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A high degree of spin polarization in half-metallic double perovskites is a prerequisite for several applications in spintronics, which depends crucially on the cationic order of the systems. This paper reports a study on tailoring the structure and morphology of nano-sized Sr2Fe1-xZnxMoO6 (x = 0.05, 0.1, 0.15) materials to improve their spin polarization. The combined analysis of synchrotron X-ray diffraction and magnetization data shows that Zn replaces Fe in the B sites. Although the majority of particles have lateral dimensions in the range 30–60 nm as observed by scanning electron microscope, the samples with x = 0.1 and 0.15 show finite-size effects with superparamagnetism below room temperature and a reduced Curie temperature (from 410 K for x = 0.05–390 K for x = 0.15). The results are due to the formation of networks of insulating Mo–O–Zn–O–Mo linkages and anti-phase boundaries, which divide the particles into smaller domains with a mean diameter of ∼11 nm as determined via a Langevin fit. The almost perfectly ordered structure in the nanodomains is responsible for a high magnetoresistance ratio. A value of -42% at 5 K in 50 kOe is recorded for the sample x = 0.15. Via fitting the magnetoresistance curve using the Inoue-Mekagawa theory, the spin polarization of 99% is determined.

Pathways to tune the electronic, chemical, mechanical, and optical properties of solids without modifying their composition represent a new paradigm in the design of functional and sustainable materials. The level of structural disorder - from perfectly crystalline to fully amorphous – for example, induces remarkable changes in material properties. Typically, disorder is introduced by altering the composition of a material, adding to the misconception that these two properties cannot be decoupled. Here, we demonstrate that striking differences in the optical, electronic, and corrosion properties of CuZr are achieved by deliberately and reproducibly engineering the level of structural disorder in pulsed laser deposited thin films of a constant composition. This approach allows tuning the structure of CuZr from polycrystalline to fully amorphous, switching the nature of charge transport from metallic to semiconductor-like, the optical properties in the visible regime from opaque to transparent, and the corrosion behavior in air from mixed oxidation to the formation of a protective Zr oxide overlayer. Our results highlight the tunability of structural disorder in alloys and its remarkable effect on material properties, providing the opportunity to design sustainable functional materials based on customizing properties beyond their composition.

Although a magnetic field gradually destroys the superconducting state in most materials, a small family of uranium compounds bucks the trend.

The transition metal dichalcogenide $\mathrm{Pd}{\mathrm{Te}}_{2}$ has attractive features based on its classification as a type-II Dirac semimetal and the occurrence of type-I superconductivity, providing a platform for discussion of a topological superconductor. Our recent work revealed that type-I superconductivity persists up to pressures of $\ensuremath{\sim}2.5$ GPa and the superconducting transition temperature ${T}_{\mathrm{c}}$ reaches a maximum at around 1 GPa, which is inconsistent with the theoretical prediction. To understand its nonmonotonic variation and investigate superconductivity at higher pressures, we performed structural analysis by x-ray diffraction at room temperature below 8 GPa and electrical resistivity measurements at low temperatures from 1 to 8 GPa. With regard to the superconductivity beyond 1 GPa, the monotonic decrease in ${T}_{\mathrm{c}}$ is reproduced without any noticeable anomalies; ${T}_{\mathrm{c}}$ changes from 1.8 K at 1 GPa to 0.82 K at 5.5 GPa with $d{T}_{\mathrm{c}}/dP\ensuremath{\sim}\ensuremath{-}0.22$ K/GPa. The crystal structure with space group $P\overline{3}m1$ is stable in the pressure range we examined. On the other hand, the normalized pressure-strain analysis (finite strain analysis) indicates that the compressibility changes around 1 GPa, suggesting that a Lifshitz transition occurs. We here discuss the effect of pressure on the superconducting and structural properties based on the comparison of these experimental results.

Nanocrystalline NbB2 powder has been synthesized via a single step solid-state reaction of precursor materials Nb2 O5 and NaBH4 . Phase composition, grain morphology, microstructure, phonon vibrations and superconducting properties were investigated by XRD, FESEM, HRTEM, Raman spectroscopy and PPMS. The results show that the grain size of nanocrystalline NbB2 is very sensitive to the reaction temperature and easy to aggregate to form micron scale of non-morphology powders. A systematic increase of the electron-phonon coupling constant with the nanoscaling grain size is observed, which we explain in terms of crystal defects and phonon vibrations. The zero-temperature critical fields, Hc1 (0) and Hc2 (0), have been estimated at 85 Oe and 7200 Oe, respectively, and the Ginzburg–Landau parameter κ is 5.1, confirming that NbB2 is a type-II superconductor.

We report a study on the influence of microstructure on the magnetic and electronic properties of nanoscale Sr2-xLaxFeMoO6 materials. The temperature dependence of the saturation magnetization of the samples in the low temperature range follows the spin wave theory which is described by a T3/2 law. The analysis of the temperature dependence of the conductivity of the sample with x = 0 shows distinct mechanisms of spin-dependent tunneling and spin-independent hoping channels. Up on La substitution, additional metallic-type conduction paths appear which reduces the resistivity of the samples and also changes its temperature dependence behavior. Spin polarization as a function of temperature was obtained via fitting the MR curves recorded at different temperatures using the spin-dependent elastic tunneling model with correction for the spin-independent conductance. At a temperature 5 K, the spin polarizations of the samples are found to be roughly proportional to the net magnetic moment. Moreover, their values are considerably higher than those derived from theoretical calculations for the double perovskite lattices with the same antisite disorder levels which reveals an inhomogeneous distribution of the antisite defects in the samples. The decay rate of the spin polarization with increasing temperature is faster for the samples with a larger amount of antisite defects.

Bi1-xSbx alloys are of special significance in topological insulator research. Here we focus on the Bi0.96Sb0.04 alloy in which the conduction band edge just touches the valence band edge. Transport measurements show quantum oscillations in the longitudinal (Shubnikov–de Haas effect) and transverse magnetoresistance originating from a spheroidal Fermi surface pocket. Further investigation of the longitudinal magnetoresistance for the magnetic field parallel to the electrical current shows a small nonmonotonic magnetoresistance that is attributed to a competition of weak-antilocalization effects and a topological term related to the chiral anomaly.

Electrons in a Dirac semimetals possess linear dispersion in all three spatial dimensions, and form part of a developing platform of novel quantum materials. Bi$_{1-x}$Sb$_x$ supports a three-dimensional Dirac cone at the Sb-induced band inversion point. Nanoscale phase-sensitive junction technology is used to induce superconductivity in this Dirac semimetal. Radio frequency irradiation experiments reveal a significant contribution of 4$\pi$-periodic Andreev bound states to the supercurrent in Nb-Bi$_{0.97}$Sb$_{0.03}$-Nb Josephson junctions. The conditions for a substantial $4\pi$ contribution to the supercurrent are favourable because of the Dirac cone's topological protection against backscattering, providing very broad transmission resonances. The large g-factor of the Zeeman effect from a magnetic field applied in the plane of the junction, allows tuning of the Josephson junctions from 0 to $\pi$ regimes., Comment: Supplementary information is included

SrMnSb$_2$ is suggested to be a magnetic topological semimetal. It contains square, 2D Sb planes with non-symmorphic crystal symmetries that could protect band crossings, offering the possibility of a quasi-2D, robust Dirac semi-metal in the form of a stable, bulk (3D) crystal. Here, we report a combined and comprehensive experimental and theoretical investigation of the electronic structure of SrMnSb$_2$, including the first ARPES data on this compound. SrMnSb$_2$ possesses a small Fermi surface originating from highly 2D, sharp and linearly dispersing bands (the Y-states) around the (0,$\pi$/a)-point in $k$-space. The ARPES Fermi surface agrees perfectly with that from bulk-sensitive Shubnikov de Haas data from the same crystals, proving the Y$-$states to be responsible for electrical conductivity in SrMnSb$_2$. DFT and tight binding (TB) methods are used to model the electronic states, and both show good agreement with the ARPES data. Despite the great promise of the latter, both theory approaches show the Y-states to be gapped above E$_F$, suggesting trivial topology. Subsequent analysis within both theory approaches shows the Berry phase to be zero, indicating the non-topological character of the transport in SrMnSb$_2$, a conclusion backed up by the analysis of the quantum oscillation data from our crystals., Comment: 26 pages, 10 figures, revised submission to SciPost after including changes requested by referees. All referee reports are open and can be viewed here: https://scipost.org/submissions/1711.07165v2/

The quantum Hall effect is studied in the topological insulator ${\mathrm{BiSbTeSe}}_{2}$. By employing top- and back-gate electric fields at high magnetic field, the Landau levels of the Dirac cones in the top and bottom topological surface states can be tuned independently. When one surface is tuned to the electron-doped side of the Dirac cone and the other surface to the hole-doped side, the quantum Hall edge channels are counter-propagating. The opposite edge mode direction, combined with the opposite helicities of top and bottom surfaces, allows for scattering between these counter-propagating edge modes. The total Hall conductance is expected to be integer valued only when the scattering is strong. For weaker interaction, a noninteger quantum Hall effect is expected and indications for this effect are measured.

The second-order phase transition into a hidden order phase in URu$_2$Si$_2$ goes along with an order parameter which is still a mystery, despite 30 years of research. However, it is understood that the symmetry of the order parameter must be related to the symmetry of the low lying local electronic $f$-states. Here we present results of a novel spectroscopy, namely core-level non-resonant inelastic x-ray scattering (NIXS). This method allows for the measurement of local high-multipole excitations and it is bulk sensitive. The observed anisotropy of the scattering function unambiguously shows that the 5$f$ ground state wave function is composed mainly, but essentially not purely, of the $\Gamma_1$ with majority $J_z$ =$|4\rangle$ + $|-4\rangle$ and/or $\Gamma_2$ singlet states., Comment: 9 pages, 6 figures

There are two prerequisites for understanding high-temperature (high-T$_c$) superconductivity: identifying the pairing interaction and a correct description of the normal state from which superconductivity emerges. The nature of the normal state of iron-pnictide superconductors, and the role played by correlations arising from partially screened interactions, are still under debate. Here we show that the normal state of carefully annealed electron-doped BaFe$_{2-x}$Co$_{x}$As$_2$ at low temperatures has all the hallmark properties of a local Fermi liquid, with a more incoherent state emerging at elevated temperatures, an identification made possible using bulk-sensitive optical spectroscopy with high frequency and temperature resolution. The frequency dependent scattering rate extracted from the optical conductivity deviates from the expected scaling $M_{2}(\omega,T)\propto(\hbar\omega)^{2}+(p\pi k_{B}T)^{2}$ with $p\approx$ 1.47 rather than $p$ = 2, indicative of the presence of residual elastic resonant scattering. Excellent agreement between the experimental results and theoretical modeling allows us to extract the characteristic Fermi liquid scale $T_{0}\approx$ 1700 K. Our results show that the electron-doped iron-pnictides should be regarded as weakly correlated Fermi liquids with a weak mass enhancement resulting from residual electron-electron scattering from thermally excited quasi-particles., Comment: 6+9pages, 3+9 figures To be published in Scientific Reports

In an ideal 3D topological insulator (TI), the bulk is insulating and the surface conducting due to the existence of metallic states that are localized on the surface; these are the topological surface states. Quaternary Bi-based compounds of Bi$_{2-x}$Sb$_{x}$Te$_{3-y}$Se$_{y}$ with finely-tuned bulk stoichiometries are good candidates for realizing ideal 3D TI behavior due to their bulk insulating character. However, despite its insulating bulk in transport experiments, the surface region of Bi$_{2-x}$Sb$_{x}$Te$_{3-y}$Se$_{y}$ crystals cleaved in ultrahigh vacuum also exhibits occupied states originating from the bulk conduction band. This is due to adsorbate-induced downward band-bending, a phenomenon known from other Bi-based 3D TIs. Here we show, using angle-resolved photoemission, how an EUV light beam of moderate flux can be used to exclude these topologically trivial states from the Fermi level of Bi$_{1.46}$Sb$_{0.54}$Te$_{1.7}$Se$_{1.3}$ single crystals, thereby re-establishing the purely topological character of the low lying electronic states of the system. We furthermore prove that this process is highly local in nature in this bulk-insulating TI, and are thus able to imprint structures in the spatial energy landscape at the surface. We illustrate this by `writing' micron-sized letters in the Dirac point energy of the system., 7 pages, 3 figures, includes Supplementary Information

The pseudobinary series U(Pt1� xPdx)3 demonstrates a wealth of magnetic and superconducting properties that are exemplary for heavy-fermion physics. In this paper, I present a survey of recent neutron-diffraction and mSR experiments, conducted to study the evolution of magnetism, and its interplay with superconductivity, in UPt3 doped with Pd. The small-moment antiferromagnetic order (SMAF) with TNB6 K reported for pure UPt3, is robust upon alloying till at least x ¼ 0:005: The small ordered moment mðxÞ grows from 0.018mB/U-atom for x ¼ 0:00 to 0.048mB/ U-atom for x ¼ 0:005: TN of the SMAF phase does not vary with Pd content. The increase of mðxÞ correlates with the splitting DTc of the superconducting transition and provides evidence for a Ginzburg–Landau scenario for unconventional superconductivity with magnetism as symmetry-breaking field. The absence of a signal of the SMAF phase in zero-field mSR spectra provides strong evidence for a moment fluctuating at a rate >10 MHz. A second largemoment antiferromagnetic phase (LMAF) is found at higher Pd concentrations. For this phase, at optimum doping (x ¼ 0:05) TN;max ¼ 5:8 K and m ¼ 0:62mB/U-atom. The critical Pd concentration for the emergence of the LMAF phase is xc;afB0:006: At the same Pd content, superconductivity is completely suppressed. The existence of a magnetic quantum critical point in the phase diagram, which coincides with the critical point for superconductivity (xc;af ¼ xc;scE0:006), yields evidence for odd-parity superconductivity mediated by ferromagnetic spinfluctuations. r 2002 Elsevier Science B.V. All rights reserved.

Cerium hexaboride (CeB6) as a heavy fermion compound displays a number of interesting low-temperature physical properties such as dense Kondo behavior and a complex magnetic phase diagram due to the interaction between itinerant and local electrons. Recently, the electron emission property of CeB6 has received much attention because it has potential applications in replacing the commercial LaB6 cathode and serving as new-generation thermal cathodes. In addition, by comparison with other metal cathodes, it also possesses some advantages, such as a low work function, low volatility, high brightness, thermal stability and high mechanical strength. However, so far the thermionic emission properties of CeB6 single crystal surfaces except for the (100) surfaces have been rarely reported. Whether the different crystal surfaces of CeB6 contribute to the various interesting emission properties is main research purpose of the present work. In this paper, the (110), (111), (210) and (310) crystal surfaces of single crystal CeB6 are determined by the X-ray Laue diffraction method, and their thermionic emission current densities are measured at different temperatures and applied voltages. As a result, the maximum emission current densities of the (110), (111), (210) and (310) crystal surfaces at 1873 K are 38.4, 11.54, 50.4 and 20.8 A/cm2, respectively. When their cathode temperatures are all 1773 K, their maximum emission current densities are 15.2, 5.43, 28.0 and 11.44 A/cm2. In addition, when the cathode temperature decreases to 1673 K, their maximum emission current densities are 4.24, 0.9, 6.2 and 2.43 A/cm2. It means that the thermionic emissions are strongly anisotropic for the different crystal surfaces. In general, the maximum emission current density of (100) crystal surface of LaB6 single crystal is about 10 A/cm2 at 1700 K. By comparing the emission current density of CeB6 single crystal at 1773 K with that of LaB6 at 1700 K, it is found that the emission properties of (210) crystal surface are maybe close to those of LaB6. The work function values of the (110), (111), (210) crystal surfaces calculated by the Richardson-Dushman formula are 2.64, 2.71 and 2.40 eV, respectively. Among these, the (210) crystal surface possesses the smallest value of the work function, which is hopeful for being used as an electron source of scanning electron microscopy. Zero-field magnetoresistivity measurments confirm the transition temperatures of TQ=3.3 K and TN=2.4 K. Field-angle dependent magnetoresistivity measurments show that the electrical resistivity varies between 69 μupΩ·cm and 73 μupΩ·cm when the crystal rotates from the[001] to the[011] direction. This indicates that the electrical resistivity in a magnetic field is also anisotropic.

Ternary semiconducting or metallic half-Heusler compounds with an atomic composition 1:1:1 are widely studied for their flexible electronic properties and functionalities. Recently, a new material property of half-Heusler compounds was predicted based on electronic structure calculations: the topological insulator. In topological insulators, the metallic surface states are protected from impurity backscattering due to spin-momentum locking. This opens up new perspectives in engineering multifunctional materials. In this article, we introduce half Heusler materials from the crystallographic and electronic structure point of view. We present an effective model Hamiltonian from which the topological state can be derived, notably from a non-trivial inverted band structure. We discuss general implications of the inverted band structure with a focus on the detection of the topological surface states in experiments by reviewing several exemplary materials. Special attention is given to superconducting half-Heusler materials, which have attracted ample attention as a platform for non-centrosymmetric and topological superconductivity., Published on MRS Bulletin, the focus issue on topological insulators. 20 Pages, 7 figures and 1 table

Rich patterns of transformations in the structure of quantum Hall (QH) effect and magnetoresistivity under tilted magnetic fields were obtained in the In x Ga 1− x As/GaAs double quantum well at mK temperatures. Local features correspond to the calculated intersections of Landau levels from different subbands and are due to the sharp motion of their crossing points with parallel field component. An incipient quenching with parallel field of the filling factor ν =3 QH state is revealed, which should be due to suppression of the interlayer connection. The observed peculiar monotonous shift in perpendicular fields with increasing in-plain field of the peak between QH states ν =1 and 2 as well as an unusual minimum on some QH plateaus are probably beyond the single particle treatment.

Development of quantum Hall peculiarities due to mobility gap between spin-split magnetic levels with addition of the parallel magnetic field component B|| is analyzed in double quantum wells (DQW) created in InGaAs/GaAs and InAs/AlSb heterosystems chosen due to their relatively large bulk g-factors. In InGaAs/GaAs DQWs, the nonmonotonous behavior of these peculiarities is observed and explained within single-electron approach in terms of competition between enhanced spin splitting and localization of electrons in the layers of DQW with increased B||. In InAs/AlSb DQW, the tunneling connection between the layers is very weak due to high barrier, nevertheless the collective odd-numbered peculiarities are revealed that exist due to spontaneous interlayer phase coherence. B|| destroys these states that is manifested, in particular, in the suppression of the peculiarity for filling factor v = 3.

We present neutron scattering data on $\mathrm{Ce}{({\mathrm{Ru}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})}_{2}{\mathrm{Ge}}_{2}$ that show that magnetic ordering in the vicinity of a quantum critical point is restricted to short length scales even though all moments that are present have lined up with their neighbors. We argue that order is not limited by crystallographic defects but rather that quantum fluctuations disorder the system and dilute the magnetic moments to such an extent that a percolation network forms. The latter naturally explains how $E∕T$ scaling is possible in a system whose apparent dimensionality is above the upper critical dimension.

The pseudobinary series U(Pt1� xPdx)3 demonstrates a wealth of magnetic and superconducting properties that are exemplary for heavy-fermion physics. In this paper, I present a survey of recent neutron-diffraction and mSR experiments, conducted to study the evolution of magnetism, and its interplay with superconductivity, in UPt3 doped with Pd. The small-moment antiferromagnetic order (SMAF) with TNB6 K reported for pure UPt3, is robust upon alloying till at least x ¼ 0:005: The small ordered moment mðxÞ grows from 0.018mB/U-atom for x ¼ 0:00 to 0.048mB/ U-atom for x ¼ 0:005: TN of the SMAF phase does not vary with Pd content. The increase of mðxÞ correlates with the splitting DTc of the superconducting transition and provides evidence for a Ginzburg–Landau scenario for unconventional superconductivity with magnetism as symmetry-breaking field. The absence of a signal of the SMAF phase in zero-field mSR spectra provides strong evidence for a moment fluctuating at a rate >10 MHz. A second largemoment antiferromagnetic phase (LMAF) is found at higher Pd concentrations. For this phase, at optimum doping (x ¼ 0:05) TN;max ¼ 5:8 K and m ¼ 0:62mB/U-atom. The critical Pd concentration for the emergence of the LMAF phase is xc;afB0:006: At the same Pd content, superconductivity is completely suppressed. The existence of a magnetic quantum critical point in the phase diagram, which coincides with the critical point for superconductivity (xc;af ¼ xc;scE0:006), yields evidence for odd-parity superconductivity mediated by ferromagnetic spinfluctuations. r 2002 Elsevier Science B.V. All rights reserved.

We have investigated the temperature dependence of resistance in the temperature range T=0.07–300K and in magnetic field up to 35T in InAs/GaAs quantum dot layers. In samples with relatively high carrier concentration quantum Hall effect—insulator transition was observed in high magnetic fields. Two-dimensional Mott variable range hopping conductivity has been observed at low temperatures in samples with low carrier concentration. The length of localization correlates very well with the quantum dot cluster size obtained by atomic force microscope. In all samples a positive persistent photoconductivity was observed.

We have investigated the resistance in the temperature range T =0 :4–300 K and magnetotransport in magnetic "elds up to 35 T in InAs/GaAs quantum dot layers. In samples with a relatively high carrier concentration the quantum Hall e'ect-to-Hall insulator transition was observed in high magnetic "elds. Two-dimensional Mott variable range hopping conductivity has been observed at low temperatures in samples with low carrier concentration. The localization length correlates very well with the quantum dot cluster size obtained by atomic force microscope. ? 2003 Elsevier Science B.V. All rights reserved.

In the past decade it has been recognized that the deloacalization of 4f or 5f electrons in cerium or uranium intermetallics occasionally gives rise to the low-temperature formation of a strongly correlated electron band close to the Fermi level. The unprecedented strong renormalization is reflected in a Fermi-liquid quasiparticle mass of the order of 100 times the free electron mass. Concurrently, the Fermi-liquid temperature is strongly renormalized: T F ~ 100 K. The enormous effective mass is built up by a variety of competing electron-electron interactions, of which the on-site Kondo effect and the inter-site Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions are thought to play the leading parts. The stabilization of the heavy-fermion state is attended by striking anomalies in the thermal and transport properties. As to the thermal properties, spin degrees of freedom turn up predominantly in the low-temperature entropy and enhance the electronic specific heat accordingly, while the strains are tightly coupled to the heavy-electron bands via anomalously large Gruneisen parameters giving rise to large coefficients of thermal expansion. As far as the transport properties are concerned, a rapid variation of the scattering processes takes place at low temperatures, as a consequence of the competing on-site and inter-site interactions. This is manifested in the electrical resistivity as a crossover from the Kondo-esque increase to the Fermi-liquid-like decrease in the coherent state, at lowering the temperature. At studying the physics of correlated electrons in heavy-fermion materials, the influence of clean external parameters, e.g. pressure and magnetic field, has received a wide attention. The pressure effects on the heavy-fermion bands are unusually large because of the strong hybridization. However, the influence of a magnetic field is relatively small, so that very strong magnetic fields are needed in order to investigate the heavy-fermion state.

We synthesised high-2D electron-density GaGs/GaAlAs heterostructures with different distance Lσ of Si delta-layer in GaAs from the heterojunction and uniform doped GaAlAs. The quantum Hall effect and Shubnikov-de Haas effect were measured for temperatures down to 0.4 K in magnetic fields up to 40 T. The enhanced 2D electron concentration achieved was 1.1*1013 cm−2 in six filled subbands. The Hall mobility depends on Lσ and has maximum for Lσ=600÷750A. From the amplitudes of the SdH oscillations and Fourier transforms the subband mobilities and electron concentration in each subband have been extracted. According to calculations intersubband electron scattering appears to be important and reduces mobilities in subbands.