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Magnetic topological materials are promising for realizing novel quantum physical phenomena. Among these, bulk Mn-rich MnSb2Te4 is ferromagnetic due to MnSb antisites and has relatively high Curie temperatures (TC), which is attractive for technological applications. We have previously reported the growth of materials with the formula (Sb2Te3)1−x(MnSb2Te4)x. Here we report their magnetic and transport properties. We show that the samples are divided into three groups based on the percent septuple layers (SLs) within the crystals and their corresponding TC values. Samples that contain less than 70% or more than 90% SLs have a single TC value of 15-20K and 30-40K, respectively, while samples with between 70–80% SLs exhibit two TC values, one at ~ 30-40K and the second (TC2) reaching values above 80K, almost twice as high as any reported value to date for these type of materials. Structural analysis shows that samples with 70–80% SLs have large regions of only SLs, which should give rise to a TC of ~ 30-40K, while other regions have isolated QLs embedded within the SL lattice. We propose that the latter regions are responsible for the higher TC2 values. Our results have important implications for the design of magnetic topological materials having optimum properties.

Hybrid organic–inorganic halides containing Bi and Sb generally exhibit higher stability and lower toxicity compared to Pb analogues. In this work, the synthesis, crystal and electronic structures and optical properties of a brand-new methylammonium bismuth iodide, (MA)4Bi6I22 (MA+ = CH3NH3+), are reported. Interestingly, we find that the presence of the HgI2 is necessary for the targeted preparation of (MA)4Bi6I22. (MA)4Bi6I22 contains isolated [Bi6I22]4− clusters made of six edge-sharing octahedral BiI6 units, which are separated by MA+ cations in its 0D crystal structure. A relatively low optical band gap of 1.9 eV was estimated for (MA)4Bi6I22 based on diffuse reflectance measurements. An intense photoluminescence peak emerges at 636 nm at low temperatures that supports the assigned band gap value. Electronic structure calculations show the presence of flat bands in the valence and conduction bands, consistent with the low-dimensional structure of (MA)4Bi6I22, and slightly indirect nature of the bandgap. Our findings suggest that the use of facilitator moieties such as HgI2 may provide a pathway to obtaining alternative methylammonium bismuth iodides to (MA)3Bi2I9.

International audience; Pinned and mobile ferroelastic domain walls are detected in response to mechanical stress in a Mn3+ complex with two-step thermal switching between the spin triplet and spin quintet forms. Single-crystal X-ray diffraction and resonant ultrasound spectroscopy on [Mn-III(3,5-diCl-sal(2)(323))]BPh4 reveal three distinct symmetry-breaking phase transitions in the polar space group series Cc -> Pc -> P1 -> P1((1/2)). The transition mechanisms involve coupling between structural and spin state order parameters, and the three transitions are Landau tricritical, first order, and first order, respectively. The two first-order phase transitions also show changes in magnetic properties and spin state ordering in the Jahn-Teller-active Mn3+ complex. On the basis of the change in symmetry from that of the parent structure, Cc, the triclinic phases are also ferroelastic, which has been confirmed by resonant ultrasound spectroscopy. Measurements of magnetoelectric coupling revealed significant changes in electric polarization at both the Pc -> P1 and P1 -> P1((1/2)) transitions, with opposite signs. All these phases are polar, while P1 is also chiral. Remanent electric polarization was detected when applying a pulsed magnetic field of 60 T in the P1 -> P1((1/2)) region of bistability at 90 K. Thus, we showcase here a rare example of multifunctionality in a spin crossover material where the strain and polarization tensors and structural and spin state order parameters are strongly coupled.

A Mn

Giant magnetostriction in insulating magnetic materials is highly required for applications but is rarely observed. Here we show that giant magnetostriction $(g1500\phantom{\rule{0.16em}{0ex}}\mathrm{ppm})$ can be achieved in an insulating transition metal oxide $\mathrm{CaBa}{\mathrm{Co}}_{4}{\mathrm{O}}_{7}$ where the ferrimagnetic ordering at ${T}_{\mathrm{C}}\ensuremath{\sim}62\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ is associated with a huge change in the lattice. Moreover, because this material is pyroelectric with a nonswitchable electric polarization ($P$), the giant magnetostriction results in a pronounced magnetoelectric effect---a huge change of electric polarization $(\mathrm{\ensuremath{\Delta}}P\ensuremath{\sim}1.6\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{C}/\mathrm{c}{\mathrm{m}}^{2})$ in response to the applied magnetic field up to 60 T. Geometric frustration as well as the orbital instability of ${\mathrm{Co}}^{2+}/{\mathrm{Co}}^{3+}$ ions is believed to play a crucial role in the giant magnetostriction. Our study provides insights on how to achieve both giant magnetostriction and pronounced magnetoelectric effect in insulating transition metal oxides.

International audience; We investigate giant magnetoelectric coupling at a Mn spin crossover in [MnL]BPh (L = (3,5-diBr-sal)323) with a field-induced permanent switching of the structural, electric, and magnetic properties. An applied magnetic field induces a first-order phase transition from a high spin/low spin (HS-LS) ordered phase to a HS-only phase at 87.5 K that remains after the field is removed. We observe this unusual effect for DC magnetic fields as low as 8.7 T. The spin-state switching driven by the magnetic field in the bistable molecular material is accompanied by a change in electric polarization amplitude and direction due to a symmetry-breaking phase transition between polar space groups. The magnetoelectric coupling occurs due to a γη coupling between the order parameter γ related to the spin-state bistability and the symmetry-breaking order parameter η responsible for the change of symmetry between polar structural phases. We also observe conductivity occurring during the spin crossover and evaluate the possibility that it results from conducting phase boundaries. We perform ab initio calculations to understand the origin of the electric polarization change as well as the conductivity during the spin crossover. Thus, we demonstrate a giant magnetoelectric effect with a field-induced electric polarization change that is 1/10 of the record for any material.

Ferrotoroidal order, which represents a spontaneous arrangement of toroidal moments, has recently been found in a few linear magnetoelectric materials. However, tuning toroidal moments in these materials is challenging. Here, we report switching between ferritoroidal and ferrotoroidal phases by a small magnetic field, in a chiral triangular-lattice magnet BaCoSiO4 with tri-spin vortices. Upon applying a magnetic field, we observe multi-stair metamagnetic transitions, characterized by equidistant steps in the net magnetic and toroidal moments. This highly unusual ferri-ferroic order appears to come as a result of an unusual hierarchy of frustrated isotropic exchange couplings revealed by first principle calculations, and the antisymmetric exchange interactions driven by the structural chirality. In contrast to the previously known toroidal materials identified via a linear magnetoelectric effect, BaCoSiO4 is a qualitatively new multiferroic with an unusual coupling between several different orders, and opens up new avenues for realizing easily tunable toroidal orders. Toroidal moments arise from vortex like spin arrangements. These moments can then interact, giving rise to ferri- or ferro-toroidal order, though controlling such order is difficult. Here, the authors demonstrate a ferri-toroidal state in BaCoSiO4, which under an applied magnetic field exhibits multiple toroidal and metamagnetic transitions.

Magnetic frustration can produce exotic spin configurations and dynamics. Here, the authors explore the seemingly simple competition between ferromagnetic nearest- and antiferromagnetic next-nearest neighbor interactions along Ising spin chains, as realized in the magnetoelectric compound Lu${}_{2}$CoMnO${}_{6}$. Commonly, this situation is described by the axial next-nearest-neighbor Ising model. For the first time, the authors measure and calculate its correlated magnetic dynamics, resulting from a characteristic fractal set of first-order phase boundaries. Experiments and Monte Carlo simulations reveal a dynamics slowdown while approaching the phase transition regime.

Alloys of hexagonal $\delta$-phase UZr$_2$ have been synthesized and studied by means of heat capacity, magnetic susceptibility, magnetization, electrical resistivity, magnetoresistance, thermoelectric power, thermal conductivity measurements, for the first time, at temperatures from 1.8 to 300 K and in magnetic fields up to 8 T. The weak temperature dependence of the magnetic susceptibility and the small value of both Seebeck (0.75 $\mu$V/K at room temperature) and of the Sommerfeld coefficient (13.5 mJ mol$^{-1}$ K$^{-2}$) point to 5$f$-electrons in this material having a delocalized nature. The electrical resistivity and magnetoresistance indicate the presence of significant electronic disorder in $\delta$-UZr$_2$, consistent with the disorder in its crystal structure. Density functional theory calculations have been performed and compared to experimental results., Comment: Accepted by Philosophical Magazine Letters

Complex high-precision mechanical devices can be fabricated using a three-dimensional printing technology with the help of computer-aided design. Using 3D stereolithography, we have constructed a cryogenic goniometer for measurements in pulsed magnetic fields of up to 100 T, at temperatures as low as 0.5 K. We review the properties of several materials tested in developing the goniometer and report on its design and performance. The goniometer allows samples to be rotated in situ to a precision of 0.2° so that the field can be applied at many different angles to the samples' symmetry directions. Following its success, we establish that 3D printing is now a viable technology for pulsed field and other cryogenic probes.

The search for new superconducting materials has been spurred on by the discovery of iron-based superconductors whose structure and composition is qualitatively different from the cuprates. The study of one such material, KxFe2-ySe2 with a critical temperature of 32 K, is made more difficult by the fact that it separates into two phases-a dominant antiferromagnetic insulating phase K2Fe4Se5, and a minority superconducting phase whose precise structure is as yet unclear. Here we perform electrical and magnetization measurements, scanning electron microscopy and microanalysis, X-ray diffraction and scanning tunnelling microscopy on KxFe2-ySe2 crystals prepared under different quenching processes to better understand the relationship between its microstructure and its superconducting phase. We identify a three-dimensional network of superconducting filaments within this material and present evidence to suggest that the superconducting phase consists of a single Fe vacancy for every eight Fe-sites arranged in a √8 x √10 parallelogram structure.

We bring together magnetization, infrared spectroscopy, and lattice dynamics calculations to uncover the magnetic field-temperature (B-T) phase diagrams and vibrational properties of the [(CH3)2NH2]M(HCOO)3 (M = Mn2+, Co2+, Ni2+) family of multiferroics. While the magnetically driven transition to the fully saturated state in [(CH3)2NH2]Mn(HCOO)3 takes place at 15.3 T, substitution with Ni or Co drives the critical fields up toward 100 T, an unexpectedly high energy scale for these compounds. Analysis of the infrared spectrum of the Mn and Ni compounds across TC reveals doublet splitting of the formate bending mode which functions as an order parameter of the ferroelectric transition. By contrast, [(CH3)2NH2]Co(HCOO)3 reveals a surprising framework rigidity across the order−disorder transition due to modest distortions around the Co2+ centers. The transition to the ferroelectric state is thus driven by the dimethylammonium cation freezing and the resulting hydrogen bonding. Under applied field, the Mn (an...

${\mathrm{Sm}\mathrm{B}}_{6}$ is a candidate topological Kondo insulator that displays surface conduction at low temperatures. Here, we perform torque magnetization measurements as a means to detect de Haas--van Alphen (dHvA) oscillations in ${\mathrm{SmB}}_{6}$ crystals grown by aluminum flux. We find that dHvA oscillations occur in single crystals containing embedded aluminum, originating from the flux used to synthesize ${\mathrm{SmB}}_{6}$. Measurements on a sample with multiple, unconnected aluminum inclusions show that aluminum crystallizes in a preferred orientation within the ${\mathrm{SmB}}_{6}$ cubic lattice. The presence of aluminum is confirmed through bulk susceptibility measurements, but does not show a signature in transport measurements. We discuss the ramifications of our results.

The field of double perovskites is now advancing to three magnetic elements on the A, B and ${\mathrm{B}}^{\ensuremath{'}}$ sites. A series of iridium-based double perovskite compounds, $L{n}_{2}{\mathrm{CoIrO}}_{6}$ $(Ln=\mathrm{Eu}, \mathrm{Tb}, \mathrm{Ho})$, with three magnetic elements was synthesized as polycrystalline samples. The compounds crystalize in monoclinic structures with the space group $P{2}_{1}/n$. Magnetic properties of these hetero-tri-spin $3d\text{\ensuremath{-}}5d\text{\ensuremath{-}}4f$ systems were studied by magnetic susceptibility and field dependent magnetization in both DC and pulsed magnetic fields. All these compounds show ferrimagnetic transitions at temperatures ${T}_{C}$ above 100 K, which are attributable to antiferromagnetic coupling between ${\mathrm{Co}}^{2+}$ and ${\mathrm{Ir}}^{4+}$ spins. For ${\mathrm{Eu}}_{2}{\mathrm{CoIrO}}_{6}$, the magnetic properties are similar to those of ${\mathrm{La}}_{2}{\mathrm{CoIrO}}_{6}$. The ${\mathrm{Eu}}^{3+}$ spins show Van Vleck paramagnetism, and do not significantly interact with transition-metal cations. In contrast, ${\mathrm{Tb}}_{2}{\mathrm{CoIrO}}_{6}$ and ${\mathrm{Ho}}_{2}{\mathrm{CoIrO}}_{6}$ reveal a second transition to antiferromagnetic order below a lower temperature ${T}_{N}$. The temperature-induced ferrimagnetic-to-antiferromagnetic phase transition might be explained by a spin-reorientation transition. Moreover, a magnetic-field-induced spin-flop-like transition with a small hysteresis was observed below ${T}_{N}$ in these two compounds. The magnetic moments of all three compounds do not saturate up to 60 T at low temperatures. A moderate magnetocaloric effect was also observed in all three compounds. Our results should motivate further investigation of the spin configuration on single crystals of these iridium-based double perovskites.

A thermoreflectance method was used to measure intragranular thermal diffusivity and conductivity of samples having a composition of U-50 wt%Zr (U–50Zr). Three phases at this composition were investigated: the high temperature γ phase, the low temperature δ phase, and the metastable ω phase. This approach uses a tightly focused laser to inject micron scale thermal waves and a second tightly focused laser to monitor the temperature distribution. The thermal properties are extracted by comparing experimental temperature profiles to an analytical heat diffusion model. The probe laser can monitor the temperature field in orthogonal directions along the surface of the sample and is well suited to measure thermal anisotropy. We show that the δ phase exhibits significant thermal anisotropy. The γ phase has the highest thermal conductivity. The higher conductivity of the γ phase is thought to be due to the presence of Zr precipitates that slightly change the stoichiometry of the γ matrix. The highly disordered ω phase appears to be thermally isotropic and has a lower conductivity than the δ phase. Both observations are likely due to the presence of γ domains that reside between ω domains. This supposition is supported by the presence of thermal heterogeneities that appear as noise in the measured signals.

SmB_{6} is a candidate topological Kondo insulator that displays surface conduction at low temperatures. Here, we perform torque magnetization measurements as a means to detect de Haas-van Alphen (dHvA) oscillations in SmB_{6} crystals grown by aluminum flux. We find that dHvA oscillations occur in single crystals containing embedded aluminum, originating from the flux used to synthesize SmB_{6}. Measurements on a sample with multiple, unconnected aluminum inclusions show that aluminum crystallizes in a preferred orientation within the SmB_{6} cubic lattice. The presence of aluminum is confirmed through bulk susceptibility measurements, but does not show a signature in transport measurements. We discuss the ramifications of our results.

We bring together magnetization, infrared spectroscopy, and lattice dynamics calculations to uncover the magnetic field-temperature ( B- T) phase diagrams and vibrational properties of the [(CH

We present a high resolution method for measuring magnetostriction in millisecond pulsed magnetic fields at cryogenic temperatures with a sensitivity of $1.11\times10^{-11}/\sqrt{\rm Hz}$. The sample is bonded to a thin piezoelectric plate, such that when the sample's length changes, it strains the piezoelectric and induces a voltage change. This method is more sensitive than a fiber-Bragg grating method. It measures two axes simultaneously instead of one. The gauge is small and versatile, functioning in DC and millisecond pulsed magnetic fields. We demonstrate its use by measuring the magnetostriction of Ca$_3$Co$_{1.03}$Mn$_{0.97}$O$_6$ single crystals in pulsed magnetic fields. By comparing our data to new and previously published results from a fiber-Bragg grating magnetostriction setup, we confirm that this method detects magnetostriction effects. We also demonstrate the small size and versatility of this technique by measuring angle dependence with respect to the applied magnetic field in a rotator probe in 65 T millisecond pulsed magnetic fields.

We investigated the Josephson effect between the conventional superconductor Pb0.7In0.3 and Ba0.4K0.6 (FeAs)2 single crystals by performing directional point-contact experiments. The junctions were irradiated with microwaves and the behavior of the Shapiro steps as a function of the square root of the microwave power was studied by means of the RSJ model extended to the nonautonomous case with an rf current-source term. The analysis reveals that, for current injection along the ab plane, the current-phase relation deviates from being simply described by the standard sin(ϕ) (as we indeed observed along the c axis) and a dominant sin(2ϕ) component is observed. The results are compared with a theory of the dc Josephson effect, formulated by using a method based on the calculation of temperature Green’s function in the junction within the tight-binding model (Burmistrova et al., Phys. Rev. B 91, 214501, 2015). The comparison shows that the experimental results here presented strongly favor an s ±-wave symmetry of the order parameter in this compound.

Electric transport and scanning tunneling spectrum (STS) have been investigated on polycrystalline samples of the new superconductor Bi4O4S3. A weak insulating behavior in the resistive curve has been induced in the normal state when the superconductivity is suppressed by applying a magnetic field. Interestingly, a kink appears on the temperature dependence of resistivity near 4 K at all high magnetic fields above 1 T when the bulk superconductivity is completely suppressed. This kink associated with the upper critical field as well as the wide range of excess conductance at low field and high temperature are explained as the possible evidence of strong superconducting fluctuation. From the tunneling spectra, a superconducting gap of about 3 meV is frequently observed yielding a ratio of 2\Delta/(kB*Tc) ~ 16.6. This value is much larger than the one predicted by the BCS theory in the weak coupling regime (2\Delta/(kB*Tc) ~ 3.53), which suggests the strong coupling superconductivity in the present system. Furthermore, the gapped feature persists on the spectra until 14 K in the STS measurement, which suggests a prominent fluctuation region of superconductivity. Such superconducting fluctuation can survive at very high magnetic fields, which are far beyond the critical fields for bulk superconductivity as inferred both from electric transport and tunneling measurements., Comment: 7 pages, 7 figures. arXiv admin note: substantial text overlap with arXiv:1207.4955

We report a high-pressure $^{75}\mathrm{As}$ NMR study on the heavily hole-doped iron pnictide superconductor ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$ (${T}_{c}\ensuremath{\approx}3.8$ K). The low-energy spin fluctuations are found to decrease with applied pressure up to 2 GPa, but then to increase again, changing in lockstep with the pressure-induced evolution of ${T}_{c}$. Their diverging nature suggests close proximity to a magnetic quantum critical point at a negative pressure of $P\ensuremath{\simeq}\ensuremath{-}0.6$ GPa. Above 2.2 GPa, the $^{75}\mathrm{As}$ satellite spectra split below 40 K, indicating a breaking of As site symmetry and an incipient charge order. These pressure-controlled phenomena demonstrate the presence of nearly critical fluctuations in both spin and charge, providing essential input for the origin of superconductivity.

The temperature and angle dependent resistivity of Ba(Fe$_{0.75}$Ru$_{0.25}$)$_2$As$_2$ single crystals were measured in magnetic fields up to 14 T. The temperature dependent resistivity with the magnetic field aligned parallel to c-axis and ab-planes allow us to derive the slope of d$H_{c2}^{ab}/dT$ and d$H_{c2}^c/dT$ near $T_c$ yielding an anisotropy ratio $\Gamma=dH_{c2}^{ab}/dT/dH_{c2}^c/dT \approx$ 2. By scaling the curves of resistivity vs. angle measured at a fixed temperature but different magnetic fields within the framework of the anisotropic Ginzburg-Landau theory, we obtained the anisotropy in an alternative way. Again we found that the anisotropy $(m_c/m_{ab})^{1/2}$ was close to 2. This value is very similar to that in Ba$_{0.6}$K$_{0.4}$Fe$_2$As$_2$ (K-doped Ba122) and Ba(Fe$_{0.92}$Co$_{0.08}$)$_2$As$_2$ (Co-doped Ba122). This suggests that the 3D warping effect of the Fermi surface in Ru-doped samples may not be stronger than that in the K-doped or Co-doped Ba122 samples, therefore the possible nodes appearing in Ru-doped samples cannot be ascribed to the 3D warping effect of the Fermi surface., Comment: 5 pages, 6 figures

Scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy have been investigated on single-crystal samples of ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$. A Van Hove singularity (VHS) has been directly observed just a few meV below the Fermi level $({E}_{\mathrm{F}})$ of superconducting ${\mathrm{KFe}}_{2}{\mathrm{As}}_{2}$, which locates in the middle of the principal axes of the first Brillouin zone. The majority of the density of states at ${E}_{\mathrm{F}}$, mainly contributed by the proximity effect of the saddle point to ${E}_{\mathrm{F}}$, is nongapped in the superconducting state. Our observation of the ungapped feature of the density of states due to the VHS points, while providing consistent explanations to many exotic behaviors previously observed in this material, suggests Cooper pairing induced by a strong-coupling mechanism.

We present theory of dc Josephson effect in contacts between Fe-based and spin-singlet $s$-wave superconductors. The method is based on the calculation of temperature Green's function in the junction within the tight-binding model. We calculate the phase dependencies of the Josephson current for different orientations of the junction relative to the crystallographic axes of Fe-based superconductor. Further, we consider the dependence of the Josephson current on the thickness of an insulating layer and on temperature. Experimental data for PbIn/Ba$_{1-x}$K$_{x}$(FeAs)$_2$ point-contact Josephson junctions are consistent with theoretical predictions for $s_{\pm}$ symmetry of an order parameter in this material. The proposed method can be further applied to calculations of the dc Josephson current in contacts with other new unconventional multiorbital superconductors, such as $Sr_2RuO_4$ and superconducting topological insulator $Cu_xBi_2Se_3$., 16 pages, 14 figures

In-plane resistivity, magnetoresistance and Hall effect measurements have been conducted on quenched K$_x$Fe$_{2-y}$Se$_2$ single crystals in order to analysis the normal-state transport properties. It is found that the Kohler's rule is well obeyed below about 80 K, but clearly violated above 80 K. Measurements of the Hall coefficient reveal a strong but non-monotonic temperature dependence with a maximum at about 80 K, in contrast to any other FeAs-based superconductors. With the two-band model analysis on the Hall coefficient, we conclude that a gap may open below 65 K. The data above 65 K are interpreted as a temperature induced crossover from a metallic state at a low temperature to an orbital-selective Mott phase at a high temperature. This is consistent with the recent data of angle resolved photoemission spectroscopy. These results call for a refined theoretical understanding, especially when the hole pockets are absent or become trivial in K$_x$Fe$_{2-y}$Se$_2$ superconductors.

An intriguing aspect of unconventional superconductivity is that it always appears in the vicinity of other competing phases, whose suppression brings the full emergence of superconductivity. In the iron-pnictides, these competing phases are marked by a tetragonal-to-orthorhombic structural transition and a collinear spin-density-wave (SDW) transition. There has been macroscopic evidence for competition between these phases and superconductivity as the magnitude of both the orthorhombicity and magnetic moment are suppressed in the superconducting state. Here, using angle-resolved photoemission spectroscopy on detwinned underdoped $Ba_{1-x}K_xFe_2As_2$, we observe a coexistence of both the SDW gap and superconducting gap in the same electronic structure. Furthermore, our data reveal that following the onset of superconductivity, the SDW gap decreases in magnitude and shifts in a direction consistent with a reduction of the orbital anisotropy. This observation provides direct spectroscopic evidence for the dynamic competition between superconductivity and both SDW and electronic nematic orders in these materials.

Wei Li,1 Chunfeng Zhang,1,* Shenghua Liu,1 Xiaxin Ding,1 Xuewei Wu,1 Xiaoyong Wang,1 Hai-Hu Wen,1 and Min Xiao1,2,† 1National Laboratory of Solid State Microstructures and School of Physics, Nanjing University, Nanjing 210093, China 2Department of Physics, University of Arkansas, Fayetteville, Arkansas 72701, USA (Received 23 June 2013; revised manuscript received 10 February 2014; published 22 April 2014)

The search for new superconducting materials has been spurred on by the discovery of iron-based superconductors whose structure and composition is qualitatively different from the cuprates. The study of one such material, KxFe2−ySe2 with a critical temperature of 32 K, is made more difficult by the fact that it separates into two phases—a dominant antiferromagnetic insulating phase K2Fe4Se5, and a minority superconducting phase whose precise structure is as yet unclear. Here we perform electrical and magnetization measurements, scanning electron microscopy and microanalysis, X-ray diffraction and scanning tunnelling microscopy on KxFe2−ySe2 crystals prepared under different quenching processes to better understand the relationship between its microstructure and its superconducting phase. We identify a three-dimensional network of superconducting filaments within this material and present evidence to suggest that the superconducting phase consists of a single Fe vacancy for every eight Fe-sites arranged in a √8 x √10 parallelogram structure., The structure of the superconducting phase of iron-based superconductor KxFe2−ySe2 is difficult to determine because it coexists with an predominant insulating phase. Ding et al. identify the superconducting filaments that provide clues to the structure of the parent phase of superconductivity.

Resistive and magnetic properties have been measured in BiS${}_{2}$-based samples CeO${}_{1\ensuremath{-}x}$F${}_{x}$BiS${}_{2}$ with a systematic substitution of O with F ($0lxl0.6$). In contrast to the band-structure calculations, it is found that the parent phase of CeOBiS${}_{2}$ is a bad metal instead of a band insulator. By doping electrons into the system, it is surprising to find that superconductivity appears together with a semiconducting normal state. This evolution is clearly different from the cuprate and the iron pnictide systems, and is interpreted as approaching the Pomeranchuk transition with a von Hove singularity and the possible charge-density-wave instability. Furthermore, ferromagnetism, which may arise from the Ce magnetic moments, has been observed in the low-temperature region in all samples, suggesting the coexistence of superconductivity and ferromagnetism in the superconducting samples.

In the superconductor LaRu$_3$Si$_2$ with the Kagome lattice of Ru, we have successfully doped the Ru with Fe and Co atoms. Contrasting behaviors of suppression to superconductivity is discovered between the Fe and the Co dopants: Fe-impurities can suppress the superconductivity completely at a doping level of only 3%, while the superconductivity is suppressed slowly with the Co dopants. A systematic magnetization measurements indicate that the doped Fe impurities lead to spin-polarized electrons yielding magnetic moments with the magnitude of 1.6 $\mu_B$\ per Fe, while the electrons given by the Co dopants have the same density of states for spin-up and spin-down leading to much weaker magnetic moments. It is the strong local magnetic moments given by the Fe-dopants that suppress the superconductivity. The band structure calculation further supports this conclusion.

We report the successful growth of and the effect of impurity scattering in single crystals of Na( )As (T = Cu, Mn). The temperature dependence of the DC magnetization at high magnetic fields is measured for different concentrations of Cu and Mn. Detailed analysis based on the Curie?Weiss law indicates that Cu doping weakens the average magnetic moments, while doping with Mn enhances the local magnetic moments greatly, suggesting that the former may produce nonmagnetic or very weak magnetic impurities, and that the latter may give rise to magnetic impurities. However, it is found that both doping with Cu and doping with Mn will enhance the residual resistivity and suppress the superconductivity, at similar rates, in the low doping region, which is consistent with the prediction of the S model. For the Cu-doped system, the superconductivity is suppressed completely at a residual resistivity of m? cm, for which a strong localization effect is observed. However, in the case of Mn doping, the suppression of becomes much weaker beyond x = 0.03 and superconductivity is maintained even up to a residual resistivity of 2.86 m? cm. Clearly the magnetic Mn impurities may even be not as detrimental as the nonmagnetic or very weak magnetic Cu impurities to the superconductivity in the high doping regime.

Domain wall motion is detected for the first time during the transition to a ferroelastic and spin state ordered phase of a spin crossover complex. Single‐crystal X‐ray diffraction and resonant ultrasound spectroscopy (RUS) revealed two distinct symmetry‐breaking phase transitions in the mononuclear Mn3+ compound [Mn(3,5‐diBr‐sal2(323))]BPh4, 1. The first at 250 K, involves the space group change Cc→Pc and is thermodynamically continuous, while the second, Pc→P1 at 85 K, is discontinuous and related to spin crossover and spin state ordering. Stress‐induced domain wall mobility was interpreted on the basis of a steep increase in acoustic loss immediately below the the Pc‐P1 transition, Stresses and strains: Domain wall motion is detected for the first time in a spin crossover crystal. Mobility of ferroelastic domain walls was interpreted on the basis of a steep increase in acoustic loss below a first order transition. Spin state ordering and domain formation is associated with collapse of the Jahn–Teller distortion on switching from the spin quintet to spin triplet form of a mononuclear Mn3+ complex.

Three-dimensional (3D) topological insulator (TI) has emerged as a unique state of quantum matter and generated enormous interests in condensed matter physics. The surfaces of a 3D TI consist of a massless Dirac cone, which is characterized by the Z2 topological invariant. Introduction of magnetism on the surface of a TI is essential to realize the quantum anomalous Hall effect and other novel magneto-electric phenomena. Here, by using a combination of first-principles calculations, magneto-transport and angle-resolved photoemission spectroscopy (ARPES), we study the electronic properties of gadolinium (Gd)-doped Sb2Te3. Our study shows that Gd doped Sb2Te3 is a spin-orbit-induced bulk band-gap material, whose surface is characterized by a single topological surface state. Our results provide a new platform to investigate the interactions between dilute magnetism and topology in magnetic doped topological materials.