperature, both co-exist even at 1.5 K. At 8 K, Er$^{3+}$/Dy$^{3+}$ moments order as $c_{y}^R$ arrangement develop, which gradually increases in intensity with decreasing temperature. At 2 K, magnetic structure associated with $c_{z}^R$ arrangement of Er$^{3+}$/Dy$^{3+}$ moments also appears. At 1.5 K the magnetic structure of Fe$^{3+}$ spins is represented by a combination of ${\Gamma}_{2}$+${\Gamma}_{4}$+${\Gamma}_{1}$, while the rare earth moments coexists as $c_{y}^R$ and $c_{z}^R$ corresponding to ${\Gamma}_{2}$ and ${\Gamma}_{1}$ representation, respectively. The observed Schottky anomaly at 2.5 K suggests that the "rare-earth ordering" is induced by polarization due to Fe$^{3+}$ spins. The Er$^{3+}$-Fe$^{3+}$ and Er$^{3+}$-Dy$^{3+}$ exchange interactions, obtained from first principle calculations, primarily cause the complicated spin-reorientation and $c_{y}^R$ rare-earth ordering, respectively, while the dipolar interactions between rare-earth moments, result in the $c_{z}^R$ type rare-earth ordering at 2 K., Comment: 15 pages, 11 figures

In present study, the magnetic structure and spin reorientation of mixed doped orthoferrite Nd$_{0.5}$Dy$_{0.5}$FeO$_3$ have been investigated. Similar to both parent compounds (NdFeO$_3$ and DyFeO$_3$), the magnetic structure of Fe$^{3+}$ belongs to ${\Gamma}_{4}$ irreducible representation (G$_{x}$, F$_{z}$) at room temperature. The experimental measurements confirmed the spin reorientation where magnetic structure of Fe$^{3+}$ changes from ${\Gamma}_{4}$ to ${\Gamma}_{2}$(F$_{x}$, G$_{z}$) between 75 and 20 \,K while maintaining G-type configuration. Such a gradual spin reorientation is unusual since the large single ion anisotropy of Dy$^{3+}$ ions causes an abrupt ${\Gamma}_{4}$${\rightarrow}$ ${\Gamma}_{1}$(G$_{y}$) spin reorientation in DyFeO$_3$. Between 20 and 10 \,K, the Fe$^{3+}$ magnetic structure is represented by ${\Gamma}_{2}$ (F$_{x}$, G$_{z}$). Unexpectedly, magnetic structure of Fe$^{3+}$ with ${\Gamma}_{4}$ representation re-emerges below 10\,K which also coincides with the development of rare-earth (Nd$^{3+}$/Dy$^{3+}$) magnetic ordering having C$_{y}$ configuration with magnetic moment of 1.8 ${\mu}_{B}$. The absence of any signature of second order phase transition in the specific heat confirms the role of $R$(Nd$^{3+}$/Dy$^{3+}$)-Fe$^{3+}$ exchange interaction in the observed "rare-earth ordering" unlike DyFeO$_3$ where Dy$^{3+}$ ordering takes place independently to the magnetic ordering of Fe$^{3+}$ magnetic structure. Our (DFT+U+SO) calculations show that the C-type arrangement of rare-earth ions (Nd$^{3+}$/Dy$^{3+}$) with ${\Gamma}_{2}$ configuration for Fe$^{3+}$ moments is the ground state whereas ${\Gamma}_{4}$ phase is energetically very close. Nd-Fe and Nd-Dy exchange interactions, estimated from DFT, are observed to have significant roles in the rare earth ordering and Fe spin reorientation corroborating our experimental results., Comment: 11 pages, 11 figures

The structural, magnetic, and electronic properties of NdFe$_{0.5}$Mn$_{0.5}$O$_3$ have been studied in detail using bulk magnetization, neutron/x-ray diffraction and first principles density functional theory calculations. The material crystallizes in the orthorhombic $Pbnm$ structure, where both Mn and Fe occupy the same crystallographic site ($4b$). Mn/Fe sublattice of the compound orders in to a G-type antiferromagnetic phase close to 250\,K where the magnetic structure belongs to ${\Gamma}_{1}$ irreducible representation with spins aligned along the crystallographic $b$ direction. This is unconventional in the sense that most of the orthoferrites and orthochromites order in the ${\Gamma}_{4}$ representation below the N\'{e}el temperature.This magnetic structure then undergoes a complete spin reorientation transition with temperature in the range 75\,K$\gtrsim$ T $\gtrsim$ 25\,K where the magnetic structure exists as a sum of two irreducible representations (${\Gamma}_{1}$+${\Gamma}_{2}$) as seen from neutron diffraction measurements. At 6\,K, the magnetic structure belongs entirely to ${\Gamma}_{2}$ representation with spins aligned antiferromagnetically along the crystallographic $c$ direction having a small ferromagnetic component ($F_x$). The unusual spin reorientation and correlation between magnetic ground state and electronic structure have been investigated using first principles calculations within GGA+U and GGA+U+SO formalisms.

Present study investigates the photoabsorption properties of single crystal rutile TiO2 (110) surfaces after they have been implanted with low fluence of Cobalt ions. The surfaces, after implantation, demonstrate fabrication of nanostructures and anisotropic nano-ripple patterns. Creation of oxygen vacancies (Ti3+ states) as well as band gap modification for these samples is also observed. Results presented here demonstrate that fabrication of self organized nanostructures and development of oxygen vacancies, upon cobalt implantation, promote the enhancement of photoabsorbance in both UV (2 times) and visible (5 times) regimes. These investigations on nanostructured TiO2 surfaces can be important for photo- catalysis., Comment: 15 pages, 6 Figures

Single phase and strained LuMnO3 thin films are discovered to display co-existing ferromagnetic and antiferromagnetic orders. A large moment ferromagnetism (approx. 1 muB), which is absent in bulk samples, is shown to display a magnetic moment distribution that is peaked at the highly-strained substrate-film interface. We further show that the strain-induced ferromagnetism and the antiferromagnetic order are coupled via an exchange field, therefore demonstrating strained rare-earth managanite thin films as promising candidate systems for new multifunctional devices., Comment: 19 pages, main text plus supplemental information

Using polarized neutron reflectometry (PNR) we have investigated a YBa2Cu3O7(10nm)/La2/3Ca1/3MnO3(9nm)]10 (YBCO/LCMO) superlattice grown by pulsed laser deposition on a La0.3Sr0.7Al0.65Ta0.35O3 (LSAT) substrate. Due to the high structural quality of the superlattice and the substrate, the specular reflectivity signal extends with a high signal-to-background ratio beyond the fourth order superlattice Bragg peak. This allows us to obtain more detailed and reliable information about the magnetic depth profile than in previous PNR studies on similar superlattices that were partially impeded by problems related to the low temperature structural transitions of the SrTiO3 substrates. In agreement with the previous reports, our PNR data reveal a strong magnetic proximity effect showing that the depth profile of the magnetic potential differs significantly from the one of the nuclear potential that is given by the YBCO and LCMO layer thickness. We present fits of the PNR data using different simple block-like models for which either a ferromagnetic moment is induced on the YBCO side of the interfaces or the ferromagnetic order is suppressed on the LCMO side. We show that a good agreement with the PNR data and with the average magnetization as obtained from dc magnetization data can only be obtained with the latter model where a so-called depleted layer with a strongly suppressed ferromagnetic moment develops on the LCMO side of the interfaces. The models with an induced ferromagnetic moment on the YBCO side fail to reproduce the details of the higher order superlattice Bragg peaks and yield a wrong magnitude of the average magnetization. We also show that the PNR data are still consistent with the small, ferromagnetic Cu moment of 0.25muB that was previously identified with x-ray magnetic circular dichroism and x-ray resonant magnetic reflectometry measurements on the same superlattice., Comment: 11 pages, 7 figures

We present a low-energy muon-spin-rotation study of the magnetic and superconducting properties of YBa2Cu3O7/PrBa2Cu3O7 trilayer and bilayer heterostructures. By determining the magnetic-field profiles throughout these structures we show that a finite superfluid density can be induced in otherwise semiconducting PrBa2Cu3O7 layers when juxtaposed to YBa2Cu3O7 "electrodes" while the intrinsic antiferromagnetic order is unaffected., Comment: 10 pages, 9 figures; figure 9 corrected in version 2

Using neutron reflectometry and resonant x-ray techniques we studied the magnetic proximity effect (MPE) in superlattices composed of superconducting YBa$_2$Cu$_3$O$_7$ (YBCO) and ferromagnetic-metallic (FM-M) La$_{0.67}$Ca$_{0.33}$MnO$_{3}$ (LCMO) or ferromagnetic-insulating (FM-I) LaMnO$_{3+\delta}$ (LMO). We find that the MPE strongly depends on the electronic state of the manganite layers, being pronounced for the FM-M LCMO and almost absent for FM-I LMO. We also detail the change of the magnetic depth profile due to the MPE and provide evidence for its intrinsic nature., Comment: 5 pages, 4 figures

The competition between collective quantum phases in materials with strongly correlated electrons depends sensitively on the dimensionality of the electron system, which is difficult to control by standard solid-state chemistry. We have fabricated superlattices of the paramagnetic metal LaNiO3 and the wide-gap insulator LaAlO3 with atomically precise layer sequences. Using optical ellipsometry and low-energy muon spin rotation, superlattices with LaNiO3 as thin as two unit cells are shown to undergo a sequence of collective metalinsulator and antiferromagnetic transitions as a function of decreasing temperature, whereas samples with thicker LaNiO3 layers remain metallic and paramagnetic at all temperatures. Metal-oxide superlattices thus allow control of the dimensionality and collective phase behavior of correlated-electron systems.

Heteroepitaxial superlattices of [YBa2Cu3O7(n)/ La0.67Ca0.33MnO3(m)]x, where n and m are the number of YBCO and LCMO monolayers and x the number of bilayer repetitions, have been grown with pulsed laser deposition on NdGaO3 (110) and Sr0.7La0.3Al0.65Ta0.35O3 (LSAT) (001). These substrates are well lattice matched with YBCO and LCMO and, unlike the commonly used SrTiO3, they do not give rise to complex and uncontrolled strain effects due to structural transitions at low temperature. The growth dynamics and the structure have been studied in-situ with reflection high energy electron diffraction (RHEED) and ex-situ with scanning transmission electron microscopy (STEM), x-ray diffraction, and neutron reflectometry. The individual layers are found to be flat and continuous over long lateral distances with sharp and coherent interfaces and with a well-defined thickness of the individual layer. The only visible defects are antiphase boundaries in the YBCO layers that originate from perovskite unit cell height steps at the interfaces with the LCMO layers. We also find that the first YBCO monolayer at the interface with LCMO has an unusual growth dynamics and is lacking the CuO chain layer while the subsequent YBCO layers have the regular Y-123 structure. Accordingly, the CuO2 bilayers at both the LCMO/YBCO and the YBCO/LCMO interfaces are lacking one of their neighboring CuO chain layers and thus half of their hole doping reservoir. Nevertheless, from electric transport measurements on asuperlattice with n=2 we obtain evidence that the interfacial CuO2 bilayers remain conducting and even exhibit the onset of a superconducting transition at very low temperature. Finally, we show from dc magnetization and neutron reflectometry measurements that the LCMO layers are strongly ferromagnetic.

We show that a multilayer analysis of the infrared c-axis response of RBa2Cu3O7-d (R=Y, Gd, Eu) provides important new information about the anomalous normal state properties of underdoped cuprate high temperature superconductors. Besides competing correlations which give rise to a pseudogap that depletes the low-energy electronic states below T*>>Tc, it enables us to identify the onset of a precursor superconducting state below Tons>Tc. We map out the doping phase diagram of Tons which reaches a maximum of ~180 K at strong underdoping and present magnetic field dependent data which confirm our conclusions., Comment: 5 pages, 3 figures

We report muon spin rotation (muSR) and infrared (IR) spectroscopy experiments on underdoped BaFe1.89Co0.11As2 which show that bulk magnetism and superconductivity (SC) coexist and compete on the nanometer length scale. Our combined data reveal a bulk magnetic order, likely due to an incommensurate spin density wave (SDW), which develops below Tmag \approx 32 K and becomes reduced in magnitude (but not in volume) below Tc = 21.7 K. A slowly fluctuating precursor of the SDW seems to develop alrady below the structural transition at Ts \approx 50 K. The bulk nature of SC is established by the muSR data which show a bulk SC vortex lattice and the IR data which reveal that the majority of low-energy states is gapped and participates in the condensate at T << Tc.

We performed combined infrared reflection and ellipsometry measurements of the in-plane optical reponse of single crystals of the pnictide high temperature superconductor BaFe$_{1.87}$Co$_{0.13}$As$_{2}$ with $T_{c}$ = 24.5 K. We observed characteristic superconductivity-induced changes which provide evidence for at least three different energy gaps. We show that a BCS-model of isotropic gaps with 2$\Delta/k_{B}T_{c}$ of 3.1, 4.7, and 9.2 reproduces the experimental data rather well. We also determine the low-temperature value of the in-plane magnetic penetration depth of 270 nm.

With infrared ellipsometry and transport measurements we investigated the electrons at the interface between LaAlO3 and SrTiO3. We obtained a sheet carrier density of Ns~5-9x 10E13 cm^-2, an effective mass of m*~3m_e, and a strongly frequency dependent mobility. The latter are similar as in bulk SrTi1-xNbxO3 and therefore suggestive of polaronic correlations of the confined carriers. We also determined the vertical density profile which has a strongly asymmetric shape with a rapid initial decay over the first 2 nm and a pronounced tail that extends to about 11 nm., Comment: 4 pages, 3 figures, 1 EPAPS file (3 figures)

We present a muon spin rotation (\muSR) study of the magnetic and superconducting properties of single crystals of electron-doped BaFe2-xCoxAs2 with x=0.08, 0.20, and 0.25 (Tc=9, 25 and 20K) and of polycrystalline hole-doped Pr1-xSrxFeAsO with x=0 and 0.2 (Tc=15 K). In the former series we observe some interesting parallels with the electron doped SmFeAsO1-xFx 1111-type system [A.J. Drew et al., to appear in Nature Materials 2009 and arXiv:0807.4876]. In particular, we obtain evidence that strongly disordered static magnetism coexists with superconductivity on a microscopic scale in underdoped samples and even at optimum doping there is a slowing down (or enhancement) of dynamic magnetic correlations below Tc\approx25K. To the contrary, for the hole-doped Pr1-xSrxFeAsO samples we obtain evidence for a mesoscopic phase segregation into regions with nearly unperturbed AF order and others that are non magnetic and most likely superconducting. The observed trend resembles the one that was previously reported for hole-doped Ba1-xKxFe2As2 [A.A. Aczel et al., Phys. Rev. B 78, 214503 (2008); J.T. Park et al., arXiv:0811.2224] and thus seems to be fairly common in these hole doped systems., Comment: 17 pages, 6 figures

The recent observation of superconductivity with critical temperatures up to 55 K in the FeAs based pnictide compounds marks the first discovery of a non copper-oxide based layered high-Tc superconductor (HTSC) [1-3]. It has raised the suspicion that these new materials share a similar pairing mechanism to the cuprates, since both exhibit superconductivity following charge doping of a magnetic parent material. Here we present a muon spin rotation study on SmFeAsO1-xFx (x=0-0.30), which shows that static magnetism persists well into the superconducting regime. The analogy with the cuprates is quite surprising since the parent compounds appear to have different magnetic ground states: itinerant spin density wave for the pnictides contrasted with the Mott-Hubbard insulator in the cuprates. Our findings suggest that proximity to magnetic order and associated soft magnetic fluctuations, rather than the strong electronic correlations in the vicinity of a Mott-Hubbard-metal-to-insulator transition, may be the key ingredients of HTSC., Comment: Accepted in Nature Materials