Single Crystalline Samples Research Articles

Apparatus design for measuring of the strain dependence of the Seebeck coefficient of single crystals.

We present the design and construction of an apparatus that measures the Seebeck coefficient of single crystals under in situ tunable strain at cryogenic temperatures. A home-built three piezostack apparatus applies uni-axial stress to a single crystalline sample and modulates anisotropic strain up to 0.7%. An alternating heater system and cernox sensor thermometry measure the Seebeck coefficient along the uniaxial stress direction. To demonstrate the efficacy of this apparatus, we applied uniaxial stress to detwin single crystals of BaFe2As2 in the orthorhombic phase. The obtained Seebeck coefficient anisotropy is in good agreement with previous measurements using a mechanical clamp.

Evolution of α phase in metastable β titanium alloys studied by small-angle X-ray scattering

Small-angle X-ray scattering (SAXS) is a technique which makes use of elastic scattering of X-ray radiation on inhomogeneities in electron density in the studied material In particular, a difference in chemical compositions between individual phases can be detected. In this research, SAXS was used to study the evolution of α particles in aged samples of a metastable β titanium alloy, Ti-6.8Mo-4.5Fe-1.5Al (LCB). In order to obtain scattering patterns for a known crystallographic orientation, the experiments were carried out on single crystals grown by a floating zone technique. Aged single-crystalline samples were measured in three different orientations, namely (001), (110) and (111) planes of the bcc β matrix oriented perpendicularly to the primary beam. Resulting scattering patterns exhibited symmetries which correlated with the orientation of the studied sample. A simple theoretical model was developed to interpret the shape and orientation of the observed scattering streaks. Good qualitative agreement between experimental data and simulation was found and the first results of the model are presented in this paper.

Multi-band Superconductivity in a misfit layered compound (SnSe)1.16(NbSe2)2

We report the discovery of superconductivity with Tc of about 5.3 K in a new misfit layered compound (SnSe)1.16(NbSe2)2. High resolution transmission electron microscopy and selected area electron diffraction of the single-crystalline samples clearly reveal the misfit layered structure. Based on the McMillan equation, the electron-phonon coupling constant λe – ph is estimated to be about 0.96, which is close to strong coupling range. The estimated out-of-plane and in-plane upper critical magnetic fields are 8.9 T and 15.5 T respectively. (0) is 1.74 times the value of Pauli paramagnetic limit. Moreover, there is a positive curvature in the curves near Tc, indicating a feature of multi-band superconductivity. The temperature dependence of specific-heat in superconducting state can be fit by a two-band BCS model, which confirms further the multi-band superconductivity. The reduced charge transfer between the two subsystems may account for the enhanced Tc comparing with (SnSe)1.16(NbSe2).

Tailoring the thermal transport properties of monolayer hexagonal boron nitride by grain size engineering

The grain size effect on the thermal transport properties of hexagonal boron nitride (h-BN) thin films was experimentally investigated using the opto-thermal Raman technique. High-quality monolayer h-BN with mean grain sizes ranging from ~7 µm to ~19 nm were successfully synthesized on Pt foil by chemical vapor deposition (CVD). The thermal conductivity (κ) of the single-crystalline h-BN was measured to be ~545 Wm−1 K−1 at 315K, well above the bulk value, and more than a factor of four higher than the value of poly-crystalline h-BN with mean grain size of ~19 nm. The very low thermal boundary conductance (deduced to be ~9.6 GW m−2 K−1) accounts for the significant reduction of κ for h-BN with small grain size. Molecular dynamics (MD) simulations reveal that due to the disordered vibrations of atoms along/near GB, the phonon scattering in poly-crystalline h-BN is greatly enhanced compared to large-grained or single-crystalline samples. These results provide a deep understanding of the thermal transport in two-dimensional systems as well as the possible technological applications.

Mechanical and correlated electronic transport properties of preferentially orientated SmNiO3 films

Although the correlated rare-earth nickelate perovskites (such as SmNiO3) exhibit remarkable metal to insulator transitions (MIT) beyond conventional semiconductors, their respective variations in mechanical properties during MIT remain yet unknown. Exploring the intrinsic mechanical properties of SmNiO3 is largely prohibited since it is theoretically impossible to grow the single crystalline samples in a large size as required for conventional characterizations. For the first time, we performed nano-indentation tests of preferentially oriented SmNiO3 films with thickness approaching micrometer scale epitaxy on single crystalline LaAlO3 substrates, and the corresponding temperature-dependent mechanical properties are characterized. As-grown SmNiO3 thick films experience sharp transitions in electrical resistivity across MIT, while their insulating phases exhibit thermistor transportation behaviors with the negative temperature dependence of resistance within a broad range of temperature. Nano-indentation tests at room and elevated temperature regions (25–200 °C) are performed for harvesting the Vickers hardness and elastic modulus at different temperatures. In addition, it is also interesting to note that these mechanical properties remain relatively stable during MIT, despite the structural twisting in their NiO6 octahedron during the MIT as previously known for SmNiO3. The SmNiO3 is shown to be relatively soft among the family of perovskite oxides, while its mechanical properties are insensitive to increase temperature across its metal to insulator transitions. It paves the way for further applying rare-earth nickelates in the field of correlation electronics.

Commensurate to incommensurate magnetic phase transition in honeycomb-lattice pyrovanadateMn2V2O7

We have synthesized single crystalline sample of Mn$_2$V$_2$O$_7$ using floating zone technique and investigated the ground state using magnetic susceptibility, heat capacity and neutron diffraction. Our magnetic susceptibility and heat capacity reveal two successive magnetic transitions at $T_{N1} =$ 19 K and $T_{N2} =$ 11.8 K indicating two distinct magnetically ordered phases. The single crystal neutron diffraction study shows that in the temperature ($T$) range 11.8 K $\le T \le$ 19 K the magnetic structure is commensurate with propagation vector $k_1 = (0, 0.5, 0)$, while upon lowering temperature below $T_{N2} =$ 11.8 K an incommensurate magnetic order emerges with $k_2 = (0.38, 0.48, 0.5)$ and the magnetic structure can be represented by cycloidal modulation of the Mn spin in $ac-$plane. We are reporting this commensurate to incommensurate transition for the first time. We discuss the role of the magnetic exchange interactions and spin-orbital coupling on the stability of the observed magnetic phase transitions.

Superconductivity under pressure in the Dirac semimetal PdTe2

The Dirac semimetal PdTe2 was recently reported to be a type-I superconductor (T c = 1.64 K, mT) with unusual superconductivity of the surface sheath. We here report a high-pressure study, GPa, of the superconducting phase diagram extracted from ac-susceptibility and transport measurements on single crystalline samples. T c (p ) shows a pronounced non-monotonous variation with a maximum T c = 1.91 K around 0.91 GPa, followed by a gradual decrease to 1.27 K at 2.5 GPa. Surface superconductivity is robust under pressure as demonstrated by the large superconducting screening signal that persists for applied dc-fields . Surprisingly, for GPa the superconducting transition temperature at the surface is larger than T c of the bulk. Therefore surface superconductivity may possibly have a non-trivial topological nature. We compare the measured pressure variation of T c with recent results from band structure calculations and discuss the importance of a Van Hove singularity.

Theoretical study and numerical simulations on the stress-induced twin interface nucleation in single-crystalline NiMnGa alloys

In this paper, the stress-induced twin interface nucleation in single-crystalline NiMnGa alloys is studied through theoretical analyses and numerical simulations. First, the mechanical governing system for a single-crystalline NiMnGa sample is formulated, which contains the mechanical field equation and the twin interface movement criteria. To complete the governing system, the nucleations of twin interfaces in the NiMnGa sample under general mechanical loads are investigated. By considering the connection conditions of some physical quantities across a “fictitious” twin interface, the explicit expression of the configurational force on the twin interface is derived. Then, based on the twin interface movement criteria, a practical scheme for predicting the twin interface nucleation is proposed. This scheme can be integrated into the iterative numerical algorithm for solving the mechanical governing system. To demonstrate the efficiency of this scheme, some simulation results are presented for two typical deformations of a NiMnGa sample (i.e., the axial compression and the bending deformation), which show good consistency with the experimental results. By analyzing the evolution properties of the configurational forces on the possible nucleation interfaces, the mechanism responsible for twin interface nucleations under different loading conditions can be revealed. The results obtained in the current work would be helpful for the design of smart devices by utilizing NiMnGa alloys.

In situ detection of stability limit of ω phase in Ti–15Mo alloy during heating

Phase transitions in a single crystal of a metastable β-titanium alloy (Ti-15Mo) were investigated in situ during heating by synchrotron X-ray diffraction. The results were compared with previous measurements of electrical resistance. Single-crystalline samples allowed different crystallographic families of ω-Ti and α-Ti phases to be distinguished. The observed evolution of the intensity of the reflections of the ω phase during heating is consistent with the evolution of electrical resistance, which proves that the resistance is affected by the presence of ω-phase particles. Between approximately 673 and 833 K, both the resistance and the intensity of ω peaks sharply decrease. At 833 K, ω reflections disappear, indicating a complete dissolution of the ω phase due to achieving the solvus temperature of the ω phase in the Ti–15Mo alloy. The synchrotron X-ray diffraction experiment proved that the disappearance of the ω phase during heating of Ti–15Mo with a heating rate of 5 K min−1 occurs by its dissolution back to the β phase and not by ω → α transformation.

(Invited) Mie-like Scattering of Phonons by a Single Vacancy in IV-VI Compounds

Defects engineering has been a widely used approach for developing low thermal conductivity materials. Also, for some materials (e.g., IV-VI compound), considerably large amount of vacancies was often observed in single crystalline samples. The atomic vacancy in dilute case was assumed as a point defect that can cause the Rayleigh scattering with its rate proportional to the fourth power of phonon frequency. Due to the large dependence of scattering rate on phonon frequency, a vacancy was considered to only weakly suppress the mode thermal conductivity of low frequency acoustic phonons. We show that this does not hold for IV-VI materials. We compare the phonon scattering by a single vacancy in group IV (diamond and Si) and IV-VI (PbTe and GeTe) crystals using an exact ab initio Green’s function approach. The phonon-vacancy scattering rates in diamond and Si follow the well-known fourth-power dependence on phonon frequency. However, PbTe and GeTe exhibit phonon scattering rates that have much weaker frequency dependence. As a result, a vacancy scatters low frequency acoustic phonons more strongly in PbTe and GeTe than in Si and diamond. This unusual behavior in PbTe and GeTe originates from the significant changes of interatomic force constants that extend up to the distance of 10 angstrom from the vacancy site. Therefore, a vacancy in IV-VI materials cannot be assumed as a point defect but its finite size should be considered for phonon scattering. In contrast, a vacancy in Si and diamond changes interatomic force constants only up to 3 angstrom range. Our study provides new insights on the phonon spectral contribution to thermal transport when vacancies exist in IV-VI materials and will help to develop low thermal conductivity materials by engineering vacancies.

An alternative approach to the measurement of anisotropy field – Single grain extraction

A fast and cost effective way to determine the anisotropy field (µ0Ha) and the saturation magnetization (µ0Ms) of a hard magnetic compound is crucial to search for new permanent magnet materials. A single crystal in a defined shape is the conventional way to determine µ0Ha. However, the growth of phase-pure single crystalline sample is not always possible for some hard magnetic compounds with poor phase stability. In this study, we propose and demonstrate a fast and cost-effective method for determining µ0Ha and µ0Ms of a main phase in complex alloys. For this purpose, a single grain in trapezoidal prism shape was micro-sampled from bulk heterogeneous (Sm0.8Zr0.2)(Fe0.8Co0.2)11.5Ti0.5 ingot using focused ion beam, from which the magnetization and the anisotropy filed of the ThMn12-type phase in the alloy was determined to be μ0Ms = 1.53 T and µ0Ha = 8.2 T. Electron backscattered diffraction (EBSD) was carried out prior to the lift-out process to select desired region with a known c-axis direction, parallel to the lift out direction in this work. The reliability of this technique was tested by comparing the result with that for the (Nd0.9Dy0.1)2Fe14B single crystal.

Order, Disorder, and Dynamics in Brownmillerite Sr2Fe2O5.

The room-temperature structure of brownmillerite-type Sr2Fe2O5 remains controversial, despite numerous published crystallographic studies utilizing X-ray, neutron, and electron diffraction data collected on single-crystalline and powder samples. The main disagreements concern the ordering of twisted FeO4 tetrahedral chains within and between the layers stacked along the b axis, and the impact of this ordering on oxide-ionic conductivity. Here, we present new data along with a reinterpretation of previously published diffraction images, including the reassignment of satellite reflections, which harmonize the results of past studies in a unified description of tetrahedral chain ordering in Sr2Fe2O5 at length scales relevant to X-ray and neutron diffraction. Implications for the prevailing model of oxide ion transport in this material are also discussed.

Small Al cluster ion implantation into Si and 4H-SiC.

Continuously downscaling integrated circuit devices requires fabrication of shallower p-n junctions. The ion implantation approach at low energy is subject to low beam current due to the Coulomb repulsion. To overcome this problem cluster ions can be used for implantation. In comparison with single ions, cluster ions possess lower energy per atom and reduced Coulomb repulsion resulting in high equivalent current. In this study to carry out low-energy implantation into single crystalline silicon and 4H-SiC samples we employ Aln - (n = 1-5) clusters with energy in the range of 5-20 keV. The Al clusters are obtained by Cs sputtering of Al rod. Time-of-flight secondary ion mass spectrometry (TOF-SIMS; IONTOF TOF.SIMS-5) is used to study aluminum and oxygen sputter depth profiles for different cluster sizes and implantation energies before and after annealing treatment. A distinguishable effect of the energy per atom in the cluster on reduction of the projected range Rp is revealed. The lowest Rp of 3 ± 1 nm has been achieved in SiC samples at the energy per atom of 1.66 keV. After annealing of Si samples, a considerable change in the Al profiles due to redistribution of Al atoms during motion of the front of recrystallization is observed. The influence of the number of atoms in the cluster at the same energy per atom within the experimental uncertainty is not observed. The transient effects of the sputtering by the primary ion beam distort the shape of the Al profiles in Si samples. In the case of SiC, due to its relatively lower surface chemical activity, more informative TOF-SIMS depth profiling of the shallow cluster implantation is feasible.

Temperature-dependent optical properties of self-doped superconducting Fe-pnictide, Sr2VO3FeAs

We performed an infrared spectroscopic study on a single crystal of Sr2VO3FeAs grown by a self-flux method. This layered material system consists of two alternative layers of [SrVO3]−1 and [SrFeAs]+1. Since the typical size of single crystalline Sr2VO3FeAs samples is 200 200 10 m3 an optical study on this material is challenging. We observed an additional interband transition around 1000 cm−1, which is absent in other doped Ba-122 Fe-pnictides. The origin of this additional transition is not clearly known yet. We also observed a hidden Fermi liquid behavior. Interestingly, we observed a Fano line-shaped phonon which appears near 555 cm−1 below 200 K and shows a strong blue-shift when the temperature is lowered. The amplitude, width, and asymmetric Fano parameter of this phonon show anomalies at 150 K, which are probably related to an electronic phase observed below 155 K recently by an NMR study (Ok et al 2017 Nat. Commun. 8 2167). Our finding may help to understand the electronic phase observed previously in the same material.

Scanning tunneling microscopic investigation on morphology of magnetic Weyl semimetal YbMnBi2**Project supported by the “Shuguang Program” from Shanghai Education Development Foundation and Shanghai Municipal Education Commission, Ministry of Science and Technology of China (Grant Nos. 2016YFA0301003 and 2016YFA0300403), the National Natural Science Foundation of China (Grant Nos. 11521404, 11634009, 11874256, 11574202, 11874258, 11790313, 11674226, U1632102, 11674222, and 11861161003), and the

YbMnBi2 is a recently discovered time-reversal-symmetry breaking type-II Weyl semimetal. However, as a representation of the new category of topological matters, the scanning tunneling microcopy (STM) results on such important material are still absent. Here, we report the STM investigations on the morphology of vacuum cleaved single crystalline YbMnBi2 samples. A hill and valley type of topography is observed on the YbMnBi2 surface, which is consistent with the non-layer nature of its crystal structure. Analysis of STM images yields the information of the index of the vicinal surface. Our results here lay a playground of future atomic scale research on YbMnBi2.

Electronic band dispersion determination in azimuthally disordered transition-metal dichalcogenide monolayers

Generally, the lack of long-range order in materials prevents from experimentally addressing their electronic band dispersion by angle-resolved photoelectron spectroscopy (ARPES), limiting such assessment to single crystalline samples. Here we demonstrate that the ARPES spectra of azimuthally disordered transition metal dichalcogenide (TMDC) monolayers with 2 H phase are dominated by their band dispersion along the two high symmetry directions Γ-K and Γ-M. We exemplify this by analyzing the ARPES spectra of four prototypical TMDCs within a mathematical framework, which allows to consistently explain the reported observations. A robust base for investigating TMDC monolayers significantly beyond single crystal samples is thus established.

Unconventional Magnetization below 25\u2009K in Nitrogen-doped Diamond provides hints for the existence of Superconductivity and Superparamagnetism

The magnetization of nitrogen-doped single crystalline diamond bulk samples shows unconventional field and temperature hysteresis loops at T {boldsymbol{lesssim }} 25 K. The results suggest the existence of superparamagnetic and superconducting regions in samples with nitrogen concentration <200 ppm. Both phases vanish at temperatures above 25 K where the samples show diamagnetic behavior similar to undoped diamond. The observation of superparamagnetism and superconductivity is attributed to the nitrogen doping and to the existence of defective regions. From particle-induced X-ray emission with ppm resolution we rule out that the main observations below 25 K are due to magnetic impurities. We investigated also the magnetic properties of ferromagnetic/high-temperature superconducting oxide bilayers. The magnetization results obtained from those bilayers show remarkable similarities to the ones in nitrogen-doped diamond.

From the Ternary Phase Ca14Zn1+δSb11 (δ ≈ 0.4) to the Quaternary Solid Solutions Ca14- xRE xZnSb11 (RE = La-Nd, Sm, Gd, x ≈ 0.9). A Tale of Electron Doping via Rare-Earth Metal Substitutions and the Concomitant Structural Transformations.

The ternary compound Ca14Zn1.37(1)Sb11 and its six rare-earth metal substituted derivatives Ca14- xRE xZnSb11 (RE = La-Nd, Sm, Gd; x ≈ 0.90 ± 0.06) have been synthesized and structurally characterized by single-crystal X-ray diffraction methods. All compounds formally crystallize in the tetragonal Ca14AlSb11 structure type (space group I41/ acd, No. 142, Z = 8). The crystal structure of Ca14Zn1.37(1)Sb11 subtly differs from the structure of the remaining six, as well as from the structure of the archetype, due to the presence of a partially occupied interstitial Zn position. The extra zinc atom is needed in this structure to alleviate the unfavorable number of valence electrons in the imaginary Ca14ZnSb11. Electron doping, via substitution of RE3+ ions on Ca2+ sites, is shown as an alternative route to achieve electron balance in these Zn-based analogs of the Ca14AlSb11 structure, which does not require the incorporation of interstitial atoms. Electrical resistivity measurements done on single-crystalline samples are in agreement with the notion that Ca14- xRE xZnSb11 moieties behave as either bad metals or heavily doped semiconductors. Magnetization measurements show Curie-Weiss paramagnetic behavior related to the local-moment magnetism of the RE3+ ions.

Hindered Quadrupole Order in PrMgNi4 with a Nonmagnetic Doublet Ground State

Structural, transport and magnetic properties of single-crystalline samples of a praseodymium-based cubic compound PrMgNi4 were studied. The single-crystal X-ray structural analysis revealed that Mg atoms are substituted for the Pr atoms at the 4a site by 4.5%. The chi(T) data follow the Curie-Weiss law with an effective moment for the Pr3+ ion. The magnetic specific heat divided by temperature, Cm/T, shows a broad maximum at around 3 K, which is reproduced by a two-level model with a ground state doublet. On cooling below 1 K, Cm/T approaches a constant value, which behavior is reproduced by a random two-level model. The twofold degeneracy of the ground state is lifted by symmetry lowering due to the substituted Mg atoms for the Pr atoms or strong hybridizations between the 4f2 electron states and conduction bands, which hinders the long-range quadrupole order.

Features of thermomagnetic transport due to the superconducting interfaces in inclination bicrystals of Bi and 3D topological insulator Bi1−xSbx

We studied the thermomagnetic power Sii (B) and Nernst–Ettingshausen effect Sij (B) in inclination bicrystals of Bi and 3D topological insulator Bi1−xSbx (0.04 < x ≤ 0.12) with superconducting nano-width interfaces (Tc ≤ 21 K). High values of thermomagnetic effects in small disorientation angle (SDA) bicrystals far exceeding values in single-crystalline samples were found. It was established that Sii (B) linearly increases in high fields without saturation and change the sign from negative in positive in bicrystals of 3D Dirac point forming (x ∼ 0.04), specifying the signature of 3D topological semimetal. Contrarily, Sii (B) in LDA bicrystals with 0.06 ≤ x ≤ 0.12 undergoes saturation or increases smoothly, the Landau level index depends linearly on 1/Bn, and extrapolate to – 0.5 if 1/Bn → 0, what is typical for the 3D topological insulators. Two new quantum oscillation harmonics are reviled in high fields; they characterize different densities of electronic states and different levels of disorder at LDA and SDA interfaces.