Magnetic Phase Transition in Two-Dimensional CrBr3 Probed by a Quantum Sensor

Recently, magnetism in two-dimensional(2 D) van der Waals(vd W) materials has attracted wide interests. It is anticipated that these materials will stimulate discovery of new physical phenomena and novel applications. The capability to quantitatively measure the magnetism of 2 D magnetic vd W materials is essential to understand these materials. Here we report on quantitative measurements of ferromagnetic-to-paramagnetic phase transition of an atomically thin(down to 11 nm) vd W magnet, namely C...     

Phase diagrams in mixed spin-3/2 and spin-2 Ising system with two alternative layers within the effective-field theory

The phase diagrams in the mixed spin-3/2 and spin-2 Ising system with two alternative layers on a honeycomb lattice are investigated and discussed by the use of the effective-field theory with correlations. The interaction of the nearest-neighbour spins of each layer is taken to be positive (ferromagnetic interaction) and the interaction of the adjacent spins of the nearest-neighbour layers is considered to be either positive or negative (ferromagnetic or anti-ferromagnetic interaction). The temperature dependence of the layer magnetizations of the system is examined to characterize the nature (continuous or discontinuous) of the phase transitions and obtain the phase transition temperatures. The system exhibits both second-and first-order phase transitions besides triple point (T P ), critical end point (E), multicritical point (A), isolated critical point (C) and reentrant behaviour depending on the interaction parameters. We have also studied the temperature dependence of the total magnetization to find the compensation points, as well as to determine the type of behaviour, and N-type behaviour in N′eel classification nomenclature existing in the system. The phase diagrams are constructed in eight different planes and it is found that the system also presents the compensation phenomena depending on the sign of the bilinear exchange interactions.     

Superconductivity of bilayer titanium/indium thin film grown on SiO_2/Si(001)

Bilayer superconducting films with tunable transition temperature(Tc) are a critical ingredient to the fabrication of high-performance transition edge sensors. Commonly chosen materials include Mo/Au, Mo/Cu, Ti/Au, and Ti/Al systems. Here in this work, titanium/indium(Ti/In) bilayer superconducting films are successfully fabricated on SiO_2/Si(001)substrates by molecular beam epitaxy(MBE). The success in the epitaxial growth of indium on titanium is achieved by lowering the substrate temperature to-150?C during indium evaporation. We measure the critical temperature under a bias current of 10 μA, and obtain different superconducting transition temperatures ranging from 645 m K to 2.7 K by adjusting the thickness ratio of Ti/In. Our results demonstrate that the transition temperature decreases as the thickness ratio of Ti/In increases.     

The Second Threshold Field of Charge-Density-Wave Conductor Rb0.3MoO3 in High Temperature Range

The switching and threshold properties of quasi-one-dimensional charge-density-wave conductor rubidium blue bronze Rb0.3MoO3 single crystals are investigated in a comparative high and large temperature range. Beyond the limit temperature 50K of Littlewood‘s theory, even up to about lOOK, typical sharp switching to negative or zero differential resistance is observed in E-I characteristic curves. Correspondingly, an obvious switching between two conducting states, from a lowly conducting state to a highly conducting state, is observed in the I-E characteristic curves in the same temperature range. Temperature dependence of the second threshold field ET2 accompanied by this kind of high field switching behaviour is firstly obtained. These new observations are discussed in the mechanism of the current inhomogeneity and redistribution due to the existence of transverse energy barriers suggested by Zhang et al. [Solid State Commun. 85 (1993) 121]     

High thermal stability of diamond-cBN-B_4C-Si composites

Improving the thermal stability of diamond and other superhard materials has great significance in various applications. Here, we report the synthesis and characterization of bulk diamond–cBN–B_4C–Si composites sintered at high pressure and high temperature(HPHT, 5.2 GPa, 1620–1680 K for 3–5 min). The results show that the diamond, cBN, B_4C,B_xSiC, SiO_2 and amorphous carbon or a little surplus Si are present in the sintered samples. The onset oxidation temperature of 1673 K in the as-synthesized sample is much higher than that of diamond, cBN, and B_4C. The high thermal stability is ascribed to the covalent bonds of B–C, C–N, and the solid-solution of B_xSiC formed during the sintering process. The results obtained in this work may be useful in preparing superhard materials with high thermal stability.     

The electronic and magnetic properties of （Mn,C）-codoped ZnO diluted magnetic semiconductor

The electronic and magnetic properties of (Mn,C)-codoped ZnO are studied in the Perdew-Burke-Ernzerhof form of generalized gradient approximation of the density functional theory. By investigating five geometrical configurations, we find that Mn doped ZnO exhibits anti-ferromagnetic or spin-glass behaviour, and there are no carriers to mediate the long range ferromagnetic (FM) interaction without acceptor co-doping. We observe that the FM interaction for (Mn,C)-codoped ZnO is due to the hybridization between C 2p and Mn 3d states, which is strong enough to lead to hole-mediated ferromagnetism at room temperature. Meanwhile, we demonstrate that ZnO co-doped with Mn and C has a stable FM ground state and show that the (Mn,C)-codoped ZnO is FM semiconductor with super-high Curie temperature (T C = 5475 K). These results are conducive to the design of dilute magnetic semiconductors with codopants for spintronics applications.     

CRITICAL CURRENT AND FLUX CREEP IN YBa_2Cu_3O_(7-x) THIN FILMS

The critical currents of YBa2Cu3O7-x thin films have been measured at 74. 3K in magnetic fields up to 2.1T and at the temperature range from 64K to Tc in the absence of the field. Jc-B and Jc-T data were fitted to the flux creep model. The values of pinning energy and number density of pinning centers were obtained from the fits. The results show that the measured critical current density is flux-creep-limited and is several times smaller than Jco, the critical current density in the absence of flux creep. The distribution of pinning energies and inhomogeneity may be responsible for the discrepancy of pinning energy values obtained from different methods.     

Magnetoresistances and magnetic entropy changes associated with negative lattice expansions in NaZn13-type compounds LaFeCoSi

Magnetoresistances and magnetic entropy changes in NaZn13-type compounds La（Fel-xCox）11.9Si1.1 （x=0.04, 0.06, and 0.08） with Curie temperatures of 243 K, 274 K, and 301 K, respectively, are studied. The ferromagnetic ordering is accompanied by a negative lattice expansion. Large magnetic entropy changes in a wide temperature range from ～230 K to ～320 K are achieved. Raising Co content increases the Curie temperature but weakens the magnetovolume effect, thereby causing a decrease in magnetic entropy change. These materials exhibit a metallic character below Tc, whereas the electrical resistance decreases abruptly and then recovers the metal-like behaviour above Tc. Application of a magnetic field retains the transitions via increasing the ferromagnetic ordering temperature. An isothermal increase in magnetic field leads to an increase in electrical resistance at temperatures near but above Tc, which is a consequence of the field-induced metamagnetic transition from a paramagnetic state to a ferromagnetic state.     

Interplay of superconductivity and d-f correlation in CeFeAslPOl_yFy

The recent discovery of high-temperature superconductivity in iron-based pnictides （chalcogenides） not only trig- gers tremendous enthusiasm in searching for new superconducting materials, but also opens a new avenue to the study of the Kondo physics. CeFeAsO is a parent compound of the 1111-type iron-based superconductors. It shows 3d- antiferromagnetic （AFM） ordering below 139 K and 4f-AFM ordering below 4 K. On the other hand, the phosphide CeFePO is a ferromagnetically corelated heavy-fermion （HF） metal with Kondo scale TK 10 K. These properties set up a new platform for research of the interplay among magnetism, Kondo effect, and superconductivity （SC）. In this review, we present the recent progress in the study of chemical pressure effect in CeFeAsOl_yFy （y = 0 and 0.05）. This P/As-doping in CeFeAsO serves as an effective controlling parameter which leads to two magnetic critical points, Xcl -- 0.4 and Xc2 - 0.92, associated with suppression of 3d and 4f magnetism, respectively. We also observe a turning point of AFM-FM ordering of Ce3＋ moment at Xc3 - 0.37. The SC is absent in the phase diagram, which is attributed to the destruction to Cooper pair by Ce-FM fluctuations in the vicinity of Xcl. We continue to investigate CeFeAsl-xPxO0.95Fo.os. With the separation of xcl and xc3, this chemical pressure results in a broad SC region 0〈 x 〈 0.53, while the original HF behavior is driven away by 5% F- doping. Different roles of P and F dopings are addressed, and the interplay between SC and Ce-4f magnetism is also discussed.     

Interplay of superconductivity and d-f correlation in CeFeAs_(1-x)P_xO_(1-y)F_y

The recent discovery of high-temperature superconductivity in iron-based pnictides (chalcogenides) not only triggers tremendous enthusiasm in searching for new superconducting materials, but also opens a new avenue to the study of the Kondo physics. CeFeAsO is a parent compound of the 1111-type iron-based superconductors. It shows 3d-antiferromagnetic (AFM) ordering below ～ 139 K and 4f-AFM ordering below ～ 4 K. On the other hand, the phosphide CeFePO is a ferromagnetically correlated heavy-fermion (HF) metal with Kondo scale T K ～ 10 K. These properties set up a new platform for research of the interplay among magnetism, Kondo effect, and superconductivity (SC). In this review, we present the recent progress in the study of chemical pressure effect in CeFeAsO 1-y F y (y = 0 and 0.05). This P/As-doping in CeFeAsO serves as an effective controlling parameter which leads to two magnetic critical points, x c1 0.4 and x c2 0.92, associated with suppression of 3d and 4f magnetism, respectively. We also observe a turning point of AFM-FM ordering of Ce 3+ moment at x c3 0.37. The SC is absent in the phase diagram, which is attributed to the destruction to Cooper pair by Ce-FM fluctuations in the vicinity of x c1 . We continue to investigate CeFeAs 1-x P x O 0.95 F 0.05 . With the separation of x c1 and x c3 , this chemical pressure results in a broad SC region 0≤ x ≤ 0.53, while the original HF behavior is driven away by 5% F doping. Different roles of P and F dopings are addressed, and the interplay between SC and Ce-4f magnetism is also discussed.     

Different effect of NiMnCo or FeNiCo on the growth of type-Ⅱa large diamonds with Ti/Cu as nitrogen getter

In order to synthesize high-quality type-Ⅱa large diamond, the selection of catalyst is very important, in addition to the nitrogen getter. In this paper, type-IIa large diamonds are grown under high pressure and high temperature(HPHT) by using the temperature gradient method(TGM), with adopting Ti/Cu as the nitrogen getter in Ni_(70)Mn_(25)Co_5(abbreviated as NiMnCo) or Fe_(55)Ni_(29)Co_(16)(abbreviated FeNiCo) catalyst. The values of nitrogen concentration(N_c) in both synthesized high-quality diamonds are less than 1 ppm, when Ti/Cu(1.6 wt%) is added in the FeNiCo or Ti/Cu(1.8 wt%) is added in the NiMnCo. The difference in solubility of nitrogen between both catalysts at HPHT is the basic reason for the different effect of Ti/Cu on eliminating nitrogen. The nitrogen-removal efficiency of Ti/Cu in the NiMnCo catalyst is less than in the FeNiCo catalyst. Additionally, a high-quality type-Ⅱa large diamond size of 5.0 mm is obtained by reducing the growth rate and keeping the nitrogen concentration of the diamond to be less than 1 ppm, when Ti/Cu(1.6 wt%) is added in the FeNiCo catalyst.     

Electrical transport properties of YCo_(1-x)Mn_xO_3(0 ≤x≤ 0.2)prepared by sol–gel process

Mn substitution compounds YCo1-xMnx O3(0 ≤ x ≤ 0.2) are synthesized by using the sol–gel process. Electrical transport properties of YCo1-xMnx O3 are investigated in the temperature range from 200 K to 780 K. The experimental results show that after Mn substitution the electrical resistivity of YCo1-xMnx O3 first increases, then decreases, which is due to the electrons introduced by Mn doping. The sign of Seebeck coefficient for YCo1-xMnx O3(x = 0) is positive or negative, which is also proved by the Hall coefficient measurement. Moreover, at about room temperature, the Seebeck coefficient of YCo1-xMnx O3 with 1% doping Mn content becomes a negative value, whose absolute value is maximum;furthermore, the absolute value gradually decreases with increasing the Mn substitution content, which can be explained by the double carrier model.     

Growth of TlBa_2Ca_2Cu_3O_9 Epitaxial Thin Films by Two-Step Method in Argon

TlBa_2 Ca_2 Cu_3 O_9(Tl-1223) films have promising applications due to their high critical temperature and strong magnetic flux pinning. Nevertheless, the preparation of pure phase Tl-1223 film is still a challenge. We successfully fabricate Tl-1223 thin films on LaAlO_3(001) substrates using dc magnetic sputtering and a post annealing two-step method in argon atmosphere. The crystallization temperature of Tl-1223 films in argon is reduced by 100℃ compared to that in oxygen. This greatly reduces the volatilization of Tl and improves the surface morphology of films. The lower annealing temperature can effectively improve the repeatability of the Tl-1223 film preparation. In addition, pure Tl-1223 phase can be obtained in a broad temperature zone,from 790℃ to 830℃. In our study, the films show homogenous and dense surface morphology using the presented method. The best critical temperature of Tl-1223 films is characterized to be 110 K, and the critical current J_c(77 K, 0 T) is up to 2.13 × 106 A/cm~2.     

Negative Thermal Expansion of GaFe(CN)_6 and Effect of Na Insertion by First-Principles Calculations

We study the negative thermal expansion(NTE) properties and effect of Na insertion on the NTE of the framework material GaFe(CN)_6 by first-principles calculations based on density functional theory within the quasi-harmonic approximation. The calculated results show that the material exhibits NTE due to the low transverse vibrational modes of the CN groups. The modes demonstrate larger negative values of the mode Grüneisen parameters. Once Na is introduced in the framework of the material, it prefers to locate at the center of the quadrates of the framework material and binds to the four N anions nearby. As a consequence, the transverse vibrational mode of the CN group is clearly hindered and the NTE of the material is weakened. Our theoretical calculations have clarified the mechanisms of NTE and the effect of the guest Na on the NTE of the framework material.     

Structural,electronic,and magnetic properties of transition-metal atom adsorbed two-dimensional GaAs nanosheet

By using first-principles calculations within the framework of density functional theory,the electronic and magnetic properties of 3d transitional metal(TM) atoms(from Sc to Zn) adsorbed monolayer Ga As nanosheets(Ga As NSs) are systematically investigated.Upon TM atom adsorption,Ga As NS,which is a nonmagnetic semiconductor,can be tuned into a magnetic semiconductor(Sc,V,and Fe adsorption),a half-metal(Mn adsorption),or a metal(Co and Cu adsorption).Our calculations show that the strong p–d hybridization between the 3d orbit of TM atoms and the 4p orbit of neighboring As atoms is responsible for the formation of chemical bonds and the origin of magnetism in the Ga As NSs with Sc,V,and Fe adsorption.However,the Mn 3d orbit with more unpaired electrons hybridizes not only with the As 4p orbit but also with the Ga 4p orbit,resulting in a stronger exchange interaction.Our results may be useful for electronic and magnetic applications of Ga As NS-based materials.     

Ferrimagnetism and abnormal spin lattice coupling in dilute magnetic ferroelectric (Bi0.46 Na0.46 Ba0.08)TiO3:Co

We have investigated the low-temperature magnetism and spin-lattice coupling in (Bi 0.46 Na 0.46 Ba 0.08)TiO 3:Co in order to understand the magnetoelectric effect in such artificially synthesized dilute magnetic ferroelectrics.It is revealed that the as-prepared (Bi 0.46 Na 0.46 Ba 0.08)TiO 3:Co at Co content of 20%～30% exhibits fascinating ferrimagnetism which is robust against magnetic field,the abnormal spin-lattice coupling characterized by a negative magnetostriction effect;and the suppressed magnetic moment within the temperature range of 30 K～50 K is identified.These magnetic behaviours at low temperatures can be explained by the competition between the ferrimagnetic response and the magnetic moment suppression induced by the abnormal spin-lattice coupling effect.Finally,the ferroelectric and magnetodielectric properties are also discussed.     

Paramagnetism in Cu-doped ZnO

Zn0.98 Cu0.02 O powders are prepared by the sol-gel method.A small number of CuO impurity phases are observed by the x-ray diffraction,indicating the solubility of Cu in ZnO is less than 2 at.%.The Zn0.98 Cu0.02 O powders exhibit diamagnetism at 300 K and paramagnetism at 5 K.After subtracting the diamagnetic contribution of ZnO bulk and the paramagnetic contribution of defects,Cu ions exhibit weak paramagnetism.By codoping Cu with Co or Mn in ZnO,only paramagnetism is observed at room temperature.     

Critical temperature and condensed fraction of Bose-Einstein condensation in optical lattices

Critical temperature and condensate fraction of Bose-Einstein condensation in the optical lattice are studied. The results show that the critical temperature in optical lattices can be characterized with an equivalent critical temperature in a single lattice, which provide a fast evaluation of critical temperature and condensate fraction of Bose-Einstein condensation confined with pure optical trap. Critical temperature can be estimated with an equivalent critical temperature. It is predicted that critical temperature is proportional to q in q number lattices for superfluid state and should be equal to that in a single lattic for Mott insulate state. Required potential depth or Rabi frequency and maximum atom number in the lattices both for superfluid state and Mott state are presented based on views of thermal mechanical statistics.     

Growth characteristics of type IIa large single crystal diamond with Ti/Cu as nitrogen getter in FeNi–C system

High-quality type IIa large diamond crystals are synthesized with Ti/Cu as nitrogen getter doped in an FeNi–C system at temperature ranging from 1230℃ to 1380℃ and at pressure 5.3–5.9 GPa by temperature gradient method. Different ratios of Ti/Cu are added to the Fe Ni–C system to investigate the best ratio for high-quality type IIa diamond. Then, the different content of nitrogen getter Ti/Cu(Ti : Cu = 4 : 3) is added to this synthesis system to explore the effect on diamond growth. The macro and micro morphologies of synthesized diamonds with Ti/Cu added, whose nitrogen concentration is determined by Fourier transform infrared(FTIR), are analyzed by optical microscopy(OM) and scanning electron microscopy(SEM), respectively. It is found that the inclusions in the obtained crystals are minimal when the Ti/Cu ratio is 4:3. Furthermore, the temperature interval for diamond growth becomes narrower when using Ti as the nitrogen getter.Moreover, the lower edge of the synthesis temperature of type IIa diamond is 25℃ higher than that of type Ib diamond.With the increase of the content of Ti/Cu(Ti : Cu = 4 : 3), the color of the synthesized crystals changes from yellow and light yellow to colorless. When the Ti/Cu content is 1.7 wt%, the nitrogen concentration of the crystal is less than 1 ppm.The SEM results show that the synthesized crystals are mainly composed by(111) and(100) surfaces, including(311)surface, when the nitrogen getter is added into the synthesis system. At the same time, there are triangular pits and dendritic growth stripes on the crystal surface. This work will contribute to the further research and development of high-quality type IIa diamond.     

Low temperature magnetic and magnetostrictive properties in Pr(Fe_(1-x)Co_x)_(1.9) cubic Laves alloys

The structures,spin reorientations,magnetic,and magnetostrictive properties of the polycrystalline Pr(Fe_(1-x)Co_x)_(1.9)(x=0–1.0)cubic laves phase alloys between 5 K and 300 K are investigated.Large low-field magnetostrictions are observed at 5 K in the alloys with x=0.2 and 0.4 due to the low magnetic anisotropies of these two alloys.A large negative magnetostriction of about-1130 ppm is found in PrCo_(1.9) alloy at 5 K.The magnetizations of the alloys with 0≤x≤0.6decrease abnormally at the spin reorientation temperature T_(sr),and an abnormity is detected in the alloy with x=1.0 at its Curie temperature T_c(45 K).The substitution of Fe by Co increases the value of T_(sr) in the alloy with x value increasing from 0.0 to 0.4,and then reduces the value of Tsr with x value further increasing to 0.6.