I. S. Lyubutin

Observation of superconductivity in hydrogen sulfide from nuclear resonant scattering

Peeking into a diamond pressure cell A defining characteristic of a superconductor is that it expels an external magnetic field. Demonstrating this effect can be tricky when the sample is under enormous pressures in a diamond anvil cell. Troyan et al. placed a tinfoil sensor inside a sample of H2S under pressure. They then bombarded it with synchrotron radiation and watched how the scattering of photons of tin nuclei changed over time. When H2S was in the normal state, an external magnetic field reached the sensor through the sample, causing the nuclear levels of tin to split. In the superconducting state, however, no splitting was observed because H2S expelled the field before it could reach the sensor. Science, this issue p. 1303 A tin foil sensor inside a pressurized superconducting sample of hydrogen sulfide is used to demonstrate the expulsion of magnetic field. [Also see Perspective by Struzhkin] High-temperature superconductivity remains a focus of experimental and theoretical research. Hydrogen sulfide (H2S) has been reported to be superconducting at high pressures and with a high transition temperature. We report on the direct observation of the expulsion of the magnetic field in H2S compressed to 153 gigapascals. A thin 119Sn film placed inside the H2S sample was used as a sensor of the magnetic field. The magnetic field on the 119Sn sensor was monitored by nuclear resonance scattering of synchrotron radiation. Our results demonstrate that an external static magnetic field of about 0.7 tesla is expelled from the volume of 119Sn foil as a result of the shielding by the H2S sample at temperatures between 4.7 K and approximately 140 K, revealing a superconducting state of H2S.

Mössbauer spectroscopy and magnetic properties of hematite/magnetite nanocomposites

A thermal reduction method has been developed to prepare magnetite/hematite nanocomposites and pure magnetite nanoparticles targeted for specific applications. The relative content of hematite α-Fe2O3 and magnetite Fe3O4 nanoparticles in the product was ensured by maintaining proper conditions in the thermal reduction of α-Fe2O3 powder in the presence of a high boiling point solvent. The structural, electronic, and magnetic properties of the nanocomposites were investigated by F57e-Mossbauer spectroscopy, x-ray diffraction, and magnetic measurements. The content of hematite and magnetite phases was evaluated at every step of the chemical and thermal treatment. It is established that not all iron ions in the octahedral B-sites of magnetite nanoparticles participate in the electron hopping Fe2+⇄Fe3+ above the Verwey temperature TV, and that the charge distribution can be expressed as (Fe3+)tet[Fe1.852.5+Fe0.153+]octO4. It is shown that surface effects, influencing the electronic states of iron ions, dominat...

Controlled oxygen removal in the high-Tc YBa2(Cu1−xFex)3O7−y system and its influence on superconductivity and local structure

Abstract The YBa 2 (Cu 0.99 57 Fe 0.01 ) 3 O 7− y system with a precisely determined oxygen content in the range 0.03≤ y ≤0.80 has bee n obtained for the first time. It is shown that 1% substitution of paramagnetic Fe atoms for Cu does not change the overall dependence of T c on y . A “shelf”-type region at T c ≅60 K is observed on the T c = f ( y ) curve at 0.2 y y =0.5, indicating that the individual YBa 2 Cu 3 O 6.5 phase is probably not superconductive. From the 57 Fe Mossbauer spectra it is shown that at least 90% of the Fe atoms are situated in Cu(1) sites, but iron does not occupy two-fold coordinated sites even in the samples with large oxygen deficiency. About 70% of the Fe atoms are in the twin domains, and about 20% in the domain boundaries. In the domains Fe atoms mainly occupy Cu(1) sites with plane square and five-fold pyramidal oxygen coordinations. The pyramidal environment of Fe atoms in the domains is structurally unstable and easily gives up one oxygen atom at oxygen removal, transforming into the planar square. Every removable oxygen atom belongs to two Fe atoms at Cu(1) sites.

Dependence of exchange interactions on chemical bond angle in a structural series: Cubic perovskite-rhombic orthoferrite-rhombohedral hematite

Mössbauer spectroscopy is used to study the hyperfine magnetic fields at tin 119Sn ions introduced as an isomorphic impurity in the lattices of the orthoferrites RFeO3. The large reduction in the field HSnhf (4.2 K) observed when R is changed from La to Lu correlates with the drop in the Néel point and indicates that the exchange interactions are decreasing over this series. A crystal chemical analysis of the structural series with the general formula ABO3 shows that the ideal structure of cubic perovskite can be converted to a rhombohedral hematite-corundum structure by simple rotation of the [BO6] octahedra if the B-O interionic distances remain unchanged. The rhombic distortions are associated with a reduction in the B-O-B bond angle from θ =180° in perovskite to ∼132° in hematite. The rare earth orthoferrites RFeO3 follow the same mechanism for structural transformations and the LaFeO3-LuFeO3 series occupies an intermediate position (157°>θ>142°) between the extreme members of the series mentioned above. A reduction in the bond angle leads to weakening of the Fe-O-Fe exchange interaction, which shows up as a drop in the Néel temperature and in the hyperfine magnetic field at the nucleus. An analysis of theoretical models shows that for a suitable choice of the exchange and transfer parameters, the angular variation in the parameters of the exchange interaction is described fairly well by the Moskvin theory over a rather wide range of angles θ. The contributions to the fields HSnhf and HFehf from the t2g-and eg-orbitals of neighboring paramagnetic ions in the orthoferrites and orthochromites are examined.

Spin reorientation effects in GdFe3(BO3)4 induced by applied field and temperature

Magnetic properties of GdFe3(BO3)4 single crystals were investigated by 57Fe-Mössbauer spectroscopy and static magnetic measurements. In the ground state, the GdFe3(BO3)4 crystal is an easy-axis compensated antiferromagnet, but the easy axis of iron moments does not coincide with the crystal C3 axis, deviating from it by about 20°. The spontaneous and field-induced spin reorientation effects were observed and studied in detail. The specific directions of iron magnetic moments were determined for different temperatures and applied fields. Large values of the angle between the Fe3+ magnetic moments and the C3 axis in the easy-axis phase and between Fe3+ moments and the a2 axis in the easy-plane phase reveal the tilted antiferromagnetic structure.

Phase transition with suppression of magnetism in BiFeO3 at high pressure

The magnetic behavior of a Bi57FeO3 powdered sample was studied at high pressures by the method of nuclear forward scattering (NFS) of synchrotron radiation. The NFS spectra from 57Fe nuclei were recorded at room temperature under high pressures up to 61.4 GPa, which were created in a diamond anvil cell. In the pressure interval 0 < P < 47 GPa, the magnetic hyperfine field HFe at the 57Fe nuclei increased reaching a value of ∼52.5 T at 30 GPa, and then it slightly decreased to ∼49.6 T at P = 47 GPa. As the pressure was increased further, the field HFe abruptly dropped to zero testifying a transition from the antiferromagnetic to a nonmagnetic state (magnetic collapse). In the pressure interval 47 < P < 61.4 GPa, the value of HFe remained zero. The field HFe recovered to the low-pressure values during decompression.

Structural and electronic transitions in gadolinium iron borate GdFe3(BO3)4 at high pressures

The optical properties and structure of gadolinium iron borate GdFe3(BO3)4 crystals are studied at high pressures produced in diamond-anvil cells. X-ray diffraction data obtained at a pressure of 25.6 GPa reveal a firstorder phase transition retaining the trigonal symmetry and increasing the unit cell volume by 8%. The equation of state is obtained and the compressibility of the crystal is estimated before and after the phase transition. The optical spectra reveal two electronic transitions at pressures ∼26 GPa and ∼43 GPa. Upon the first transition, the optical gap decreases jumpwise from 3.1 to ∼2.25 eV. Upon the second transition at P=43 GPa, the optical gap deceases down to ∼0.7 eV, demonstrating a dielectric-semiconductor transition. By using the theoretical model developed for a FeBO3 crystal and taking into account some structural analogs of these materials, the anomalies of the high-pressure optical spectra are explained.

Optimization of the conditions of synchrotron Mössbauer experiment for studying electronic transitions at high pressures by the example of (Mg, Fe)O magnesiowustite

The effect of the experimental conditions on the shape of the nuclear resonant forward scattering (NFS) from (Mg0.75Fe0.25)O magnesiowustite has been studied at high pressures up to 100 GPa in diamond anvil cells by the method of the NFS of synchrotron radiation from the Fe-57 nuclei at room temperature. The behavior of the system in the electronic transition of the Fe2+ ion from the high-spin to low-spin state (spin crossover) near 62 GPa is analyzed as a function of the sample thickness, degree of nonhydrostaticity, and focusing and collimation conditions of a synchrotron beam. It is found that the inclusion of dynamical beats associated with the sample thickness is very important in the approximation of the experimental NFS spectra. It is shown that the electronic transition occurs in a much narrower pressure range (±6 GPa) rather than in a broad range as erroneously follows from experiments with thick samples under strongly nonhydrostatic conditions.

Electronic and structural transitions in NdFeO3 orthoferrite under high pressures

The effect of high pressure up to 65 GPa on the crystal structure and optical absorption spectra of NdFeO3 orthoferrite single crystals is studied in diamond anvil cells. At P∼37.5 GPa, an electronic transition at which the optical absorption edge jumps from ∼2.2 to ∼0.75 eV is observed. The equation of state V(P) is studied on the basis of the X-ray diffraction data obtained under pressure. This study reveals a first-order structural phase transition at P∼37 GPa with a jump of ∼4% in the unit cell volume. It is shown that the phase transition observed in rare-earth orthoferrites at 30–40 GPa is a transition of the insulator-to-semiconductor type.

High-spin-low-spin transition and the sequence of the phase transformations in the BiFeO3 crystal at high pressures

The transition of Fe3+ ions from the high-spin (HS) state (S = 5/2) to the low-spin (LS) state (S = 1/2) has been observed in the BiFeO3 multiferroic crystal at high pressures in the range 45–55 GPa. This effect has been studied in high-pressure diamond-anvil cells by means of two experimental methods using synchrotron radiation: nuclear resonant forward scattering (NFS or synchrotron Mössbauer spectroscopy) and FeKβ high-resolution X-ray emission spectroscopy (XES). The HS-LS transition correlates with anomalies in the magnetic, optical, transport, and structural properties of the crystal. At room temperature, the transition is not stepwise, but is extended in a pressure range of about 10 GPa due to thermal fluctuations between the high-spin and low-spin states. It has been found that the transition of the BiFeO3 insulator to the metal occurs only in the low-spin phase and the cause of all phase transitions is the HS-LS crossover.