L. C. Gupta

Synthesis and Properties of SmO0.5F0.5BiS2 and Enhancement in Tc in La1–ySmyO0.5F0.5BiS2

The crystal structure and properties of a new member of the oxybismuth sulfide family SmO(0.5)F(0.5)BiS(2) are reported here. The compounds SmO(1-x)F(x)BiS(2) (x = 0.0 and 0.5) are isostructural with LaOBiS(2) and crystallize in the CeOBiS(2)-type structure (P4/nmm). Sm substitution in LaO(0.5)F(0.5)BiS(2) (La1-ySmyO(0.5)F(0.5)BiS(2)) leads to a gradual decrease in the a-lattice constant; however, the c-lattice constant does not show such a gradual trend. Enhancement in T(c) is achieved upon partial substitution of La by the smaller Sm ion. A maximum T(c) ∼ 4.6 K was observed for composition with y = 0.8. Disobeying this trend, Tc disappears unexpectedly in the composition SmO(0.5)F(0.5)BiS(2) (y = 1.0). Both the undoped and F-doped (x = 0.0 and 0.5) compounds are paramagnetic, exhibiting semiconducting behavior down to 2 K.

Pressure enhanced superconductivity at 10 K in La doped EuBiS2F

We have investigated the effect of La doping and high pressure on superconducting properties of EuBiS2F, which is a newly discovered superconducting material (T-c similar to 0.3 K) (Zhai et al (2014) Phys. Rev. B 90 064518). An enhancement of T-c to 2.2 K is observed in Eu0.5La0.5BiS2F. Upon application of pressure T-c is further enhanced up to similar to 10 K (P = 2.5 GPa). Eu0.5La0.5BiS2F is semiconducting down to 3 K. The onset of a superconductivity-like feature is seen at 2.2 K at ambient pressure. At a pressure above 1.38 GPa, the T-c(onset) remains invariant at 10 K but the T-c(rho = 0) is increased to above 8.2 K. There is a possible crystallographic transformation by application of pressure from a structure of low T-c to a structure corresponding to high T-c.

CuFeAs : A New Member in the 111-Family of Iron-Pnictides

We have synthesized CuFeAs, a new iron-pnictide compound with a layered tetragonal Cu2Sb type structure [space group P4/nmm: a = b = 3.7442(2) A and c = 5.8925(4) A] that is identical to that of 111-type iron-based superconductors. Our measurements suggest that in low applied magnetic field it undergoes an antiferromagnetic transition below TN ∼ 9 K. When compared with the ground state of CuFeSb, recently reported 111-type ferromagnetic material (TC ∼ 375 K), it has important implication with regard to the nature of Fe–Fe magnetic interaction in Fe–As materials. CuFeAs does not exhibit superconductivity down to 2 K.

Coexistence of superconductivity and ferromagnetism in Sr 0.5 Ce 0.5 FBiS 2- x Se x ( x = 0.5 and 1.0), a non-U material with T c FM

We have carried out detailed magnetic and transport studies of the new Sr0.5Ce0.5FBiS2-xSex (0.0u2009≤u2009xu2009≤u20091.0) superconductors derived by doping Se in Sr0.5Ce0.5FBiS2. Se–doping produces several effects: it suppresses semiconducting–like behavior observed in the undoped Sr0.5Ce0.5FBiS2, the ferromagnetic ordering temperature, TFM, decreases considerably from 7.5u2009K (in Sr0.5Ce0.5FBiS2) to 3.5u2009K and the superconducting transition temperature, Tc, gets enhanced slightly to 2.9–3.3u2009K. Thus in these Se–doped materials, TFM is marginally higher than Tc. Magnetization studies provide evidence of bulk superconductivity in Sr0.5Ce0.5FBiS2-xSex at xu2009≥u20090.5 in contrast to the undoped Sr0.5Ce0.5FBiS2 (xu2009=u20090) where magnetization measurements indicate a small superconducting volume fraction. Quite remarkably, as compared with the effective paramagnetic Ce–moment (~2.2u2009μB), the ferromagnetically ordered Ce–moment in the superconducting state is rather small (~0.1u2009μB) suggesting itinerant ferromagnetism. To the best of our knowledge, Sr0.5Ce0.5FBiS2-x Sex (xu2009=u20090.5 and 1.0) are distinctive Ce–based bulk superconducting itinerant ferromagnetic materials with Tcu2009<u2009TFM. Furthermore, a novel feature of these materials is that they exhibit a dual and quite unusual hysteresis loop corresponding to both the ferromagnetism and the coexisting bulk superconductivity.

Structural and magnetic properties of a new and ordered quaternary alloy MnNiCuSb (SG: F 4¯3m)

Abstract We have synthesized a new crystallographically ordered quaternary Heusler alloy, MnNiCuSb. The crystal structure of the alloy has been determined by Rietveld refinement of the powder X-ray diffraction data. This alloy crystallizes in the LiMgPdSb type structure with F 4 ¯ 3 m space group. MnNiCuSb is a ferromagnet with a high T C ~690xa0K and magnetic moment of 3.85xa0µB/f.u. Besides this we have also studied two other off-stoichiometric compositions; one Cu rich and the other Ni rich (MnNi 0.9 Cu 1.1 Sb and MnNi 1.1 Cu 0.9 Sb) which are also ferromagnets. It must be stressed that MnNiCuSb is one of the very few known, non-Fe containing quaternary Heusler alloys with 1:1:1:1 composition.

Magnetic susceptibility of CeRu3Si2 and a scaling effect in mixed-valence compounds

Abstract To investigate the mixed-valence (MV) state in CeRu 3 Si 2 , we measured the magnetic susceptibility χ ( T ) in the temperature range 8–300xa0K. The magnetic susceptibility is constant below 100xa0K, and slightly increases with increasing temperature. We estimated the spin fluctuation temperature T sf by plotting χ ( T ) T / C against T , where C is the Curie constant of a Ce 3+ ion. CeRu 3 Si 2 is found to be a typical MV system, indicating that χ ( T ) T / C is a function of T / T sf . This scaling relation is applied to the other MV compounds such as CeSn 3 .

Magnetic Susceptibility Study of LaRu3Si2

We have measured the magnetization of LaRu 3 Si 2 to investigate the electronic states in the normal and superconducting states. LaRu 3 Si 2 is a typical type II superconductor with the superconducting transition temperature T s =6.5 K. We estimated the lower critical field, upper critical field, coherence length, penetration depth and Ginzburg–Landau parameter at 2.0 and 4.2 K. We also derived these values at T =0 K according to Ginzburg–Landau theory. In the normal state, the intrinsic magnetic susceptibility after subtracting the impurity part depends on temperature. The fact is considered to be due to a high density of states and a narrow band.

Unusual Mixed Valence of Eu in Two Materials—EuSr2Bi2S4F4 and Eu2SrBi2S4F4: Mössbauer and X-ray Photoemission Spectroscopy Investigations

We have synthesized two new Eu-based compounds, EuSr2Bi2S4F4 and Eu2SrBi2S4F4, which are derivatives of Eu3Bi2S4F4, an intrinsic superconductor with Tc = 1.5 K. They belong to a tetragonal structure (SG: I4/mmm, Z = 2), similar to the parent compound Eu3Bi2S4F4. Our structural and 151Eu Mössbauer spectroscopy studies show that, in EuSr2Bi2S4F4, Eu-atoms exclusively occupy the crystallographic 2a-sites. In Eu2SrBi2S4F4, 2a-sites are fully occupied by Eu-atoms and the other half of Eu-atoms and Sr-atoms together fully occupy 4e-sites in a statistical distribution. In both compounds Eu atoms occupying the crystallographic 2a-sites are in a homogeneous mixed valent state ∼2.6-2.7. From our magnetization studies in an applied H ≤ 9 T, we infer that the valence of Eu-atoms in Eu2SrBi2S4F4 at the 2a-sites exhibits a shift toward 2+. Our XPS studies corroborate the occurrence of valence fluctuations of Eu and after Ar-ion sputtering show evidence of enhanced population of Eu2+-states. Resistivity measurements, down to 2 K, suggest a semimetallic nature for both compounds.

Dramatic variation of the multiferroic properties in Sr doped Ca1-xSrxMn7O12

CaMn7O12 is a magnetic multiferroic material, in which large ferroelectric polarization has reportedly been induced by magnetic ordering. In this work, we observe remarkable changes in the spontaneous ferroelectric polarization PS as well as the magnetization M with only 2% Sr doping. In Ca0.98Sr0.02Mn7O12, PS dramatically becomes more than double the PS in the un-doped material and concomitantly M is reduced to less than half of its value therein. Increase of PS together with the decrease of M points out clearly the coupling of PS and M in CaMn7O12. We stress here that as Ca and Sr are isovalent, no charge carriers (electrons and holes) are added in the system due to Sr-doping. X-ray diffraction shows that all our Sr-doped materials Ca1-xSrxMn7O12 (x = 0.01, 0.02, 0.05, 0.10) are free from secondary phases. From our work, it becomes clear why SrMn7O12 exhibits no or weak ferroelectricity.

Pr2FeCrO6: A Type I Multiferroic

We synthesized double perovskite Pr2FeCrO6 by solid-state method. Analysis of its X-ray powder diffraction shows that the compound crystallizes in a centrosymmetric structure with space group Pbnm. Our X-ray photoelectron spectroscopy (XPS) studies show that all the cations are present in +3 oxidation state. Magnetization studies of Pr2FeCrO6 show that the material is paramagnetic at room temperature and undergoes a magnetic transition below TCM = 250 K. We observe clear magnetic hysteresis loop, for example, below 150 K. A low remnant magnetization Mr, ∼0.05 μB/f. u., is inferred from the observed magnetic hysteresis loop. 57Fe Mössbauer study at 25 K shows a high hyperfine magnetic field of ∼53 T at the Fe nucleus, which corresponds to a magnetic moment of ∼6-7 μB/Fe. These two results together suggest a ferrimagnetic (nearly compensated or canted) ordering of the Fe moments. Mössbauer studies close to the ferrimagnetic ordering temperature suggest interesting magnetic relaxation effects. A dielectric anomaly observed at TCE = 453 K signals a ferroelectric ↔ paraelectric phase transition. We observe at room temperature a clear and well-defined ferroelectric hysteresis loop, PS = 1.04 μC/cm2, establishing ferroelectricity in the material. From these results, we conclude that Pr2FeCrO6 is a type I multiferroic (TCE > TCM).