The coexistence of charge density wave (CDW) and superconductivity in tantalum disulfide (2H−TaS2) at low temperature is boosted by applying hydrostatic pressures to study both vibrational and ...magnetic transport properties. Around Pc, we observe a superconducting dome with a maximum superconducting transition temperature Tc=9.1 K. First-principles calculations of the electronic structure predict that, under ambient conditions, the undistorted structure is characterized by a phonon instability at finite momentum close to the experimental CDW wave vector. Upon compression, this instability is found to disappear, indicating the suppression of CDW order. The calculations reveal an electronic topological transition (ETT), which occurs before the suppression of the phonon instability, suggesting that the ETT alone is not directly causing the structural change in the system. The temperature dependence of the first vortex penetration field has been experimentally obtained by two independent methods. While a d wave and single-gap BCS prediction cannot describe the lower critical field Hc1 data, the temperature dependence of the Hc1 can be well described by a single-gap anisotropic s-wave order parameter.
Low-temperature neutron diffraction experiments at Formula: see text GPa have been conducted to investigate the magnetic structures of metallic Holmium at high pressures by employing a long d-spacing ...high-flux diffractometer and a Paris-Edinburgh press cell inside a cryostat. We find that at Formula: see text GPa and Formula: see text K, no nuclear symmetry change is observed, keeping therefore the hexagonal closed packed (hcp) symmetry at high pressure. Our neutron diffraction data confirm that the ferromagnetic state does not exist. The magnetic structure corresponding to the helimagnetic order, which survives down to 5 K, is fully described by the magnetic superspace group formalism. These results are consistent with those previously published using magnetization experiments.
The ferromagnetic state of the spin-polarized ferromagnet La1−xSrxMnO3 is stabilized in the metallic region by strong coupling between localized spins in the t2g orbital and conduction electrons in ...the eg orbital. We prepared polycrystalline La1−xSrxMnO3 films (x = 0.15, 0.25, or 0.30) by deposition on an oxidized Si substrate. The three types of La1−xSrxMnO3 films were in the ferromagnetic rhombohedral phase, and their Curie temperatures, TC, evaluated from the midpoint of ac magnetization, were 305 K, 335 K, and 338 K, respectively. By applying expansion-mode acoustic vibration to the crystal structure of La1−xSrxMnO3, we observed a remarkable decrease (as large as 70 K) in TC. The applied structural perturbation causes a decrease in the possibility of conduction electron hopping and an increase in the Jahn–Teller distortion. The former is more effective for decreasing TC than the latter.
It is well known that the archetype chiral magnet MnSi stabilizes a skyrmion lattice, termed “A-phase,” in a narrow temperature range in the vicinity of the paramagnetic boundary around Tc ∼ 29 K and ...Hc ∼ 2 kOe. Recently, it has been predicted that at much lower temperatures below Tc, the conical helicoid and the forced ferromagnetic (FFM) states could be separated by a new “unknown state.” In order to detect this “unknown state,” we explored the phase diagram of MnSi oriented single crystals as a function of the d.c. magnetic field (H⃗dc) and the temperature (T) by using a.c. magnetization measurements. For H⃗dc∥ 〈111〉, we observed a new region, termed “B-phase,” in the magnetic phase diagram, characterized by a flat-valley-like anomaly on the in-phase component of the a.c. magnetization (m′), over 3.5 ≤ Hdc ≤ 6.2 kOe just below the low temperature (T < 6 K) FFM boundary. The observed frequency independence over 0.3–1000 Hz and the absence of any measurable absorption in the a.c. magnetization (m″) in the “B-phase” suggest a static nature. The “B-phase” was not observed for either H⃗dc∥ 〈100〉 or 〈110〉, revealing that the magnetic anisotropy could play a role in the stabilization of the phase. The “B-phase” could be compatible with the theoretical predictions if the new magnetic state is supposedly related with a relative reorientation of the four helices in MnSi.
A monoaxial chiral magnet forms a kind of spin superlattice structure, termed chiral soliton lattice (CSL), by the application of a magnetic field H perpendicular to a helical chiral axis. It has ...been reported that the CSL accompanies the magnetoresistance effect as well as a discrete change in magnetization and magnetoresistance. In order to verify the effect of the structural modification on the CSL state, we measured the magnetoresistance under the dynamic stress (DS) with a frequency of the order of MHz, which was applied by a piezoelectric ceramic oscillator. The steady application of DS while decreasing H resulted in a suppression of the insertion of chiral soliton. On the other hand, the application of a pulse-like DS while H decreased assisted the insertion of chiral soliton. These results demonstrate that DS modifies the spin structure of the monoaxial chiral magnet, and we can therefore change the activation energy for the insertion of chiral soliton while H is decreased.
The constraint nondegeneracy condition is one of the most relevant and useful constraint qualifications in nonlinear semidefinite programming. It can be characterized in terms of any fixed ...orthonormal basis of the, let us say,
ℓ
-dimensional kernel of the constraint matrix, by the linear independence of a set of
ℓ
(
ℓ
+
1
)
/
2
derivative vectors. We show that this linear independence requirement can be equivalently formulated in a smaller set, of
ℓ
derivative vectors, by considering all orthonormal bases of the kernel instead. This allows us to identify that not all bases are relevant for a constraint qualification to be defined, giving rise to a strictly weaker variant of nondegeneracy related to the global convergence of an external penalty method. We use some of these ideas to revisit an approach of Forsgren (Math Program 88, 105–128, 2000) for exploiting the sparsity structure of a transformation of the constraints to define a constraint qualification, which led us to develop another relaxed notion of nondegeneracy using a simpler transformation. If the zeros of the derivatives of the constraint function at a given point are considered, instead of the zeros of the function itself in a neighborhood of that point, we obtain an even weaker constraint qualification that connects Forsgren’s condition and ours.
Sequential optimality conditions play a major role in proving stronger global convergence results of numerical algorithms for nonlinear programming. Several extensions are described in conic ...contexts, in which many open questions have arisen. In this paper, we present new sequential optimality conditions in the context of a general nonlinear conic framework, which explains and improves several known results for specific cases, such as semidefinite programming, second-order cone programming, and nonlinear programming. In particular, we show that feasible limit points of sequences generated by the augmented Lagrangian method satisfy the so-called approximate gradient projection optimality condition and, under an additional smoothness assumption, the so-called complementary approximate Karush–Kuhn–Tucker condition. The first result was unknown even for nonlinear programming, and the second one was unknown, for instance, for semidefinite programming.
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