arXiv subjects

M. J. Kramer

Publications and source records attributed to M. J. Kramer.

12 recordsLinked to original sources

arXiv2020

Short- and medium-range orders in Al90Tb10 glass and their relation to the structures of competing crystalline phases

Molecular dynamics simulations using an interatomic potential developed by artificial neural network deep machine learning are performed to study the local structural order in Al90Tb10 metallic glass. We show that more than 80% of the Tb-centered clusters in Al90Tb10 glass have short-range order (SRO) with their 17 first coordination shell atoms stacked in a '3661' or '15551' sequence. Medium-range order (MRO) in Bergman-type packing extended out to the second and third coordination shells is also clearly observed. Analysis of the network formed by the '3661' and '15551' clusters show that ~82% of such SRO units share their faces or vertexes, while only ~6% of neighboring SRO pairs are interpenetrating. Such a network topology is consistent with the Bergman-type MRO around the Tb-centers. Moreover, crystal structure searches using genetic algorithm and the neural network interatomic potential reveal several low-energy metastable crystalline structures in the composition range close to Al90Tb10. Some of these crystalline structures have the '3661' SRO while others have the '15551' SRO. While the crystalline structures with the '3661' SRO also exhibit the MRO very similar to that observed in the glass, the ones with the '15551' SRO have very different atomic packing in the second and third shells around the Tb centers from that of the Bergman-type MRO observed in the glassy phase.

cond-mat.mtrl-sci↗

arXiv2020

Development of Interatomic Potential for Al-Tb Alloy by Deep Neural Network Learning Method

An interatomic potential for Al-Tb alloy around the composition of Al90Tb10 was developed using the deep neural network (DNN) learning method. The atomic configurations and the corresponding total potential energies and forces on each atom obtained from ab initio molecular dynamics (AIMD) simulations are collected to train a DNN model to construct the interatomic potential for Al-Tb alloy. We show the obtained DNN model can well reproduce the energies and forces calculated by AIMD. Molecular dynamics (MD) simulations using the DNN interatomic potential also accurately describe the structural properties of Al90Tb10 liquid, such as the partial pair correlation functions (PPCFs) and the bond angle distributions, in comparison with the results from AIMD. Furthermore, the developed DNN interatomic potential predicts the formation energies of crystalline phases of Al-Tb system with the accuracy comparable to ab initio calculations. The structure factor of Al90Tb10 metallic glass obtained by MD simulation using the developed DNN interatomic potential is also in good agreement with the experimental X-ray diffraction data.

cond-mat.mtrl-sci↗

arXiv2017

Observation of Al41Sm5: new evidence for structural hierarchy in Al-Sm alloys

Using an effective genetic algorithm, we uncover the structure of a metastable Al41Sm5 phase that supplements its family sharing similar short-range orders. The phase evolves upon heating an amorphous Al-9.7at.%Sm ribbon, produced by melt-spinning. The dynamical phase selection is discussed with respect to the structural connections between the short-range packing motifs in the amorphous precursor and those observed in the selected phases. The phase elucidated here is one of several newly discovered large-unit-cell phases found to form during devitrification from the glass in this binary system, further illustrating the power and efficiency of our approach, the important role of structural hierarchy in phase selection, and the richness of the metastable phase landscape accessible from the glassy structure.

cond-mat.mtrl-sci↗

arXiv2014

Crystalline 'Genes' in Metallic Liquids

The underlying structural order that transcends the liquid, glass and crystalline states is identified using an efficient genetic algorithm (GA). GA identifies the most common energetically favorable packing motif in crystalline structures close to the alloy's Al-10 at.% Sm composition. These motifs are in turn compared to the observed packing motifs in the actual liquid structures using a cluster-alignment method which reveals the average topology. Conventional descriptions of the short-range order, such as Voronoi tessellation, are too rigid in their analysis of the configurational poly-types when describing the chemical and topological ordering during transition from undercooled metallic liquids to crystalline phases or glass. Our approach here brings new insight into describing mesoscopic order-disorder transitions in condensed matter physics.

physics.comp-ph↗

arXiv2014

Stripe-like nanoscale structural phase separation and optimal inhomogeneity in superconducting BaPb$_{1-x}$Bi$_x$O$_3$

Structural phase separation in the form of partially disordered stripes, with characteristic length scales in the nanometer range, is observed for superconducting BaPb$_{1-x}$Bi$_x$O$_3$. The evolution of the superconducting coherence length with composition relative to the size of these stripes suggests an important role of the nanostructure in determining the shape of the superconducting dome. It is proposed that the maximum $T_c$ is determined by a kind of "optimal inhomogeneity", characterized by a crossover from an inhomogeneous macroscopic superconductor to a granular superconductor for which phase fluctuations suppress $T_c$.

cond-mat.supr-con↗

arXiv2012

Field-Tuned Superconductor-Insulator Transition in BaPb1-xBixO3

BaPb$_{1-x}$Bi$_x$O$_3$ is found to exhibit a field-tuned superconductor to insulator transition for Bi compositions 0.24 $\leq x \leq$ 0.29. The magnetoresistance of optimally doped samples manifests a temperature-independent crossing point and scaling of the form $ρ(T,H)=ρ_c F(|H-H_{c}|T^{-1/zν})$, where $H_c$ is the field determined by the temperature-independent crossing point, and $zν$ = 0.69 $\pm$ 0.03. High resolution transmission electron microscopy measurements reveal a complex intergrown nanostructure comprising tetragonal and orthorhombic polymorphs. Data are analyzed in terms of both a classical effective medium theory and a field-tuned quantum phase transition, neither of which provides a completely satisfactory explanation for this remarkable phenomenology.

cond-mat.supr-con↗

arXiv2011

Stabilization of an ambient pressure, collapsed tetragonal phase in CaFe2As2 and tuning of the orthorhombic / antiferromagnetic transition temperature by over 70 K by control of nano-precipitates

We have found a remarkably large response of the transition temperature of CaFe2As2 single crystals grown out of excess FeAs to annealing / quenching temperature. Whereas crystals that are annealed at 400 C exhibit a first order phase transition from a high temperature tetragonal to a low temperature orthorhombic and antiferromagnetic state near 170 K, crystals that have been quenched from 960 C exhibit a transition from a high temperature tetragonal phase to a low temperature, non-magnetic, collapsed tetragonal phase below 100 K. By use of temperature dependent electrical resistivity, magnetic susceptibility, X-ray diffraction, Mossbauer spectroscopy and nuclear magnetic resonance measurements we have been able to demonstrate that the transition temperature can be reduced in a monotonic fashion by varying the annealing / quenching temperature from 400 to 850 C with the low temperature state remaining antiferromagnetic for transition temperatures larger than 100 K and becoming collapsed tetragonal / non-magnetic for transition temperatures below 90 K. This suppression of the orthorhombic / antiferromagnetic phase transition and its ultimate replacement with the collapsed tetragonal / non-magnetic phase is similar to what has been observed for CaFe2As2 under hydrostatic pressure. Transmission electron microscopy studies indicate that there is a temperature dependent, width of formation of CaFe2As2 with a decreasing amount of excess Fe and As being soluble in the single crystal at lower annealing temperatures. For samples quenched from 960 C there is a fine (of order 10 nm), semi-uniform distribution of precipitate that can be associated with an average strain field whereas for samples annealed at 400 C the excess Fe and As form mesoscopic grains that induce little strain throughout the CaFe2As2 lattice.

cond-mat.supr-con↗

arXiv2010

Anomalous scattering in superconducting indium-doped tin telluride

Results of resistivity, Hall effect, magnetoresistance, susceptibility and heat capacity measurements are presented for single crystals of indium-doped tin telluride with compositions Sn$_{.988-x}$In$_x$Te where $0 \leq x \leq 8.4 %$, along with microstructural analysis based on transmission electron microscopy. For small indium concentrations, $x \leq 0.9 %$ the material does not superconduct above 0.3 K, and the transport properties are consistent with simple metallic behavior. For $x \geq 2.7 %$ the material exhibits anomalous low temperature scattering and for $x \geq 6.1 %$ bulk superconductivity is observed with critical temperatures close to 2 K. Intermediate indium concentrations $2.7% \leq x \leq 3.8%$ do not exhibit bulk superconductivity above 0.7 K. Susceptibility data indicate the absence of magnetic impurities, while magnetoresistance data are inconsistent with localization effects, leading to the conclusion that indium-doped SnTe is a candidate charge Kondo system, similar to thallium-doped PbTe.

cond-mat.supr-con↗

arXiv2008

Multiple charge density wave transitions in Gd$_2$Te$_5$

Diffraction measurements performed via transmission electron microscopy and high resolution X-ray scattering reveal two distinct charge density wave transitions in Gd$_2$Te$_5$ at $T_{c1}$ = 410(3) and $T_{c2}$ = 532(3) K, associated with the \textit{on}-axis incommensurate lattice modulation and \textit{off}-axis commensurate lattice modulation respectively. Analysis of the temperature dependence of the order parameters indicates a non-vanishing coupling between these two distinct CDW states.

cond-mat.str-el↗

arXiv2007

Charge density wave formation in $R_{2}$Te$_{5}$ ($R$=Nd, Sm and Gd)

The rare earth ($R$) tellurides $R_2$Te$_5$ have a crystal structure intermediate between that of $R$Te$_2$ and $R$Te$_3$, consisting of alternating single and double Te planes sandwiched between $R$Te block layers. We have successfully grown single crystals of Nd$_2$Te$_5$, Sm$_2$Te$_5$ and Gd$_2$Te$_5$ from a self flux, and describe here the first evidence for charge density wave formation in these materials. The superlattice patterns for all three compounds are relatively complex, consisting at room temperature of at least two independent wavevectors. Consideration of the electronic structure indicates that to a large extent these wave vectors are separately associated with sheets of the Fermi surface which are principally derived from the single and double Te layers.

cond-mat.str-el↗

arXiv2004

Superconductivity in MgB_2 doped with Ti and C

Measurements of the superconducting upper critical field, H_{c2}, and critical current density, J_c, have been carried out for MgB_2 doped with Ti and/or C in order to explore the problems encountered if these dopants are used to enhance the superconducting performance. Carbon replaces boron in the MgB_2 lattice and apparently shortens the electronic mean free path thereby raising H_c2. Titanium forms precipitates of either TiB or TiB_2 that enhance the flux pinning and raise J_c. Most of these precipitates are intra-granular in the MgB_2 phase. If approximately 0.5% Ti and approximately 2% C are co-deposited with B to form doped boron fibers and these fibers are in turn reacted in Mg vapor to form MgB_2, the resulting superconductor has H_{c2}(T=0) ~ 25 T and J_c ~ 10,000 A/cm**2 at 5 K and 2.2 T.

cond-mat.supr-con↗

arXiv2003

Intra-granular Ti precipitates in MgB2

Superconducting magnesium diboride has been prepared with intra-granular TiB precipitates that have appropriate size and spacing for the pinning of superconducting vortices. The TiB particles have a size ranging from 1 nm to 20 nm and a spacing about 5 times as large. By co-depositing Ti and B on a commercial carbon coated silicon carbide substrate with chemical vapor deposition methods, it was possible to obtain a starting fiber with Ti well dispersed in the B. Reacting these fibers in a Mg vapor then transformed the fiber into a superconductor capable of carrying 10,000 A/cm^2 at 25K and 1 T. The Ti-B bonds are strong enough that there is relatively little migration of the Ti as the MgB2 grains form around the TiB precipitates. TiB2 precipitates, if they are present, can not be seen in the selected area diffraction pictures that show the TiB precipitates.

cond-mat.supr-con↗