Modulation Wave Vector Research Articles

The chiral dependence of effective masses of polaron spin states in chiral structures

The renormalization of effective masses of polaron spin states in chiral structures is studied based on the direct spin–phonon coupling, in which the simple formulas for the renormalization of effective masses are derived for polarons with spin-up and spin-down states using a linear-combination operator method. The theoretical calculations show that the effective masses are renormalized significantly with increasing the cut-off wave vector of phonon modes and the renormalized magnitudes are different for right-handed and left-handed structures. Moreover, the renormalized magnitude for the spin-down state is larger than the spin-up one. This asymmetrical behavior naturally gives rise to very different conductivities for spin-up and spin-down states as electrons transmitted through the systems. These results not only enrich the microscopic origin of CISS mechanism, but also enlighten more experimental ways to explore CISS.

Nb<sub>0.8</sub>CoSb ordering transformation caused by <i>in situ</i> heating-induced Nb diffusion

<sec>This study focuses on the investigation of Nb<sub>0.8</sub>CoSb half-Heusler alloy covered with Nb films. By employing <i>in-situ</i> heating transmission electron microscopy (TEM) technique, diffusion of Nb is observed at high temperature, showing the ordering transformation from Nb<sub>0.8</sub>CoSb to Nb<sub>0.8+δ</sub>CoSb. Through observations of high-angle annular dark-field (HAADF) images and selected-area electron diffraction (SAED) patterns, it is found that under elevated temperatures, the diffuse streaks representing short-range disorder in Nb<sub>0.8</sub>CoSb sample transform into superlattice diffraction spots representing long-range order. The modulation wave vector of this superstructure is determined to be <inline-formula><tex-math id="M3">\begin{document}$ q={1}/{3}({a}^{*}+{b}^{*}-{c}^{*}) $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240325_M3.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240325_M3.png"/></alternatives></inline-formula>. This structural evolution primarily arises from the diffusion of Nb atoms from the Nb film into the Nb<sub>0.8</sub>CoSb sample at high temperature, leading to compositional changes in Sb and Nb.</sec><sec>Further comparative analysis reveals significant differences between <i>in-situ</i> synthesized Nb<sub>0.8+δ</sub>CoSb samples and <i>ex-situ</i> synthesized Nb<sub>0.84</sub>CoSb samples despite both exhibiting superstructures. In the <i>ex-situ</i> synthesized Nb<sub>0.84</sub>CoSb, the modulation wave vector of the superstructure is <inline-formula><tex-math id="M4">\begin{document}$ q={1}/{3}({2a}^{*}-2{c}^{*}) $\end{document}</tex-math><alternatives><graphic specific-use="online" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240325_M4.jpg"/><graphic specific-use="print" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="11-20240325_M4.png"/></alternatives></inline-formula>, which is mainly attributed to Nb compositional variations. Moreover, the superstructure in Nb<sub>0.84</sub>CoSb sample can remain stable from room temperature to high temperature, whereas in Nb<sub>0.8+δ</sub>CoSb samples, it only exists at elevated temperatures and gradually weakens as the temperature decreases, suggesting that it may be a metastable structure between Nb<sub>0.8</sub>CoSb and Nb<sub>0.84</sub>CoSb.</sec><sec>This study reveals the diversity of superstructures induced by compositional variations and the complexity of structural phase transitions in half-Heusler alloys, enriching the understanding of these materials and providing important guidance for the design and functional control of phase-change materials.</sec>

Shinkolobweite, from the Shinkolobwe Mine, Democratic Republic of Congo: A New Mineral Containing Uranium in the Rare Pentavalent Oxidation State

ABSTRACT Shinkolobweite, Pb1.333[U5+O(OH)(UO2)5O4.67(OH)5.33](H2O)5, is a new lead uranyl oxide-hydroxide hydrate mineral containing hexavalent and pentavalent uranium from the Shinkolobwe mine, Democratic Republic of Congo. Crystals of shinkolobweite are dark reddish-brown prisms up to 0.5 mm in length, occurring on a matrix of massive uraninite associated with fourmarierite, rutherfordine, soddyite, and sklodowskite. Crystals are translucent with subadamantine luster and light bronze-yellow streak, are flattened on {010}, are elongated on [001], and exhibit the forms {100}, {010}, {101}, and . The mineral is non-fluorescent under both longwave and shortwave ultraviolet illumination. It has a Mohs hardness of ∼2 and exhibits brittle tenacity with perfect cleavage on {010}, imperfect cleavage on {100}, and even fracture. The calculated density is 5.853 g/cm3 based on the empirical formula. Electron probe microanalysis provided the empirical formula Pb1.290U6+4.876U5+1.166O27H16.633 based on 27 O apfu and U5+:U6+ determined by X-ray photoelectron spectroscopy. Shinkolobweite is orthorhombic, superspace group Pnnm(0b0)000, a = 14.4808(4), b = 7.0681(8), c = 11.9423(3) Å, V = 1222.32(15) Å3, modulation wave vector [0 1/3 0], and Z = 2. The structure was refined from 8959 reflections to a final R1 = 0.0736 for all reflections. Uranyl oxide-hydroxide sheets in shinkolobweite adopt the β-U3O8 topology and possess (3 + 1) commensurately modulated ordering that results from the long-range ordering of U5+ and U6+ in the sheet, as well as the position and occupancy of interlayer Pb2+ cations. Observations of a topological transition between α-U3O8 and β-U3O8 type sheets in shinkolobweite supplements our understanding of U5+ mineral oxidation and stability.

ETG turbulence in a tokamak pedestal

This paper explores the fundamental characteristics of electron-temperature-gradient (ETG)-driven turbulence in the tokamak pedestal. The extreme gradients in the pedestal produce linear instabilities and nonlinear turbulence that are distinct from the corresponding ETG phenomenology in the core plasma. The linear system exhibits multiple (greater than ten) unstable eigenmodes at each perpendicular wave vector, representing different toroidal and slab branches of the ETG instability. Proper orthogonal decomposition of the nonlinear fluctuations reveals no clear one-to-one correspondence between the linear and nonlinear modes for most wave vectors. Moreover, nonlinear frequencies deviate strongly from those of the linear instabilities, with spectra peaking at positive frequencies, which is opposite the sign of the ETG instability. The picture that emerges is one in which the linear properties are preserved only in a narrow range of k-space. Outside this range, nonlinear processes produce strong deviations from both the linear frequencies and eigenmode structures. This is interpreted in the context of critical balance, which enforces alignment between the parallel scales and fluctuation frequencies. We also investigate the nonlinear saturation processes. We observe a direct energy cascade from the injection scale to smaller scales in both perpendicular directions. However, in the bi-normal direction, there is also nonlocal inverse energy transfer to larger scales. Neither streamers nor zonal flows dominate the saturation.

Engineerable wavevector diagrams of non-Moiré geometry-based photonic crystals for beam steering applications

The wavevector diagrams or eigenfrequency contours (EFCs) (also called dispersion surfaces) are the best tools to explore the optical properties of photonic crystals (PhCs). Many optical phenomena, such as self-collimation, super-prism, negative refraction, and lensing, have been extensively explored in PhCs based on EFCs. Also, several approaches have been continuingly pursued to modulate the EFCs of PhCs for molding the flow of light. This work presents the modulated wavevector diagrams of PhCs formed by asymmetric non-Moiré (NM) patterns. The NM patterns are contours of trigonometric functions that generate attractive tiles and shapes. Employing such shapes to design a PhC tailors the dispersion of PhCs with stretching, squeezing, and shape-modulated EFCs. Based on the modulated EFCs of the proposed structures, we demonstrate the direction-dependent beam steering phenomenon. The ray tracing, full-wave electromagnetic simulations, far-field patterns, and electric field profiles corroborate the beam steering application of the modulated EFCs. We anticipate that the modulated EFCs of non-Moiré pattern-based PhCs are useful for reconfigurable wave optics and beam steering applications.

As predicted and more: modulated channel occupation in YZn5+x.

Like many complex intermetallic phases, the crystal structures of REZn5+x compounds (RE = lanthanide or Group 3 element) based on the EuMg5 type have gradually unfolded. The original reports described a complex hexagonal structure with an unusual combination of tetrahedrally close-packed regions and open spaces, as well as observations of superstructure reflections. More recently, we reinvestigated the structure of YZn5, reclassifying it as the EuMg5+x-type compound YZn5+x (x ≃ 0.2), in which disordered channels run along c through the spaces formerly considered open. In addition, DFT-chemical pressure (DFT-CP) analysis of ordered models of YZn5+x highlighted paths for communication between neighboring channels setting the stage for superstructure formation. Herein, the experimental elucidation of this effect is presented with the synthesis and structure determination of a modulated form of YZn5+x. By slow-cooling samples of YZn5+x from the annealing temperature, crystals were obtained that exhibit satellite reflections with the modulation wavevector q = {1\over 3}a* + {1\over 3}b* + 0.3041c*. Structure solution and refinement using a (3+1)D model in superspace group P31c({1\over 3}\,\!{1\over 3}σ3)00s reveals incommensurate order in the structure's channels. Here, two Zn sites associated with the channels are present, each with discontinuous atomic domains that are slanted in the x3x4 plane. Their slanting corresponds to adjustments along the c axis for the presence or absence of close neighbors along that axis, while the occupation patterns of neighboring channels are shifted by {1\over 3} of the modulation period. These features follow earlier predictions from CP analysis, highlighting how this approach can be used predictively in search of new phenomena.

Spin-wave dynamics controlled by tunable ac magnonic crystal

The magnonic crystal, which has a spatial modulation wave vector $q$, couples the spin wave with wave vector $k$ to the one with wave vector $k\ensuremath{-}q$. For a conventional magnonic crystal with direct current (dc) supply, the spin waves around $q/2$ are resonantly coupled to the waves near $\ensuremath{-}q/2$, and a band gap is opened at $k=\ifmmode\pm\else\textpm\fi{}q/2$. If instead of the dc current the magnonic crystal is supplied with an alternating current (ac), then the band gap is shifted to $k$ satisfying $|{\ensuremath{\omega}}_{s}(k)\ensuremath{-}{\ensuremath{\omega}}_{s}(k\ensuremath{-}q)|={\ensuremath{\omega}}_{\mathrm{ac}}$; here ${\ensuremath{\omega}}_{s}(k)$ is the dispersion of the spin wave, while ${\ensuremath{\omega}}_{\mathrm{ac}}$ is the frequency of the ac modulation. The resulting gap in the case of the ac magnonic crystal is the half of the one caused by the dc with the same amplitude of modulation. The time evolution of the resonantly coupled spin waves controlled by properly suited ac pulses can be well interpreted as the motion on a Bloch sphere. The tunability of the ac magnonic crystal broadens the perspective of spin-wave computing.

Asymmetric rotations and dimerization driven by normal to modulated phase transition in 4-biphenylcarboxy coupled L-phenylalaninate.

Amongst the derivatives of 4-biphenylcarboxylic acid and amino acid esters, the crystal structure of 4-biphenylcarboxy-(L)-phenylalaninate is unusual owing to its monoclinic symmetry within a pseudo-orthorhombic crystal system. The distortion is described by a disparate rotational property around the chiral centers (ϕchiral ≃ -129° and 58°) of the two molecules in the asymmetric unit. Each of these molecules comprises planar biphenyl moieties (ϕbiphenyl = 0°). Using temperature-dependent single-crystal X-ray diffraction experiments we show that the compound undergoes a phase transition below T ∼ 124 K that is characterized by a commensurate modulation wavevector, q = δ(101), δ = ½. The (3+1)-dimensional modulated structure at T = 100 K suggests that the phase transition drives the biphenyl moieties towards noncoplanar conformations with significant variation of internal torsion angle (ϕmaxbiphenyl ≤ 20°). These intramolecular rotations lead to dimerization of the molecular stacks that are described predominantly by distortions in intermolecular tilts (θmax ≤ 20°) and small variations in intermolecular distances (Δdmax ≃ 0.05 Å) between biphenyl molecules. Atypical of modulated structures and superstructures of biphenyl and other polyphenyls, the rotations of individual molecules are asymmetric (Δϕbiphenyl ≈ 5°) while ϕbiphenyl of one independent molecule is two to four times larger than the other. Crystal-chemical analysis and phase relations in superspace suggest multiple competing factors involving intramolecular steric factors, intermolecular H-C...C-H contacts and weak C-H...O hydrogen bonds that govern the distinctively unequal torsional properties of the molecules.

Peculiarities of incommensurate modulation behavior during superstructure creation in case of potential symmetry n = 4

Type or paste your abstract here as prescribed by the journal’s instructions for authors. Type or paste your abstract here as prescribed by the journal’s instructions for authors. Type or paste your abstract here as prescribed by the journal’s instructions for authors. Type or paste your abstract here. This paper deals with dynamics of incommensurate superstructure at the moment of its formation. Lyapunov exponent value spectra, phase portraits, incommensurate modulation wave vector behavior, Fourier spectra and maps of incommensurate phase dynamic modes under changed parameters of long-range interaction (Т) and anisotropic interaction (K) in interval 0.0 ÷ 1.0 were analyzed. The calculations were made in Python environment applying Skipy, JiTCODE libraries. Under low values of Т and K parameters (0.0015 ÷ 0.03) a chaotic state occurs in this system. It is determined by the interaction between different spatial areas of the correlated motion of tetrahedral groups. The interaction of spatial areas of the correlated motion of tetrahedral groups violates the modulation pattern of tetrahedral group motion causing the superstructure chaotic state. The study of Fourier spectra has shown that interaction of spatial areas of tetrahedral group correlated motion of causes the chaotic state that is non-stable and is destroyed by long-range interaction. The impact of surface energy on the interaction of spatial areas of tetrahedral group correlated motion is determined by decreased amplitude of vibrations of tetrahedral groups causing the disappearance of the chaotic state.

Pseudogap phase and fractionalization: Predictions for Josephson junction setup

The pseudogap regime of the underdoped cuprates arguably remains one of the most enigmatic phenomena of correlated quantum matter. Recent theoretical ideas suggest that a pair density wave (PDW) or a "fractionalized PDW" could be a key ingredient for the understanding of the pseudogap physics. These ideas are to be contrasted to the scenario where charge density wave order and superconductivity coexist at low temperatures. In this paper, we present a few tests to compare the two scenarios in a Josephson junction setup. For a PDW scenario, we observe a beat-like structure of AC Josephson current. The additional frequencies for the AC Josephson current appear at the half-odd integer multiple of the standard Josephson frequency. We can extract the modulation wavevector of the PDW state by studying the average Josephson current. Furthermore, the usual sharp Shapiro steps break down. In contrast, these signatures are absent for the simple coexistence of orders. Any detection of such signatures in a similar experimental setup will strongly support the PDW scenario for the pseudogap phase.

Terahertz transverse electric modes in graphene with DC current in hydrodynamic regime

The dispersion, excitation, and amplification of electromagnetic transverse electric (TE) modes at terahertz (THz) frequencies in graphene in the hydrodynamic (HD) regime, with a direct electric current flowing perpendicular to the TE mode wavevector, were theoretically investigated. The expression for the nonlocal HD conductivity of graphene with a direct electric current flowing perpendicular to the TE mode wavevector was derived. The direct electric current in graphene leads to the capacitive nature of the graphene HD conductivity at THz frequencies, which makes TE modes exist in this frequency range. The excitation of TE modes in graphene by an incident THz wave was modeled for the attenuated total reflection geometry. A new physical mechanism of TE mode amplification in graphene effective for a low value of carrier drift velocity was predicted. THz lasing regimes with TE modes in graphene structure with direct electric current were found. The results of this work can be used to create miniature technologically feasible sources and amplifiers of THz radiation.

Orthorhombic charge density wave on the tetragonal lattice of EuAl4.

EuAl4 possesses the BaAl4 crystal structure type with tetragonal symmetry I4/mmm. It undergoes a charge density wave (CDW) transition at T CDW= 145 K and features four consecutive antiferromagnetic phase transitions below 16 K. Here we use single-crystal X-ray diffraction to determine the incommensurately modulated crystal structure of EuAl4 in its CDW state. The CDW is shown to be incommensurate with modulation wave vector q= (0,0,0.1781 (3)) at 70 K. The symmetry of the incommensurately modulated crystal structure is orthorhombic with superspace group Fmmm(00σ)s00, where Fmmm is a subgroup of I4/mmm of index 2. Both the lattice and the atomic coordinates of the basic structure remain tetragonal. Symmetry breaking is entirely due to the modulation wave, where atoms Eu and Al1 have displacements exclusively along a, while the fourfold rotation would require equal displacement amplitudes along a and b. The calculated band structure of the basic structure and interatomic distances in the modulated crystal structure both indicate the Al atoms as the location of the CDW. The tem-per-ature dependence of the specific heat reveals an anomaly at T CDW= 145 K of a magnitude similar to canonical CDW systems. The present discovery of orthorhombic symmetry for the CDW state of EuAl4 leads to the suggestion of monoclinic instead of orthorhombic symmetry for the third AFM state.

Charge, bond, and pair density wave orders in a strongly correlated system

The coexistence of multiple quasi-degenerate orders is the hallmark of the strongly correlated materials. Experiments often reveal several spatially modulated orders in the underdoped cuprates. This has come to the forefront with the possible detection of the pair density wave states. However, microscopic calculations often struggle to stabilize such spatially modulating orders as the ground state in the strong correlation limit. This work uses the $t-t^\prime-J$-model with an additional nearest-neighbor repulsion to stabilize spatially oscillating charge, bond, and pairing orders in the underdoped regime. We employ the standard Gutzwiller approach while treating the inhomogeneity for the spatial orders using the self-consistent Hartree-Fock-Bogoliubov methodology. Our calculations reveal that unidirectional bond density states coexisting with charge and pairing modulations can have lower energy than the uniform superconducting state over an extensive doping range. These modulating states vanish monotonically as the modulation wavevector becomes shorter with increased dopings. The finite momentum orders melt upon increasing doping to a vestigial nematic state which breaks the rotational symmetry of the system. The spatial features of the ground state at each doping reveal multiple wavevectors, which potentially drives the incommensuration of charge orders. Interestingly, the spatially modulating states are absent when the strong correlations criteria are relaxed, suggesting that the removal of double occupancy aids the stabilization of density wave orders.

Incommensurate Phase in Λ-cobalt (III) Sepulchrate Trinitrate Governed by Highly Competitive N-H⋅⋅⋅O and C-H⋅⋅⋅O Hydrogen Bond Networks.

Phase transitions in molecular crystals are often determined by intermolecular interactions. The cage complex of [Co(C12H30N8)]3+ ⋅ 3 NO3 − is reported to undergo a disorder‐order phase transition at T c1 ≈133 K upon cooling. Temperature‐dependent neutron and synchrotron diffraction experiments revealed satellite reflections in addition to main reflections in the diffraction patterns below T c1. The modulation wave vector varies as function of temperature and locks in at T c3≈98 K. Here, we demonstrate that the crystal symmetry lowers from hexagonal to monoclinic in the incommensurately modulated phases in T c1<T<T c3. Distinctive levels of competitions: trade‐off between longer N−H⋅⋅⋅O and shorter C−H⋅⋅⋅O hydrogen bonds; steric constraints to dense C−H⋅⋅⋅O bonds give rise to pronounced modulation of the basic structure. Severely frustrated crystal packing in the incommensurate phase is precursor to optimal balance of intermolecular interactions in the lock‐in phase.

Spin and quadrupole correlations by three-spin interaction in the frustrated pyrochlore magnetTb2+xTi2−xO7+y

We have investigated the origin of the magnetic dipole correlations $\langle \sigma_{\boldsymbol{Q}}^z \sigma_{\boldsymbol{-Q}}^z \rangle$ characterized by the modulation wave vector $\boldsymbol{k} \sim (\tfrac{1}{2},\tfrac{1}{2},\tfrac{1}{2})$ observed in the frustrated pyrochlore magnet Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$. This magnetic short-range order cannot be accounted for by adding further-neighbor exchange interactions to the nearest-neighbor pseudospin-$\tfrac{1}{2}$ Hamiltonian for quantum pyrochlore magnets. Using classical Monte Carlo simulation and quantum simulation based on thermally pure quantum (TPQ) states we have shown that the spin correlations with $\boldsymbol{k} \sim (\tfrac{1}{2},\tfrac{1}{2},\tfrac{1}{2})$ are induced at low temperatures by a three-spin interaction of a form $\sigma_{\boldsymbol{r}}^{\pm} \sigma_{\boldsymbol{r}^{\prime}}^z \sigma_{\boldsymbol{r}^{\prime \prime}}^z $, which is a correction to the Hamiltonian due to the low crystal-field excitation. Simulations using TPQ states have shown that the spin correlations coexist with electric quadrupole correlations $\langle \sigma_{\boldsymbol{Q}}^{\alpha} \sigma_{\boldsymbol{-Q}}^{\beta} \rangle$ ($\alpha, \beta = x, y$) with $\boldsymbol{k} \sim \boldsymbol{0}$. These results suggest that the putative quantum spin liquid state of Tb$_{2+x}$Ti$_{2-x}$O$_{7+y}$ is located close to phase boundaries of the spin-ice, quadrupole-ordered, and magnetic-ordered states in the classical approximation, and that the three-spin interaction brings about a quantum disordered ground state with both spin and quadrupole correlations.

Disentangling the phase sequence and correlated critical properties in Bi0.7La0.3FeO3 by structural studies

This work addresses the study of the high-temperature phase sequence of ${\mathrm{Bi}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{FeO}}_{3}$ by undertaking temperature-dependent high-resolution neutron powder diffraction (NPD) and Raman spectroscopy measurements. A determination of lattice parameters, phase fractions, and modulation wave vector was performed by Pawley refinement of the NPD data. The analysis revealed that ${\mathrm{Bi}}_{0.7}{\mathrm{La}}_{0.3}{\mathrm{FeO}}_{3}$ exhibits an incommensurate modulated orthorhombic $Pn{2}_{1}a(00\ensuremath{\gamma})000$ structure at room temperature, with a weak ferromagnetic behavior, likely arising from a canted antiferromagnetic ordering. Above ${T}_{1}=543\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, the low-temperature modulated $Pn{2}_{1}a(00\ensuremath{\gamma})000$ evolves monotonically into a fractionally growing Pnma structure up to ${T}_{\mathrm{N}}=663\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. At 663 K, the low-temperature canted antiferromagnetic phase is suppressed concurrently with the switching of the former into a nonmodulated $Pn{2}_{1}a$ structure that continues to coexist with the Pnma one, until the latter is expected to reach the 100% fraction of the sample volume at high temperatures above 733 K. The $Pn{2}_{1}a$ space group is obtained from the Pnma one through the ${\mathrm{\ensuremath{\Gamma}}}_{4}^{\ensuremath{-}}$ polar distortion. Neutron diffraction and Raman spectroscopy results provide evidence for the emergence of noteworthy linear spin-phonon coupling. In this regard, magnetostructural coupling is observed below ${T}_{\mathrm{N}}$, revealed by the relation between the weak ferromagnetism of the canted iron spins and the ${\mathrm{FeO}}_{6}$ octahedra symmetric stretching mode. The correlation between magnetization and structural results from NPD provides definite evidence for the magnetic origin of the structural modulation. The analysis of the temperature-dependent magnetization and the magnetic peak intensity as well yields a critical exponent (\ensuremath{\beta}) value of 0.38. The lower limit of the phase coexistence temperature ${T}_{1}=543\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, marking the emergence of the Pnma phase, is also associated with the temperature whereupon the modulation magnitude starts to decrease.

Low-dimensional antiferromagnetic fluctuations in the heavy-fermion paramagnetic ladder compound UTe2

Inelastic-neutron-scattering measurements were performed on a single crystal of the heavy-fermion paramagnet UTe$_2$ above its superconducting temperature. We confirm the presence of antiferromagnetic fluctuations with the incommensurate wavevector $\mathbf{k}_1=(0,0.57,0)$. A quasielastic signal is found, whose momentum-transfer dependence is compatible with fluctuations of magnetic moments $\mu\parallel\mathbf{a}$, with a sine-wave modulation of wavevector $\mathbf{k}_1$ and in-phase moments on the nearest U atoms. Low dimensionality of the magnetic fluctuations, consequence of the ladder structure, is indicated by weak correlations along the direction $\mathbf{c}$. These fluctuations saturate below the temperature $T_1^*\simeq15$~K, in possible relation with anomalies observed in thermodynamic, electrical-transport and nuclear-magnetic-resonance measurements. The absence or weakness of ferromagnetic fluctuations, in our data collected at temperatures down to 2.1 K and energy transfers from 0.6 to 7.5 meV, is emphasized. These results constitute constraints for models of magnetically-mediated superconductivity in UTe$_2$.

Anisotropy and the Onset of the Thermoconvective Instability in a Vertical Porous Layer

Abstract The thermoconvective instability of the parallel vertical flow in a fluid saturated porous layer bounded by parallel open boundaries is studied. The open boundaries are assumed to be kept at constant uniform pressure while their temperatures are uniform and different, thus forcing a horizontal temperature gradient across the layer. The anisotropic permeability of the porous layer is accounted for by assuming the principal axes to be oriented along the directions perpendicular and parallel to the layer boundaries. A linear stability analysis based on the Fourier normal modes of perturbation is carried out by testing the effect of the inclination of the normal mode wavevector to the vertical. The neutral stability curves and the critical Rayleigh number for the onset of the instability are evaluated by solving numerically the stability eigenvalue problem.

Evolving Devil's Staircase Magnetization from Tunable Charge Density Waves in Nonsymmorphic Dirac Semimetals.

While several magnetic topological semimetals have been discovered in recent years, their band structures are far from ideal, often obscured by trivial bands at the Fermi energy. Square-net materials with clean, linearly dispersing bands show potential to circumvent this issue. CeSbTe, a square-net material, features multiple magnetic-field-controllable topological phases. Here, it is shown that in this material, even higher degrees of tunability can be achieved by changing the electron count at the square-net motif. Increased electron filling results in structural distortion and formation of charge density waves (CDWs). The modulation wave-vector evolves continuously leading to a region of multiple discrete CDWs and a corresponding complex "Devil's staircase" magnetic ground state. A series of fractionally quantized magnetization plateaus is observed, which implies direct coupling between CDW and a collective spin-excitation. It is further shown that the CDW creates a robust idealized nonsymmorphic Dirac semimetal, thus providing access to topological systems with richmagnetism.

Impact of the interfacial Dzyaloshinskii-Moriya interaction on the band structure of one-dimensional artificial magnonic crystals: A micromagnetic study

We present the results of a systematic micromagnetic study of the effect of the Dzyaloshinskii-Moriya interaction (DMI) on the spin wave band structure of two one-dimensional magnonic crystals (MCs), both with the same periodicity p = 300 nm, but different implementation of the DMI modulation. In the first system the artificial periodicity was achieved by modulating the interfacial DMI constant D, while in the second system also the sample morphology was modulated. Due to the folding property of the band structure in the dispersion relations of the magnonic crystals it is possible to extend the sensitivity of Brillouin light scattering towards weak DMI strength (D in the range from 0 to 0.5 mJ/m2), by measuring the frequency splitting of folded modes in high-order artificial Brillouin zones, since the splitting increases almost linearly with the band index. For relatively large values of the DMI (D in the range from 1.0 to 2.0 mJ/m2) the spin waves dispersion relations present flat modes for positive wavevectors, separated by forbidden frequency gaps whose amplitude depend on the value of D. These frequency gaps are more pronounced for the sample with morphology modulation. The non-reciprocal, localised, spatial profiles of these modes in both MCs are discussed with reference to spin waves in plain films and in isolated stripes of the same thickness.