Spin Density Wave Phase Research Articles

Kinetic-arrest-induced phase coexistence and metastability in(Mn,Fe)2(P,Si)

Neutron diffraction, M\"ossbauer spectroscopy, magnetometry, and in-field x-ray diffraction are employed to investigate the magnetoelastic phase transition in hexagonal ${(\mathrm{Mn},\mathrm{Fe})}_{2}(\mathrm{P},\mathrm{Si})$ compounds. ${(\mathrm{Mn},\mathrm{Fe})}_{2}(\mathrm{P},\mathrm{Si})$ compounds undergo for certain compositions a second-order paramagnetic (PM) to a spin-density-wave (SDW) phase transition before further transforming into a ferromagnetic (FM) phase via a first-order phase transition. The SDW-FM transition can be kinetically arrested, causing the coexistence of FM and untransformed SDW phases at low temperatures. Our in-field x-ray diffraction and magnetic relaxation measurements clearly reveal the metastability of the untransformed SDW phase. This unusual magnetic configuration originates from the strong magnetoelastic coupling and the mixed magnetism in hexagonal ${(\mathrm{Mn},\mathrm{Fe})}_{2}(\mathrm{P},\mathrm{Si})$ compounds.

Heavy electron doping induced antiferromagnetic phase as the parent for the iron oxypnictide superconductorLaFeAsO1−xHx

We perform transport measurements and band structure calculations of electron-doped LaFeAsO1-xHx over a wide range of x from 0.01 to 0.66. The T^2 and sqrt(T) dependency of the resistivity are observed at x ~ 0.17 and 0.41, respectively. The sign change of RH without opening of the spin-density-wave gap for 0.45 < x < 0.58 and T < TN as well as the calculated non-nested Fermi surface at x = 0.5 indicate the more localized nature of the AF2 as compared to spin density wave phase at non-doped sample. Considering the results from band calculations and the finite size of Hund' s rule coupling, the change of the normal conducting state with x is reasonably explained by a strong depression of the coherent scale owing to the increased effective Coulomb repulsion in the narrow anti-bonding 3dxy band that approaches the half-filled regime. The following transition from the paramagnet to AF2 is understood as the quenching or ordering of the less-screened spins with a large entropy at low temperature. These results suggest that the normal conducting properties over a wide doping range for the LaFeAsO1-xHx are strongly influenced by the local spins in the incoherent region. Thus, we conclude that the parent phase in LaFeAsO1-xHx is not the spin-density wave but the AF2, which may be primarily responsible for the singular superconducting properties of the 1111 type compared with other iron pnictides.

Spin density wave instabilities in theNbS2monolayer

In the present work, we study the magnetic properties of the NbS2 monolayer by first-principles calculations. The transition metal dichalcogenides (TMDC) are a family of laminar materials presenting exciting properties such as charge density waves (CDW), superconductivity and metal-insulating transitions among others. 2H-NbS2 is a particular case within the family, because it is the only one that is superconductor without exhibiting a CDW order. Although no long range magnetic order was experimentally observed in the TMDC, we show here that the single monolayer of NbS2 is on the verge of a spin density wave (SDW) phase. Our calculations indicate that a wave-like magnetic order is stabilized in the NbS2 monolayer in the presence of magnetic defects or within zig-zag nanoribbons, due to the presence of unpaired electrons. We calculate the real part of the bare electronic susceptibilty and the corresponding nesting function of the clean NbS2 monolayer, showing that there are strong electronic instabilities at the same wavevector asociated with the calculated SDWs, also corresponding with one of the main nesting vectors of the Fermi surface. We conclude that the physical mechanism behind the spin-wave instabilities are the nesting properties, accentuated by the quasi 2D character of this system, and the rather strong Coulomb interactions of the 4d band of the Nb atom. We also estimate the amplitude of the spin-fluctuations and find that they are rather large, as expected for a system on the verge of a quantum critical transition.

Incommensurate spin-density-wave antiferromagnetism inCeRu2Al2B

The newly discovered Ising-type ferromagnet CeRu$_2$Al$_2$B exhibits an additional phase transition at $T_\textrm{N}$ = 14.2 K before entering the ferromagnetic ground state at $T_\textrm{C}$ = 12.8 K. We clarify the nature of this transition through high resolution neutron diffraction measurements. The data reveal the presence of a longitudinal incommensurate spin-density wave (SDW) in the temperature range of $T_\textrm{C}<T<T_\textrm{N}$. The propagation vector ${\bf q} \sim (0,0,0.148)$ is nearly temperature independent in this region and discontinuously locks into ${\bf q} = 0$ at $T_\textrm{C}$. Mean-field calculations of an effective Ising model indicate that the modulated SDW phase is stabilized by a strong competition between ferromagnetic and antiferromagnetic exchange interactions. This makes CeRu$_2$Al$_2$B a particularly attractive model system to study the global phase diagram of ferromagnetic heavy-fermion metals under influence of magnetic frustration.

Quantum phase transition in a one-dimensional Holstein-Hubbard model at half-filling in the thermodynamic limit: A quantum entanglement approach

The quantum phase transition from a spin-density wave phase to a charge-density wave phase is studied within the framework of the one-dimensional Holstein-Hubbard model. The phonons are first eliminated by using a variational phonon state and the effective electronic Hamiltonian is then exactly solved using the Bethe ansatz technique to get the ground state energy. The entanglement entropy is finally calculated to show the possibility of existence of an intervening metallic phase at the cross-over region of the spin-density and charge-density wave phases in the thermodynamic limit at half-filling.

Optical study of BaFe2As2 under pressure: Coexistence of spin-density-wave gap and superconductivity

Temperature-dependent reflectivity measurements in the frequency range 85 - 7000 cm$^{-1}$ were performed on BaFe$_2$As$_2$ single crystals under pressure up to $\sim$5 GPa. The corresponding pressure- and temperature-dependent optical conductivity was analyzed with the Drude-Lorentz model to extract the coherent and incoherent contributions. The gradual suppression of the spin-density wave (SDW) state with increasing pressure and the appearance of the superconducting phase coexisting with the SDW phase at 3.6~GPa were observed. At 3.6~GPa, the reflectivity reaches unity below $\sim$95~cm$^{-1}$ indicating the opening of the superconducting gap and shows a full gap tendency at 6 K.

Dimensionality-tuned electronic structure of nickelate superlattices explored by soft-x-ray angle-resolved photoelectron spectroscopy

The electronic and magnetic properties of epitaxial LaNiO3/LaAlO3 superlattices can be tuned by layer thickness and substrate-induced strain. Here, we report on direct measurements of the k-space-resolved electronic structure of buried nickelate layers in superlattices under compressive strain by soft x-ray photoemission. After disentangling strong extrinsic contributions to the angle-dependent signal caused by photoelectron diffraction, we are able to extract Fermi surface information from our data. We find that with decreasing LaNiO3 thickness down to two unit cells (2 uc) quasiparticle coherence becomes strongly reduced, in accord with the dimension-induced metal-to-insulator transition seen in transport measurements. Nonetheless, on top of a strongly incoherent background a residual Fermi surface can be identified in the 2 uc superlattice whose nesting properties are consistent with the spin-density wave (SDW) instability recently reported. The overall behavior of the Ni 3d spectra and the absence of a complete gap opening indicate that the SDW phase is dominated by strong order parameter fluctuations.

Influence of the Kondo effect on spin density waves in the Kondo lattice model

We analyze the influence of the Kondo effect on the spin density wave (SDW) phase in the Kondo lattice model (KLM) for a two-dimensional square lattice. We demonstrate that in the isotropic KLM the SDW phase is divided into a small Fermi-surface region and a large Fermi-surface region and vanishes for strong interaction. On the other hand, in the Ising-spin KLM, where the Kondo effect is absent, we only find a small Fermi-surface SDW phase, which does not vanish at strong coupling. This demonstrates the importance of the Kondo effect for the large Fermi surface SDW phase as well as for the existence of the quantum critical transition between the SDW phase and the paramagnetic phase.

Field-induced spin density wave and spiral phases in a layered antiferromagnet

We determine the low-field ordered magnetic phases of the $S=1$ dimerized antiferromagnet ${\mathrm{Ba}}_{3}{\mathrm{Mn}}_{2}{\mathrm{O}}_{8}$ using single-crystal neutron diffraction. We find that for magnetic fields between ${\ensuremath{\mu}}_{0}H=8.80\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ and $10.56\phantom{\rule{0.28em}{0ex}}\mathrm{T}$ applied along the $[1\overline{1}0]$ direction the system exhibits spin density wave order with incommensurate wave vectors of type $(\ensuremath{\eta},\ensuremath{\eta},\ensuremath{\epsilon})$. For ${\ensuremath{\mu}}_{0}Hg10.56\phantom{\rule{0.28em}{0ex}}\mathrm{T}$, the magnetic order changes to a spiral phase with incommensurate wave vectors only along the $[hh0]$ direction. For both field-induced ordered phases, the magnetic moments are lying in the plane perpendicular to the field direction. The nature of these two transitions is fundamentally different: the low-field transition is a second-order transition to a spin density wave ground state, while the one at higher field, toward the spiral phase, is of first order.

Incommensurate spin density wave at a ferromagnetic quantum critical point in a three-dimensional parabolic semimetal

We explore the ferromagnetic quantum critical point in a three-dimensional semimetallic system with upward- and downward-dispersing bands touching at the Fermi level. Evaluating the static spin susceptibility to leading order in the coupling between the fermions and the fluctuating ferromagnetic order parameter, we find that the ferromagnetic quantum critical point is masked by an incommensurate, longitudinal spin density wave phase. We first analyze an idealized model which, despite having strong spin-orbit coupling, still possesses O(3) rotational symmetry generated by the total angular momentum operator. In this case, the direction of the incommensurate spin density wave propagation can point anywhere, while the magnetic moment is aligned along the direction of propagation. Including symmetry-allowed anisotropies in the fermion dispersion and the coupling to the order parameter field, however, the ordering wavevector instead breaks a discrete symmetry and aligns along either the [111] or [100] direction, depending on the signs and magnitudes of these two types of anisotropy.

Combined external pressure and Cu-substitution studies on BaFe2As2 single crystals

We report a combined study of external pressure and Cu-substitution on BaFe2As2 single crystals grown by the in-flux technique. At ambient pressure, the Cu-substitution is known to suppress the spin density wave (SDW) phase in pure BaFe2As2(TSDW ≈ 140 K) and to induce a superconducting (SC) dome with a maximum transition temperature . This is much lower than the Tc ∼ 15–28 K achieved in the case of Ru, Ni and Co substitutions. Such a lower Tc is attributed to a Cu2+ magnetic pair-breaking effect. The latter is strongly suppressed by applied pressure, as shown herein, Tc can be significantly enhanced by applying high pressures. In this work, we investigated the pressure effects on Cu2+ magnetic pair-breaking in the BaFe2−xCuxAs2 series. Around the optimal concentration (xopd = 0.11), all samples showed a substantial increase of Tc as a function of pressure. Yet for those samples with a slightly higher doping level (over-doped regime), Tc presented a dome-like shape with maximum Tc ≃ 8 K. Remarkably interesting, the under-doped samples, e.g. x = 0.02 display a maximum pressure induced Tc ≃ 30 K which is comparable to the maximum Tc's found for the pure compound under external pressures. Furthermore, the magnetoresistance effect as a function of pressure in the normal state of the x = 0.02 sample also presented an evolution consistent with the screening of the Cu2+ local moments. These findings demonstrate that the Cu2+ magnetic pair-breaking effect is completely suppressed by applying pressure in the low concentration regime of Cu2+ substituted BaFe2As2.

Spin excitations in the nematic phase and the metallic stripe spin-density wave phase of iron pnictides

We present a general study of the magnetic excitations within a weak-coupling five-orbital model relevant to itinerant iron pnictides. As a function of enhanced electronic correlations, the spin excitations in the symmetry-broken spin-density wave phase evolve from broad low-energy modes in the limit of weak interactions to sharply dispersing spin waves prevailing to higher energies at larger interaction strengths. We show how the resulting spin response at high energies depends qualitatively on the magnitude of the interactions. We also calculate the magnetic excitations in the nematic phase by including an orbital splitting, and find a pronounced ${C}_{2}$-symmetric excitation spectrum right above the transition to long-range magnetic order. Finally, we discuss the ${C}_{2}$ versus ${C}_{4}$ symmetry of the spin excitations as a function of energy for both the nematic and the spin-density wave phase.

Phase diagram of (Li(1-x)Fe(x))OHFeSe: a bridge between iron selenide and arsenide superconductors.

Previous experimental results have shown important differences between iron selenide and arsenide superconductors which seem to suggest that the high-temperature superconductivity in these two subgroups of iron-based families may arise from different electronic ground states. Here we report the complete phase diagram of a newly synthesized superconducting (SC) system, (Li1-xFex)OHFeSe, with a structure similar to that of FeAs-based superconductors. In the non-SC samples, an antiferromagnetic (AFM) spin-density-wave (SDW) transition occurs at ∼127 K. This is the first example to demonstrate such an SDW phase in an FeSe-based superconductor system. Transmission electron microscopy shows that a well-known √5×√5 iron vacancy ordered state, resulting in an AFM order at ∼500 K in AyFe2-xSe2 (A = metal ions) superconductor systems, is absent in both non-SC and SC samples, but a unique superstructure with a modulation wave vector q = (1)/2(1,1,0), identical to that seen in the SC phase of KyFe2-xSe2, is dominant in the optimal SC sample (with an SC transition temperature Tc = 40 K). Hence, we conclude that the high-Tc superconductivity in (Li1-xFex)OHFeSe stems from the similarly weak AFM fluctuations as FeAs-based superconductors, suggesting a universal physical picture for both iron selenide and arsenide superconductors.

Spontaneous Peierls dimerization and emergent bond order in one-dimensional dipolar gases

We investigate the effect of dipolar interactions in one-dimensional systems in connection with the possibility of observing exotic many-body effects with trapped atomic and molecular dipolar gases. By combining analytical and numerical methods, we show how the competition between short- and long-range interactions gives rise to frustrating effects which lead to the stabilization of spontaneously dimerized phases characterized by a bond-ordering. This genuine quantum order is sharply distinguished from Mott and spin-density wave phases, and can be unambiguously probed by measuring non local order parameters in-situ imaging techniques.

Tunable Charge and Spin Order in PrNiO_{3} Thin Films and Superlattices.

We use polarized Raman scattering to probe lattice vibrations and charge ordering in 12nm thick, epitaxially strained PrNiO_{3} films, and in superlattices of PrNiO_{3} with the band insulator PrAlO_{3}. A carefully adjusted confocal geometry is used to eliminate the substrate contribution to the Raman spectra. In films and superlattices under tensile strain which undergo a metal-insulator transition upon cooling, the Raman spectra reveal phonon modes characteristic of charge ordering. These anomalous phonons do not appear in compressively strained films, which remain metallic at all temperatures. For superlattices under compressive strain, the Raman spectra show no evidence of anomalous phonons indicative of charge ordering, while complementary resonant x-ray scattering experiments reveal antiferromagnetic order associated with a modest increase in resistivity upon cooling. This confirms theoretical predictions of a spin density wave phase driven by spatial confinement of the conduction electrons.

Site specific spin dynamics in BaFe2As2: tuning the ground state by orbital differentiation.

The role of orbital differentiation on the emergence of superconductivity in the Fe-based superconductors remains an open question to the scientific community. In this investigation, we employ a suitable microscopic spin probe technique, namely Electron Spin Resonance (ESR), to investigate this issue on selected chemically substituted BaFe2As2 single crystals. As the spin-density wave (SDW) phase is suppressed, we observe a clear increase of the Fe 3d bands anisotropy along with their localization at the FeAs plane. Such an increase of the planar orbital content is interestingly independent of the chemical substitution responsible for suppressing the SDW phase. As a consequence, the magnetic fluctuations in combination with this particular symmetry of the Fe 3d bands are propitious ingredients for the emergence of superconductivity in this class of materials.

Versatile module for experiments with focussing neutron guides

We report the development of a versatile module that permits fast and reliable use of focussing neutron guides under varying scattering angles. A simple procedure for setting up the module and neutron guides is illustrated by typical intensity patterns to highlight operational aspects as well as typical parasitic artefacts. Combining a high-precision alignment table with separate housings for the neutron guides on kinematic mounts, the change-over between neutron guides with different focussing characteristics requires no readjustments of the experimental setup. Exploiting substantial gain factors, we demonstrate the performance of this versatile neutron scattering module in a study of the effects of uniaxial stress on the domain populations in the transverse spin density wave phase of single crystal Cr.

Characterization of Doped Na(Fe1−x T x )As Single Crystals with T = Pd, Ni, Cr, and Mn

Single crystals of Na1−δ (Fe1−x T x )As with T = Pd, Ni, Cr, and Mn were investigated by magnetic susceptibility, specific heat, and electronic transport measurements. The structural and spin density wave phase transitions are completely suppressed when Fe in NaFeAs is substituted by those different heterovalent dopings. While superconductivity is only observed for formal electron doping (viz. by substituting Fe with either Pd or Ni), a non-superconducting phase down to 1.8 K is achieved by formal hole doping when Fe is substituted by Cr or Mn in NaFeAs along with the formation of a magnetic phase. This magnetic phase has also strong impact on the material properties.

Superconductivity in titanium-based pnictide oxide compounds

Superconductivity in a novel class of layered materials, Ti -based pnictide oxides, was recently discovered. These compounds have attracted interest since they combine features of copper oxide and iron pnictide superconductors. Here the transition metal (titanium) forms two-dimensional Ti 2 O layers (anti structure to the CuO 2 planes), capped by pnictogen ions (similar to Fe 2 As 2 layers). The pnictide oxide compounds show a spin or charge density wave phase which coexists with superconductivity in some members of the family. Unlike the cuprates, but similar to iron pnictides, the parent compounds of pnictide oxides are metals with specific nesting properties of the Fermi surface which leads to the density wave instability. The nature of the superconductivity, coexisting with the density wave order, and the possible competition or mutual interaction between both states is one of the central questions of recent studies. This short review summarizes the current knowledge from an experimental as well as theoretical point of view and discusses some of the open questions and possible future developments.

Coexistence of spin density waves and superconductivity in (TMTSF)2PF6.

We present simultaneous measurements of angular-dependent magnetoresistance and thermopower along all three crystal axes in (TMTSF)2PF6 for pressures to 7.4kbar and magnetic fields to 35T. (TMTSF)2PF6 under pressure shows the coexistence of spin density wave and metal-superconducting orders. We suggest that this coexistence results neither in microscopic coexistence nor in a new soliton wall phase, contrary to previous suggestions, but in phase separation into domains of the high-pressure metal and the low-pressure spin density wave phases. Simultaneous measurement of transport along all crystal axes allows us to unambiguously describe the domain structure, whereas the superconducting transition temperature and four independent Fermi surface-sensitive magnetoresistance signatures allow us to unambiguously characterize the coexisting metallic domains.