XPS Study in BiFeO3 Surface Modified by Argon Etching

This paper reports an XPS surface study of pure phase BiFeO3 thin film produced and later etched by pure argon ions. Analysis of high-resolution spectra from Fe 2p, Bi 4f and 5d, O 1s, and the valence band, exhibited mainly Fe3+ and Bi3+ components, but also reveal Fe2+. High-energy argon etching induces the growth of Fe(0) and Bi(0) and an increment of Fe2+, as expected. The BiFeO3 semiconductor character is preserved despite the oxygen loss, an interesting aspect for the study of the photovoltaic effect through oxygen vacancies in some ceramic films. The metal-oxygen bonds in O 1s spectra are related only to one binding energy contrary to the split from bismuth and iron reported in other works. All these data evidence that the low-pressure argon atmosphere is proved to be efficient to produce pure phase BiFeO3, even after argon etching.     

Study of Magnetic Properties and Relaxation Time of Nanoparticle Fe3O4-SiO2

The magnetic properties and relaxation time of Fe₃O₄ nanoparticles, and their encapsulation with silicon dioxide (Fe₃O₄-SiO₂), have been successfully investigated by analyzing the temperature dependence of magnetization (M(T)) and the time dependence of magnetization (M(t)), using the SQUID magnetometer measurement. The M(T) measurement results can determine the magnetic parameters and magnetic irreversibility of Fe₃O₄ and Fe₃O₄-SiO₂ samples. The values of Curie constant (C), effective magnetic moment (μeff), and Weiss temperature (θP) are 4.2 (emu.K.Oe/mol), 5.77 μB, and −349 K, respectively, for the Fe₃O₄ samples, and 81.3 (emu.K.Oe/mol), 25.49 μB, and −2440 K, respectively, for the Fe₃O₄-SiO₂ samples. After encapsulation, the broadening peak deviation decreased from 281.6 K to 279 K, indicating that the superparamagnetic interactions increased with the encapsulation process. The magnetic parameters and irreversibility values showed that the superparamagnetic properties increased significantly after encapsulation (Fe₃O₄-SiO₂). From the results of the M(t) measurement, it was found that there was a decrease in the magnetic relaxation time after the encapsulation process, which indicated that the distribution of the nanoparticle size and anisotropy energy increased.     

(RE)Ba2Cu3O7−δ and the Roeser-Huber Formula

We apply the Roeser–Huber formula to the (RE)Ba2Cu3O7−δ (REBCO with RE= rare earths) high-Tc superconducting material class to calculate the superconducting transition temperature, Tc, using the electronic configuration and the crystallographic data. In a former publication (H. P. Roeser et al., Acta Astronautica 2008, 62, 733–736), the basic idea was described and Tc was successfully calculated for the YBa2Cu3O7−δ compound with two oxygen doping levels δ= 0.04 and 0.45, but several open questions remained. One of the problems remaining was the determination of Tc for the δ= 0.45 sample, which can be explained regarding the various oxygen arrangements being possible within the copper-oxide plane. Having established this proper relation and using the various crystallographic data on the REBCO system available in the literature, we show that the Roeser–Huber equation is capable to calculate the Tc of the various REBCO compounds and the effects of strain and pressure on Tc, when preparing thin film samples. Furthermore, the characteristic length, x, determined for the REBCO systems sheds light on the size of the δTc-pinning sites being responsible for additional flux pinning and the peak effect.     

Mid-Infrared Laser Generation of Zn1−xMnxSe and Zn1−xMgxSe (x ≈ 0.3) Single Crystals Co-Doped by Cr2+ and Fe2+ Ions—Comparison of Different Excitation Wavelengths

Two different mid-infrared (mid-IR) solid-state crystalline laser active media of Cr2+, Fe2+:Zn1−xMnxSe and Cr2+, Fe2+:Zn1−xMgxSe with similar amounts of manganese or magnesium ions of x ≈ 0.3 were investigated at cryogenic temperatures for three different excitation wavelengths: Q-switched Er:YLF laser at the wavelength of 1.73 μm, Q-switched Er:YAG laser at 2.94 μm, and the gain-switched Fe:ZnSe laser operated at a liquid nitrogen temperature of 78 K at ∼4.05 μm. The temperature dependence of spectral and laser characteristics was measured. Depending on the excitation wavelength and the selected output coupler, both laser systems were able to generate radiation by Cr2+ or by Fe2+ ions under direct excitation or indirectly by the Cr2+→ Fe2+ energy transfer mechanism. Laser generation of Fe2+ ions in Cr2+, Fe2+:Zn1−xMnxSe and Cr2+, Fe2+:Zn1−xMgxSe (x ≈ 0.3) crystals at the wavelengths of ∼4.4 and ∼4.8 μm at a temperature of 78 K was achieved, respectively. The excitation of Fe2+ ions in both samples by direct 2.94 μm as well as ∼4.05 μm radiation or indirectly via the Cr2+→ Fe2+ ions’ energy transfer-based mechanism by 1.73 μm radiation was demonstrated. Based on the obtained results, the possibility of developing novel coherent laser systems in mid-IR regions (∼2.3–2.5 and ∼4.4–4.9 μm) based on AIIBVI matrices was presented.     

Effects of JFET Region Design and Gate Oxide Thickness on the Static and Dynamic Performance of 650 V SiC Planar Power MOSFETs

650 V SiC planar MOSFETs with various JFET widths, JFET doping concentrations, and gate oxide thicknesses were fabricated by a commercial SiC foundry on two six-inch SiC epitaxial wafers. An orthogonal P+ layout was used for the 650 V SiC MOSFETs to reduce the ON-resistance. The devices were packaged into open-cavity TO-247 packages for evaluation. Trade-off analysis of the static and dynamic performance of the 650 V SiC power MOSFETs was conducted. The measurement results show that a short JFET region with an enhanced JFET doping concentration reduces specific ON-resistance (Ron,sp) and lowers the gate-drain capacitance (Cgd). It was experimentally shown that a thinner gate oxide further reduces Ron,sp, although with a penalty in terms of increased Cgd. A design with 0.5 μm half JFET width, enhanced JFET doping concentration of 5.5×1016 cm⁻³, and thin gate oxide produces an excellent high-frequency figure of merit (HF-FOM) among recently published studies on 650 V SiC devices.     

Pressure Tuning of Superconductivity of LaPt4Ge12 and PrPt4Ge12 Single Crystals

We carried out electrical resistivity and X-ray diffraction (XRD) studies on the filled skutterudite superconductors LaPt4Ge12 and PrPt4Ge12 under hydrostatic pressure. The superconducting transition temperature Tc is linearly suppressed upon increasing pressure, though the effect of pressure on Tc is rather weak. From the analysis of the XRD data, we obtain bulk moduli of B=106 GPa and B=83 GPa for LaPt4Ge12 and PrPt4Ge12, respectively. The knowledge of the bulk modulus allows us to compare the dependence of Tc on the unit-cell volume from our pressure study directly with that found in the substitution series La1−xPrxPt4Ge12. We find that application of hydrostatic pressure can be characterized mainly as a volume effect in LaPt4Ge12 and PrPt4Ge12, while substitution of Pr for La in La1−xPrxPt4Ge12 yields features going beyond a simple picture.     

Crystallographic Calculations and First-Principles Calculations of Heterogeneous Nucleation Potency of γ-Fe on La2O2S Particles

Rare earth (RE) inclusions with high melting points as heterogeneous nucleation in liquid steel have stimulated many recent studies. Evaluating the potency of RE inclusions as heterogeneous nucleation sites of the primary phase is still a challenge. In this work, the edge-to-edge matching (E2EM) model was employed to calculate the atomic matching mismatch and predict the orientation relationship between La₂O₂S and γ-Fe from a crystallographic point of view. A rough orientation relationship (OR) was predicted with the minimum values of fr=9.43% and fd=20.72% as follows: [21¯1¯0]La2O2S∥[100]γ-Fe and (0003¯)La2O2S∥(002¯)γ-Fe. The interface energy and bonding characteristics between La₂O₂S and γ-Fe were calculated on the atomic scale based on a crystallographic study using the first-principles calculation method. The calculations of the interface energy showed that the S-terminated and La(S)-terminated interface structures were more stable. The results of difference charge density, electron localization function (ELF), the Bader charges and the partial density of states (PDOS) study indicated that the La(S)-terminated interface possessed metallic bonds and ionic bonds, and the S-terminated interface exhibited metallic bond and covalent bond characteristics. This work addressed the stability and the characteristics of the La₂O₂S/γ-Fe interface structure from the standpoint of crystallography and energetics, which provides an effective theoretical support to the study the heterogeneous nucleation mechanism. As a result, La₂O₂S particles are not an effective heterogeneous nucleation site for the γ-Fe matrix from crystallography and energetics points of view.     

Band Structure Studies of the R5Rh6Sn18 (R = Sc,Y, Lu) Quasiskutteridite Superconductors

We report on X-ray photoelectron spectroscopy and ab initio electronic structure investigations of the skutterudite-related R5Rh6Sn18 superconductors, where R = Sc, Y, and Lu. These compounds crystallise with a tetragonal structure (space group I41/acd) and are characterised by a deficiency of R atoms in their formula unit (R5−δRh6Sn18, δ≪1). Recently, we documented that the vacancies δ and atomic local defects (often induced by doping) are a reason for the enhancement in the superconducting transition temperature Tc of these materials, as well as metallic (δ=0) or semimetallic (δ≠0) behaviours in their normal state. Our band structure calculations show the pseudogap at a binding energy of −0.3 eV for the stoichiometric compounds, which can be easily moved towards the Fermi level by vacancies δ. As a result, dychotomic nature in electric transport of R5Rh6Sn18 (metallic or semimetallic resistivity) depends on δ, which has not been interpreted before. We have shown that the densities of states are very similar for various R5Rh6Sn18 compounds, and they practically do not depend on the metal R, while they are determined by the Rh d-and Sn s- and p-electron states. The band structure calculations for Sc5Rh6Sn18 have not been reported yet. We also found that the electronic specific heat coefficients γ0 for the stoichiometric samples were always larger with respect to the γ0 of the respective samples with vacancies at the R sites, which correlates with the results of ab initio calculations.     

VOx Phase Mixture of Reduced Single Crystalline V2O5: VO2 Resistive Switching

The strongly correlated electron material, vanadium dioxide (VO2), has seen considerable attention and research application in metal-oxide electronics due to its metal-to-insulator transition close to room temperature. Vacuum annealing a V2O5(010) single crystal results in Wadsley phases (VnO2n+1, n > 1) and VO2. The resistance changes by a factor of 20 at 342 K, corresponding to the metal-to-insulator phase transition of VO2. Macroscopic voltage-current measurements with a probe separation on the millimetre scale result in Joule heating-induced resistive switching at extremely low voltages of under a volt. This can reduce the hysteresis and facilitate low temperature operation of VO2 devices, of potential benefit for switching speed and device stability. This is correlated to the low resistance of the system at temperatures below the transition. High-resolution transmission electron microscopy measurements reveal a complex structural relationship between V2O5, VO2 and V6O13 crystallites. Percolation paths incorporating both VO2 and metallic V6O13 are revealed, which can reduce the resistance below the transition and result in exceptionally low voltage resistive switching.     

Modularity of PGL2(𝔽p)-representations over totally real fields

We study an analog of Serre’s modularity conjecture for projective representations ρ¯:Gal(K¯/K)→PGL2(k), where K is a totally real number field. We prove cases of this conjecture when k=F5.

Let K be a number field, and consider a continuous representationρ:GK→GL2(k),where k is a finite field. (Here GK denotes the absolute Galois group of K; for this and other notation, see 1.A. Notation below.) We say that ρ is of Serre type, or S type, if it is absolutely irreducible and totally odd, in the sense that for each real place v of K and each associated complex conjugation cv∈GK, detρ(cv)=−1.Serre’s conjecture and its generalizations assert that any ρ of S type should be automorphic (see for example refs. 1 and 2 in the case K=Q, ref. 3 when K is totally real, and ref. 4 for a general number field K). The meaning of the word “automorphic” depends on the context but when K is totally real, for example, we can ask for ρ to be associated to a cuspidal automorphic representation π of GL2(AK), which is regular algebraic of weight 0 (2.A. Automorphy of Linear and Projective Representations). Serre’s conjecture is now a theorem when K=Q (5, 6). For a totally real field K, some results are available when k is “small.” These are summarized in Theorem 1.1, which relies upon refs. 2 and 7–10:Theorem 1.1.LetKbe a totally real number field, and letρ:GK→GL2(k)be a representation ofS type. Then ρ is automorphic provided |k|∈{2,3,4,5,7,9}.One can equally consider continuous representationsσ:GK→PGL2(k),where again k is a finite field. We say that σ is of S type if it is absolutely irreducible and totally odd, in the sense that if k has odd characteristic, then for each real place v of K, σ(cv) is nontrivial. One could formulate a projective analog of Serre’s conjecture, asking that any representation σ of S type be automorphic. A theorem of Tate implies that σ lifts to a linear representation valued in GL2(k′) for some finite extension k′/k, and by σ being automorphic we mean that a lift of it to a linear representation is automorphic (2.A. Automorphy of Linear and Projective Representations). Thus, if k is allowed to vary, this conjecture is equivalent to Serre’s conjecture, since any representation ρ has an associated projective representation Proj(ρ), and any projective representation σ lifts to a representation valued in GL2(k′) for some finite extension k′/k; moreover, ρ is of S type if and only if Proj(ρ) is, and ρ is automorphic if and only if Proj(ρ) is.However, for fixed k the two conjectures are not equivalent: Certainly if ρ is valued in GL2(k) then Proj(ρ) takes values in PGL2(k), but it is not true that any representation σ:GK→PGL2(k) admits a lift valued in GL2(k), and in fact in general the determination of the minimal extension k′/k such that there is a lift to GL2(k′) is somewhat subtle. It is therefore of interest to ask whether the consideration of projective representations allows one to expand the list of “known” cases of Serre’s conjecture.Our main theorem affirms that this is indeed the case. Before giving the statement we need to introduce one more piece of notation. We write Δ:PGL2(k)→k×/(k×)2 for the homomorphism induced by the determinant. We say that a homomorphism GK→k×/(k×)2 is totally even (resp. totally odd) if each complex conjugation in GK is a trivial (resp. nontrivial) image.Theorem 1.2. LetKbe a totally real number field, and letσ:GK→PGL2(k)be a representation ofStype. Thenσis automorphic provided that one of the following conditions is satisfied:

• 1)|k|∈{2,3,4}.
• 2)|k|=5, [K(ζ5):K]=4, andΔ○σis totally even.
• 3)|k|=5, [K(ζ5):K]=4, andΔ○σis totally odd.
• 4)|k|=7andΔ○σis totally odd.
• 5)|k|=9andΔ○σis totally even.

We note the exceptional isomorphisms PSL2(F9)=A6, PGL2(F5)=S5, PGL2(F3)=S4, PGL2(F2)=S3, which link our results to showing that splitting fields of polynomials of small degree over K arise automorphically.The proof of Theorem 1.2 falls into three cases. The first one is when |k| is even or k=F3. When |k| is even, the homomorphism GL2(k)→PGL2(k) splits, so we reduce easily to Theorem 1.1. When k=F3, the homomorphism PGL2(Z[−2])→PGL2(F3) splits and we can use the Langlands–Tunnell theorem (7) to establish the automorphy of σ.The second case is when |k| is odd and −1 is a square in k (resp. a nonsquare in k) and Δ○σ is totally even (resp. totally odd). In this case we are able to construct the following data:

• •A solvable totally real extension L/K and a representation ρ¯1:GL→GL2(k) such that Proj(ρ¯1)=σ|GL (by showing that L/K can be chosen to kill the Galois cohomological obstruction to lifting).
• •A representation ρ2:GK→GL2(Q¯p) such that Proj(ρ¯2) and σ are conjugate in PGL2(F¯p) (by choosing an arbitrary lift of σ to GL2(F¯p) and applying the Khare–Wintenberger method).

We can then use Theorem 1.1 to verify the automorphy of ρ¯1 and hence the residual automorphy of ρ¯2|GL. An automorphy lifting theorem then implies the automorphy of ρ2|GL, hence ρ2 itself by solvable descent, and hence finally of σ.The final case is when k=F5 and Δ○σ is totally odd. In this case there does not exist any totally real extension L/K such that σ|GL lifts to a representation valued in GL2(k) (there is a local obstruction at the real places). However, it is possible to find a CM extension L/K such that σ|GL lifts to a representation valued in GL2(k) with determinant the cyclotomic character. (By definition, a CM number field is a quadratic, totally imaginary extension of a totally real field.) When k=F5, such a representation necessarily appears in the group of 5-torsion points of an elliptic curve over L (8) and so we can use the automorphy results over CM fields established in ref. 11 together with a solvable descent argument to obtain the automorphy of σ.Remark 1.3: In the final case above of a representation σ:GK→PGL2(F5) with nonsolvable image, the residual automorphy of the lift ρ:GL→GL2(F5) ultimately depends on ref. 11, theorem 7.1, which proves the automorphy of certain residually dihedral 2-adic Galois representations. The residual automorphy of these 2-adic representations is verified using automorphic induction. In particular, our proof in this case does not depend on the use of the Langlands–Tunnell theorem. This is in contrast to the argument used in, e.g., ref. 8, theorem 4.1 to establish the automorphy of representations ρ′:GK→GL2(F5) with cyclotomic determinant.This “2–3 switch” strategy can also be used to prove the automorphy of representations σ:GK→PGL2(F3) with Δ○σ totally odd using the 2-adic automorphy theorems proved in ref. 12; see Theorem 3.1. This class of representations includes the projective representations associated to the Galois action on the 3-torsion points of an elliptic curve over K. This gives a way to verify the modulo 3 residual automorphy of elliptic curves over K, which does not rely on the Langlands–Tunnell theorem (and in particular refs. 13 and 14) but only on the Saito–Shintani lifting for holomorphic Hilbert modular forms (15). (We note that we do need to use the Langlands–Tunnell theorem to prove the automorphy of representations σ:GK→PGL2(F3) with Δ○σ totally even; cf. Theorem 2.12.)We now describe the structure of this paper. We begin in 2. Lifting Representations by studying the lifts of projective representations and collecting various results about the existence of characteristic 0 lifts of residual representations and their automorphy. We are then able to give the proofs of Theorem 1.1 and the first two cases in the proof of Theorem 1.2 described above. In 3. Modularity of Mod 3 Representations we expand on Remark 1.3 by showing how the main theorems of ref. 12 can be used to give another proof of the automorphy of S-type representations σ:GK→PGL2(F3) (still under the hypothesis that K is totally real and Δ○σ is totally odd). Finally, in 4. Modularity of Mod 5 Representations we use similar arguments, now based on the main theorems of ref. 11, to complete the proof of Theorem 1.2.A. Notation.If K is a perfect field, then we write GK=Gal(K¯/K) for the Galois group of K with respect to a fixed choice of algebraic closure. If K is a number field and v is a place of K, then we write Kv for the completion of K at v and fix an embedding K¯→K¯v extending the natural embedding K→Kv; this determines an injective homomorphism GKv→GK. If v is a finite place of K, then we write Frobv∈GKv for a lift of the geometric Frobenius, k(v) for the residue field of Kv, and qv for the cardinality of Kv; if v is a real place, then we write cv∈GKv for complex conjugation. Any homomorphism from a Galois group GK to another topological group will be assumed to be continuous.If p is a prime and K is a field of characteristic 0, then we write ϵ:GK→Zp× for the p-adic cyclotomic character, ϵ¯:GK→Fp× for its reduction modulo p, and ω:GK→Fp×/(Fp×)2 for the character ϵ¯mod(Fp×)2. More generally, if ρ:GK→GLn(Q¯p) is a representation, then we write ρ¯:GK→GLn(F¯p) for the associated semisimple residual representation (uniquely determined up to conjugation).If k is a field, then we write Proj:GLn(k)→PGLn(k) for the natural projection and Δ:PGLn(k)→k×/(k×)n for the character induced by the determinant. We use these maps only in the case n=2.If K is a field of characteristic 0, E is an elliptic curve over K, and p is a prime, then we write ρ¯E,p:GK→GL2(Fp) for the representation associated to H1(EK¯,Fp) after a choice of basis. Thus detρ¯E,p=ϵ¯−1.     

Brief Report: Lattices in Tate modules

Refining a theorem of Zarhin, we prove that, given a g-dimensional abelian variety X and an endomorphism u of X, there exists a matrix A∈M2g(ℤ) such that each Tate module TℓX has a ℤℓ-basis on which the action of u is given by A, and similarly for the covariant Dieudonné module if over a perfect field of characteristic p.     

Effect of Sr Doping on Structural and Transport Properties of Bi2Te3

Search for doped superconducting topological insulators is of prime importance for new quantum technologies. We report on fabrication of Sr-doped Bi2Te3 single crystals. We found that Bridgman grown samples have p-type conductivity in the low 1019 cm−3, high mobility of 4000 cm2V−1s−1, crystal structure independent on nominal dopant content, and no signs of superconductivity. We also studied molecular beam epitaxy grown SrxBi2−xTe3 films on lattice matched (1 1 1) BaF2 polar surface. Contrary to the bulk crystals thin films have n-type conductivity. Carrier concentration, mobility and c-lattice constant demonstrate pronounced dependence on Sr concentration x. Variation of the parameters did not lead to superconductivity. We revealed, that transport and structural parameters are governed by Sr dopants incorporation in randomly inserted Bi bilayers into the parent matrix. Thus, our data shed light on the structural position of dopant in Bi2Te3 and should be helpful for further design of topological insulator-based superconductors.     

Strongly Correlated Quantum Spin Liquids versus Heavy Fermion Metals: A Review

This review considers the topological fermion condensation quantum phase transition (FCQPT) that explains the complex behavior of strongly correlated Fermi systems, such as frustrated insulators with quantum spin liquid and heavy fermion metals. The review contrasts theoretical consideration with recent experimental data collected on both heavy fermion metals (HF) and frustrated insulators. Such a method allows to understand experimental data. We also consider experimental data collected on quantum spin liquid in Lu3Cu2Sb3O14 and quasi-one dimensional (1D) quantum spin liquid in both YbAlO3 and Cu(C4H4N2)(NO3)2 with the aim to establish a sound theoretical explanation for the observed scaling laws, Landau Fermi liquid (LFL) and non-Fermi-liquid (NFL) behavior exhibited by these frustrated insulators. The recent experimental data on the heavy-fermion metal α−YbAl1−xFexB4, with x=0.014, and on its sister compounds β−YbAlB4 and YbCo2Ge4, carried out under the application of magnetic field as a control parameter are analyzed. We show that the thermodynamic and transport properties as well as the empirical scaling laws follow from the fermion condensation theory. We explain how both the similarity and the difference in the thermodynamic and transport properties of α−YbAl1−xFexB4 and in its sister compounds β−YbAlB4 and YbCo2Ge4 emerge, as well as establish connection of these (HF) metals with insulators Lu3Cu2Sb3O14, Cu(C4H4N2)(NO3)2 and YbAlO3. We demonstrate that the universal LFL and NFL behavior emerge because the HF compounds and the frustrated insulators are located near the topological FCQPT or are driven by the application of magnetic fields.     

Large Vertical Piezoelectricity in a Janus Cr2I3F3 Monolayer

Two-dimensional (2D) materials have potential applications in nanoscale sensors and spintronic devices. Herein, motivated by experimental synthesis of a CrI3 monolayer possessing intrinsic magnetism and a Janus MoSSe monolayer with piezoelectricity, we propose a 2D Janus Cr2I3F3 monolayer as a multifunctional material exhibiting both piezoelectricity and ferromagnetism. Using density functional theory calculations, we systematically investigated the structural stability and the electronic, magnetic, and piezoelectric properties of the Janus Cr2I3F3 monolayer. We predicted that a vertical polarization of up to −0.155 × 10−10 C/m is induced in the Cr2I3F3 monolayer due to the breaking of symmetry. The origination mechanism of polarization was demonstrated in terms of a local dipole moment calculated by maximally localized Wannier functions. Meanwhile, it was found that a remarkable piezoelectric response can be produced under a uniaxial strain in the basal plane. The calculated piezoelectric coefficients of the Cr2I3F3 monolayer compare favorably with those of the frequently used bulk piezoelectric materials such as α–quartz and wurtzite AlN. Particularly, the e31 and d31 values of the Cr2I3F3 monolayer are nearly 10 times as large as that of Mo-based transition metal dichalcogenides. We also found that the magnitude of e31 mainly arises from the ionic contribution, while the electronic contribution can be nearly neglected. The considerable piezoelectric response combined with the intrinsic magnetism make the Janus Cr2I3F3 monolayer a potential candidate for novel multifunctional devices integrating both piezoelectric and spintronic applications.     

Magnetic Collapse in Fe3Se4 under High Pressure

Electronic structure and magnetic properties of Fe3Se4 are calculated using the density functional approach. Due to the metallic properties, magnetic moments of the iron atoms in two nonequivalent positions in the unit cell are different from ionic values for Fe3+ and Fe2+ and are equal to M1=2.071μB and M2=−2.042μB, making the system ferrimagnetic. The total magnetic moment for the unit cell is 2.135μB. Under isotropic compression, the total magnetic moment decreases non-monotonically and correlates with the non-monotonic dependence of the density of states at the Fermi level N(EF). For 7% compression, the magnetic order changes from the ferrimagnetic to the ferromagnetic. At 14% compression, the magnetic order disappears and the total magnetic moment becomes zero, leaving the system in a paramagnetic state. This compression corresponds to the pressure of 114 GPa. The magnetic ordering changes faster upon application of an isotropic external pressure due to the sizeable anisotropy of the chemical bondings in Fe3Se4. The ferrimagnetic and paramagnetic states occur under pressures of 5.0 and 8.0 GPa, respectively. The system remains in the metallic state for all values of compression.     

Effect of Internal Donors on Raman and IR Spectroscopic Fingerprints of MgCl2/TiCl4 Nanoclusters Determined by Machine Learning and DFT

To go deep into the origin of MgCl2 supported Ziegler-Natta catalysis we need to fully understand the structure and properties of precatalytic nanoclusters MgCl2/TiCl4 in presence of Lewis bases as internal donors (ID). In this work MgCl2/TiCl4 nanoplatelets derived by machine learning and DFT calculations have been used to model the interaction with ethyl-benzoate EB as ID, with available exposed sites of binary TixCly/MgCl2 systems. The influence of vicinal Ti2Cl8 and coadsorbed TiCl4 on energetic, structural and spectroscopic behaviour of EB has been considered. The adsorption of homogeneous-like TiCl4EB and TiCl4(EB)2 at the various surface sites have been also simulated. B3LYP-D2 and M06 functionals combined with TZVP quality basis set have been adopted for calculations. The adducts have been characterized by computing IR and Raman spectra that have been found to provide specific fingerprints useful to identify surface species; IR spectra have been successfully compared to available experimental data.     

Magnetoelastic standing waves induced in UO2 by microsecond magnetic field pulses

Magnetoelastic dilatometry of the piezomagnetic antiferromagnet UO₂ was performed via the fiber Bragg grating method in magnetic fields up to 150 T generated by a single-turn coil setup. We show that in microsecond timescales, pulsed-magnetic fields excite mechanical resonances at temperatures ranging from 10 to 300 K, in the paramagnetic as well as within the robust antiferromagnetic state of the material. These resonances, which are barely attenuated within the 100-µs observation window, are attributed to the strong magnetoelastic coupling in UO₂ combined with the high crystalline quality of the single crystal samples. They compare well with mechanical resonances obtained by a resonant ultrasound technique and superimpose on the known nonmonotonic magnetostriction background. A clear phase shift of π in the lattice oscillations is observed in the antiferromagnetic state when the magnetic field overcomes the piezomagnetic switch field Hc=−18 T. We present a theoretical argument that explains this unexpected behavior as a result of the reversal of the antiferromagnetic order parameter at Hc.

The antiferromagnetic (AFM) insulator uranium dioxide UO₂ has been the subject of extensive research during the last decades predominantly due to its widespread use as nuclear fuel in commercial power reactors (1). Besides efforts to understand the unusually poor thermal conductivity of UO₂, which impacts its performance as nuclear fuel (2), a recent magnetostriction study in pulsed magnetic fields up to 92 T uncovered linear magnetostriction in UO₂ (3), a hallmark of piezomagnetism.Piezomagnetism is characterized by the induction of a magnetic polarization by application of mechanical strain, which, in the case of UO₂, is enabled by broken time-reversal symmetry in the 3-k AFM structure that emerges below TN=30.8 K (4–7) and is accompanied by a Jahn–Teller distortion of the oxygen cage (8–11). This also leads to a complex hysteretic magnetoelastic memory behavior where magnetic domain switching occurs at fields around ±18 T at T=2.5 K. Interestingly, the very large applied magnetic fields proved unable to suppress the AFM state that sets in at TN (3). These earlier results provide direct evidence for the unusually high energy scale of spin-lattice interactions and call for further studies in higher magnetic fields.Here we present axial magnetostriction data obtained in a UO₂ single crystal in magnetic fields to 150 T. These ultrahigh fields were produced by single-turn coil pulsed resistive magnets (12, 13) and applied along the [111] crystallographic axis at various temperatures between 10 K and room temperature. At all temperatures, we observe a dominant negative magnetostriction proportional to H² accompanied by unexpectedly strong oscillations that establish a mechanical resonance in the sample virtually instantly upon delivery of the 102 T/μs pulsed magnetic field rate of change. The oscillations are long-lasting due to very low losses and match mechanical resonances obtained with a resonant ultrasound spectroscopy (RUS) technique (14). Mechanical resonances were suggested to explain anomalies in magnetostriction measurements during single-turn pulses (15, 16); however, their potential to elucidate magnetic dynamics was not explored so far. When the sample is cooled below room temperature, the frequencies soften, consistent with observations in studies of the UO₂ elastic constant c₄₄ as a function of temperature (17, 18).In the AFM state, i.e., T<30.8 K, the characteristic magnetic field sign switch in our single-turn coil magnet (a feature of destructive magnets) results in applied field values in excess of the UO₂ AFM domain switch field of Hc≈−18 T. This field sign switch exposes yet another unexpected result, namely, a π (180°) phase shift in the magnetoelastic oscillations. We use a driven harmonic oscillator and an analytical model to shed light on the origin of the observed phase shift.     

Numerical Simulation and Optimization of Highly Stable and Efficient Lead-Free Perovskite FA1−xCsxSnI3-Based Solar Cells Using SCAPS

Formamidinium tin iodide (FASnI₃)-based perovskite solar cells (PSCs) have achieved significant progress in the past several years. However, these devices still suffer from low power conversion efficiency (PCE=6%) and poor stability. Recently, Cesium (Cs)-doped Formamidinium tin iodide (FA1−xCsxSnI3) showed enhanced air, thermal, and illumination stability of PSCs. Hence, in this work, FA1−xCsxSnI3 PSCs have been rigorously studied and compared to pure FASnI₃ PSCs using a solar cell capacitance simulator (SCAPS) for the first time. The aim was to replace the conventional electron transport layer (ETL) TiO₂ that reduces PSC stability under solar irradiation. Therefore, FA1−xCsxSnI3 PSCs with different Cs contents were analyzed with TiO₂ and stable ZnOS as the ETLs. Perovskite light absorber parameters including Cs content, defect density, doping concentration and thickness, and the defect density at the interface were tuned to optimize the photovoltaic performance of the PSCs. The simulation results showed that the device efficiency was strongly governed by the ETL material, Cs content in the perovskite and its defect density. All the simulated devices with ZnOS ETL exhibited PCEs exceeding 20% when the defect density of the absorber layer was below 1015 cm−3, and deteriorated drastically at higher values. The optimized structure with FA75Cs25SnI3 as light absorber and ZnOS as ETL showed the highest PCE of 22% with an open circuit voltage Voc of 0.89 V, short-circuit current density Jsc of 31.4 mA·cm−2, and fill factor FF of 78.7%. Our results obtained from the first numerical simulation on Cs-doped FASnI₃ could greatly increase its potential for practical production.