arXiv subjects

Despina Louca

Publications and source records attributed to Despina Louca.

35 records · Page 2Linked to original sources

arXiv2021

US Muon Workshop 2021: A road map for a future Muon Facility

The workshop titled "US Muon Workshop 2021: A road map for a future Muon Facility" was held virtually on February 1-2, 2021. The workshop aimed to bring together world experts in muon spectroscopy ($μ$SR) and other techniques along with interested stakeholders to evaluate the scientific need to construct a new $μ$SR facility in the United States (US). The more than 200 participants highlighted several key scientific areas for $μ$SR research, including quantum materials, hydrogen chemistry, and battery materials, and how each area could benefit from a new, high flux pulsed muon source. Experts also discussed aspects of the $μ$SR technique, such as low-energy $mu$SR, novel software developments, and beam and detector technologies that could enable revolutionary advances in $μ$SR at a next-generation facility. The workshop concluded with discussion of a concept being developed for a new $μ$SR facility at the Spallation Neutron Source (SNS) of Oak Ridge National Laboratory (ORNL). That novel design concept was first envisioned by many of the same $μ$SR experts at a workshop held previously at ORNL in 2016. The participants expressed that the current design had the potential to be a world-leading $μ$SR facility, and strongly encouraged the principal investigators to continue their work in order to refine the concept and determine instrument parameters that would enable new scientific opportunities.

physics.ins-det↗

arXiv2021

Bi kagome sublattice distortions by quenching and flux pinning in superconducting RbBi$_2$

The properties of RbBi$_2$, a 4.15 K superconductor, were investigated using magnetic field, pressure and neutron diffraction. Under hydrostatic pressure, an almost 50 % reduction of T$_c$ is observed, linked to a low Debye temperature estimated at 165 K. The resistivity and magnetic susceptibility were measured on quenched and slow-cooled polycrystalline samples. The resistivity follows a low temperature power-law dependence in both types of samples, while the diamagnetic susceptibility, $χ$, is dependent on the sample cooling history. Slow-cooled samples have a $χ= -1$ while quenched samples have $χ< -1$ due to grain size differences. Evidence of the effects of the cooling rate is also discerned from the local structure, obtained by neutron diffraction and the pair density function analysis. Slow-cooled samples have structurally symmetric Bi hexagons, in contrast to quenched samples in which disorder is manifested in periodic distortions of the Bi hexagonal rings of the kagome sublattice. Disorder may lead to flux pinning that reduces vortex mobility, but T$_c$ remains unaffected by the cooling rate.

cond-mat.supr-con↗

arXiv2020

T$_{d}$ to 1T$^{\prime}$ structural phase transition in WTe$_{2}$ Weyl semimetal

Elastic neutron scattering on a single crystal and powder X-ray diffraction measurements were carried out to investigate how the crystal structure evolves as a function of temperature in the Weyl semimetal WTe$_{2}$. A sharp transition from the low-temperature orthorhombic phase (T$_{d}$) to the high-temperature monoclinic phase (1T$^{\prime}$) was observed at ambient pressure in the single crystal near $\sim$565 K. Unlike in MoTe$_{2}$, the solid-solid transition from T$_{d}$ to 1T$^{\prime}$ occurs without the cell doubling of the intermediate T$_{d}^{*}$ phase with AABB (or ABBA) layer stacking. In powders however, the thermal transition from the T$_{d}$ to the 1T$^{\prime}$ phase is broadened and a two phase coexistence was observed until 700K, well above the structural transition.

cond-mat.mtrl-sci↗

arXiv2020

Evolution of the structural transition in Mo$_{1-x}$W$_{x}$Te$_{2}$

The composition dependence of the structural transition between the monoclinic 1T$^{\prime}$ and orthorhombic T$_{d}$ phases in the Mo$_{1-x}$W$_{x}$Te$_{2}$ Weyl semimetal was investigated by elastic neutron scattering on single crystals up to $x \approx 0.54$. First observed in MoTe$_{2}$, the transition from T$_{d}$ to 1T$^{\prime}$ is accompanied by an intermediate pseudo-orthorhombic phase, T$_{d}^{*}$. Upon doping with W, the T$_{d}^{*}$ phase vanishes by $x \approx 0.34$. Above this concentration, a phase coexistence behavior with both T$_{d}$ and 1T$^{\prime}$ is observed instead. The interlayer in-plane positioning parameter $δ$, which relates to the 1T$^{\prime}$ $β$ angle, decreases with temperature as well as with W substitution, likely due to strong anharmonicity in the interlayer interactions. The temperature width of the phase coexistence remains almost constant up to $x \approx 0.54$, in contrast to the broadening reported under pressure.

cond-mat.mtrl-sci↗

arXiv2020

Charge Density Modulation and Defect Ordering in Na$_x$MnBi$_y$ magnetic semimetal

The I-Mn-V antiferromagnet, NaMnBi, develops a very large positive magnetoresistance (MR) up to 10,000% at 2 K and 9 T in crystals showing a semiconductor-to-metal transition (SMT). In the absence of an SMT, a modest (20%) MR is achieved. Here, we show that upon cooling below the magnetic transition, a spatial modulation appears giving rise to new Bragg peaks due to charge and defect ordering in a checkerboard pattern, with two kinds of modulation vectors, $q_1$=($\frac23$, 0, 1) and $q_2$=($\frac23, \frac13, \frac12$). This constitutes a superlattice transition ($T_s$) that lowers the symmetry from the high temperature centrosymmetric P4/nmm to the non-centrosymmetric P$\overline4$m2. In crystals with a large MR, a close to room temperature $T_s$ is observed with $q_1$ appearing first, followed by $q_2$. In crystals with low MR however, $T_s$ is much lower and only $q_1$ is observed. The charge modulation and spin fluctuations may both contribute to the enhancement of MR.

cond-mat.mtrl-sci↗

arXiv2019

Appearance of T$_d^*$ phase across the T$_{d}$-1T$^{\prime}$ phase boundary in Weyl semimetal MoTe$_{2}$

Using elastic neutron scattering on single crystals of MoTe$_{2}$ and Mo$_{1-x}$W$_{x}$Te$_{2}$ ($x \lesssim 0.01$), the temperature dependence of the recently discovered T$_{d}^{*}$ phase, present between the low temperature orthorhombic T$_{d}$ phase and high temperature monoclinic 1T$^{\prime}$ phase, is explored. The T$_{d}^{*}$ phase appears only on warming from T$_{d}$ and is observed in the hysteresis region prior to the 1T$^{\prime}$ transition. This phase consists of four layers in its unit cell, and is constructed by an "AABB" sequence of layer stacking operations rather than the "AB" and "AA" sequences of the 1T$^{\prime}$ and T$_{d}$ phases, respectively. Though the T$_{d}^{*}$ phase emerges without disorder on warming from T$_{d}$, on cooling from 1T$^{\prime}$ diffuse scattering is observed that suggests a frustrated tendency toward the "AABB" stacking.

cond-mat.mtrl-sci↗

arXiv2019

Emergence of topologically protected states in MoTe$_{2}$ Weyl semimetal with layer stacking order

Electronic tunability in crystals with weakly-bound layers can be achieved through layer stacking order. One such example is MoTe$_2$, where the low-temperature orthorhombic T$_d$ phase is topological and host to Weyl quasiparticles. The transition mechanism to the non-trivial topology is elucidated by single crystal neutron diffraction. Upon cooling from the monoclinic 1T$\prime$ to the T$_d$ phase, diffuse scattering accompanies the transition, arising from random, in-plane layer displacements, and dissipates upon entering the T$_d$ phase. Diffuse scattering is observed only in the H0L plane due to irreversible layer shifts along the c-axis that break the centrosymmetry of the monoclinic lattice.

cond-mat.mtrl-sci↗

arXiv2017

Elastic and electronic tuning of magnetoresistance in MoTe$_2$

Quasi-two dimensional transition metal dichalcogenides (TMD) exhibit dramatic properties that may transform electronic and photonic devices. We report on how the anomalously large magnetoresistance (MR) observed under high magnetic field in MoTe$_2$, a type II Weyl semimetal, can be reversibly controlled under tensile strain. The MR is enhanced by as much as ~ 30 % at low temperatures and high magnetic fields, when uniaxial strain is applied along the $a$-crystallographic direction and reduced by about the same amount when strain is applied along the $b$-direction. We show that the large in-plane electric anisotropy is coupled with the structural transition from the 1T' monoclinic to the Td orthorhombic Weyl phase. A shift of the Td - 1T' phase boundary is achieved by minimal tensile strain. The sensitivity of the MR to tensile strain suggests the possibility of a nontrivial spin-orbital texture of the electron and hole pockets in the vicinity of Weyl points. Our ab initio calculations indeed show a significant orbital mixing on the Fermi surface, which is modified by the tensile strains.

cond-mat.mtrl-sci↗

arXiv2017

Charge Fluctuations in the NdO$_{1-x}$F$_{x}$BiS$_{2}$ Superconductors

The local atomic structure of superconducting NdO$_{1-x}$F$_{x}$BiS$_{2}$ ($x$=0.2 and 0.4) is investigated using neutron diffraction and the pair density function analysis technique. In the non-superconducting $x$=0.2 composition, ferrodistortive displacements of the pyramidal sulfur ions break the tetragonal symmetry and a superlattice structure emerges with peaks appearing at $h+k$ odd reflections superimposed on the even reflections of the $P4/nmm$ symmetry. In the superconducting $x$=0.4 composition, similar ferrodistortive displacements are observed but with different magnitudes coupled with in-plane Bi distortions. The distortions are indicative of charge fluctuations that may lead to charge localization and suppression of superconductivity.

cond-mat.supr-con↗

arXiv2017

Superconductivity at the vacancy disorder boundary in K$_x$Fe$_{2-y}$Se$_2$

The role of phase separation in the emergence of superconductivity in alkali metal doped iron selenides A$_{x}$Fe$_{2-y}$Se$_{2}$ (A = K, Rb, Cs) is revisited. High energy X-ray diffraction and Monte Carlo simulation were used to investigate the crystal structure of quenched superconducting (SC) and as-grown non-superconducting (NSC) K$_{x}$Fe$_{2-y}$Se$_{2}$ single crystals. The coexistence of superlattice structures with the in-plane $\sqrt{2}\times\sqrt{2}$ K-vacancy ordering and the $\sqrt{5}\times\sqrt{5}$ Fe-vacancy ordering were observed in SC and NSC crystals along side the \textit{I4/mmm} Fe-vacancy free phase. Moreover, in the SC crystal an Fe-vacancy disordered phase is additionally present. It appears at the boundary between the \textit{I4/mmm} vacancy free phase and the \textit{I4/m} vacancy ordered phase ($\sqrt{5}\times\sqrt{5}$). The vacancy disordered phase is most likely the host of superconductivity.

cond-mat.supr-con↗

arXiv2015

In-plane Charge Fluctuations in Bismuth Sulfide Superconductors

Evidence for local charge fluctuations linked to a charge disproportionation of the Bi ions in the distorted lattice of superconducting LaO$_{1-x}$F$_{x}$ BiS$_{2}$ is presented. In-plane short-range distortions of sulfur atoms up to 0.3 Å in magnitude break site symmetry and create two distinct environments around Bi. Out-of-plane motion of apical sulfur brings it closer to the La-O/F doping layer with increasing $x$ that may lead to a charge transfer conduit between the doping layers and the superconducting BiS$_{2}$ planes. The mechanism for superconductivity may arise from the interplay between charge density fluctuations and an enhanced spin-orbit coupling suggested theoretically, that induces spin polarization.

cond-mat.supr-con↗

arXiv2014

Structure and composition of the superconducting phase in alkali iron selenide K$_y$Fe$_{1.6+x}$Se$_2$

We use neutron diffraction to study the temperature evolution of the average structure and local lattice distortions in insulating and superconducting potassium iron selenide K$_y$Fe$_{1.6+x}$Se$_2$. In the high temperature paramagnetic state, both materials have a single phase with crystal structure similar to that of the BaFe$_2$As$_2$ family of iron pnictides. While the insulating K$_y$Fe$_{1.6+x}$Se$_2$ forms a $\sqrt{5}\times\sqrt{5}$ iron vacancy ordered block antiferromagnetic (AF) structure at low-temperature, the superconducting compounds spontaneously phase separate into an insulating part with $\sqrt{5}\times\sqrt{5}$ iron vacancy order and a superconducting phase with chemical composition of K$_z$Fe$_{2}$Se$_2$ and BaFe$_2$As$_2$ structure. Therefore, superconductivity in alkaline iron selenides arises from alkali deficient K$_z$Fe$_{2}$Se$_2$ in the matrix of the insulating block AF phase.

cond-mat.supr-con↗

arXiv2013

The hybrid lattice of KxFe2-ySe2: why superconductivity and magnetism can coexist

Much remains unknown of the microscopic origin of superconductivity when it materializes in atomically disordered systems as in amorphous alloys (1) or in crystals riddled with defects(2). A manifestation of this conundrum is envisaged in the highly defective iron chalcogenide superconductors of KxFe2-ySe2 (3-6). How can superconductivity survive under such crude conditions that call for strong electron localization (7)? With vacancies present both at the K and Fe sites, superconductivity is bordering a semi-metallic region below x ~ 0.7 and an insulating and antiferromagnetic region above x ~ 0.85 (8,9). Here, we report on the bulk local atomic structure and show that the Fe sublattice is locally distorted in a way that it accommodates two kinds of Fe valence environments giving rise to a bimodal bond distribution. While the bond length distribution is driven by K and Fe contents, the superconducting state is characterized by the coexistence of both short (metallic) and long (insulating) Fe bond environments and is not phase separated. In contrast to other Fe-based materials in which only one kind of Fe to Fe bond is present, the dual nature of the Fe correlations explains why superconductivity is intertwined with magnetic order. Such a hybrid state is most likely present in cuprate superconductors as well (10,11) while our results point to the importance of the local atomic symmetry by which the exchange interactions between local moments can materialize (12).

cond-mat.supr-con↗

arXiv2010

Possible link of a structurally driven spin flip transition and the insulator-metal transition in the perovskite La$_{1-x}$Ba$_{x}$CoO$_{3}$

The complex nature of the magnetic ground state in La$_{1-x}$A$_{x}$CoO$_{3}$ (A = Ca, Sr, Ba) has been investigated via neutron scattering. It was previously observed that ferromagnetic (FM) as well as antiferromagnetic (AFM) correlations can coexist prior to the insulator-metal transition (IMT). We focused on a unique region in the Ba phase diagram, from x = 0.17 - 0.22, in which a commensurate AFM phase appears first with a propagation vector, k = (0, -0.5, 0.5), and the Co moment in the (001)$_{R}$ plane of the rhombohedral lattice. With increasing x, the AFM component weakens while an FM order appears with the FM Co moment directed along the (001)$_{R}$ (=(111)$_{C}$) axis. By x = 0.22, a spin flip to new FM component appears as the crystal fully transforms to an orthorhombic (Pnma) structure, with the Co moments pointing along a new direction, (001)$_{O}$ (=(110)$_{C}$). It is the emergence of the magnetic Pnma phase that leads to IMT.

cond-mat.str-el↗

arXiv2009

The local atomic structure of superconducting Fe-Se-Te

We report on the first local atomic structure study via the pair density function (PDF) analysis of neutron diffraction data and show a direct correlation of local coordinates to TC in the newly discovered superconducting FeSe1-xTex. The isovalent substitution of Te for Se such as in FeSe0.5Te0.5 increases Tc by twofold in comparison to a-FeSe without changing the carrier concentration but, on average, decreases the chalcogen-Fe bond angle. However, we find that the local symmetry is lower than the average P4/nmm crystal symmetry, because the Se and Te ions do not share the same site, leading to two distinct z-coordinates that exhibit two types of bond angles with Fe. The angle indeed increases from ~ 104.02o in FeSe to ~105.20o in FeSe0.5Te0.5 between Fe and Se. Simultaneously, ab-initio calculations based on spin density function theory yielded an optimized structure with distinct z-coordinates for Se and Te, in agreement with the experiment. The valence charge distribution in the Fe-Se bonds was found to be different from that in the Fe-Te bonds. Thus, superconductivity in this chalcogenide is closely related to the local structural environment, with direct implications on the multiband magnetism where modulations of the ionic lattice can change the distribution of valence electrons.

cond-mat.supr-con↗

arXiv2005

Nano-magnetic droplets and implications to orbital ordering in La1-xSrxCoO3

Inelastic cold neutron scattering on LaCoO3 provided evidence for a distinct low energy excitation at 0.6 meV coincident with the thermally induced magnetic transition. Coexisting strong ferromagnetic (FM) and weaker antiferromagnetic (AFM) correlations that are dynamic follow the activation to the excited state, identified as the intermediate S=1 spin triplet. This is indicative of dynamical orbital ordering favoring the observed magnetic interactions. With hole doping as in La1-xSrxCoO3, the FM correlations between Co spins become static and isotropically distributed due to the formation of FM droplets. The correlation length and condensation temperature of these droplets increase rapidly with metallicity due to the double exchange mechanism.

cond-mat.str-el↗

arXiv2003

Dynamical disorder of spin-induced Jahn-Teller orbitals with the insulator-metal transition in cobaltates

Using elastic and inelastic neutron scattering, we investigated the evolution of the local atomic structure and lattice dynamics of La(1-x)Sr(x)CoO3 (x=0-0.5) as it crosses over with x from an insulator to a ferromagnetic metal (FMM). Our pair density function analysis indicates that in the paramagnetic insulating (PMI) phase for all x, spin activation of Co3+ ions induces local static Jahn-Teller (JT) distortions. The size of the JT lattice increases almost linearly with x. However, in the FMM phase, static JT distortions are absent for x <= 30 %. This coincides with narrowing of hw=22 and 24 meV modes in the phonon spectrum in which we argue is due to localized dynamical JT fluctuations. For x > 30%, static JT distortions reappear along with broadening of the phonon modes because of weakened charge-lattice interactions.

cond-mat.str-el↗