Low-energy Magnetic Fluctuations Research Articles

Infinite critical boson non-Fermi liquid on heterostructure interfaces

We study the emergence of non-Fermi liquid on heterostructure interfaces where there exists an infinite number of critical boson modes accounting for the magnetic fluctuations in two spatial dimensions. The interfacial Dzyaloshinskii-Moriya interaction naturally arises in magnetic interactions due to the absence of inversion symmetry, resulting in a degenerate contour for the low-energy bosonic modes in the momentum space which simultaneously becomes critical near the magnetic phase transition. The itinerant electrons are scattered by the critical boson contour via the Yukawa coupling. When the boson contour is much smaller than the Fermi surface, it is shown that, there exists a regime with a dynamic critical exponent {z=3} while the boson contour still controls the low-energy magnetic fluctuations. Using a self-consistent renormalization calculation for this regime, we uncover a prominent non-Fermi liquid behavior in the resistivity with a characteristic temperature scaling power. These findings open up new avenues for understanding boson-fermion interactions and the novel fermionic quantum criticality.

Gradual emergence of superconductivity in underdoped La2−xSrxCuO4

We present triple-axis neutron scattering studies of low-energy magnetic fluctuations in strongly underdoped La$_{2-x}$Sr$_{x}$CuO$_{4}$ with $x=0.05$, $0.06$ and $0.07$, providing quantitative evidence for a direct competition between these fluctuations and superconductivity. At dopings $x=0.06$ and $x=0.07$, three-dimensional superconductivity is found, while only a very weak signature of two-dimensional superconductivity residing in the CuO$_2$ planes is detectable for $x=0.05$. We find a surprising suppression of the low-energy fluctuations by an external magnetic field at all three dopings. This implies that the response of two-dimensional superconductivity to a magnetic field is similar to that of a bulk superconductor. Our results provide direct evidence of a very gradual onset of superconductivity in cuprates.

Large Dynamical Axion Field in Topological Antiferromagnetic Insulator Mn2Bi2Te5Supported by the Fundamental Research Funds for the Central Universities (Grant No. 020414380149), the Natural Science Foundation of China (Grant Nos. 11674165, 11834006 and 11774065), the Fok Ying-Tong Education Foundation of China (Grant No. 161006), the National Key R&D Program of China (Grant Nos. 2016YFA0300703 and 2019YFA0308404), the Shanghai Municipal Science and

The dynamical axion field is a new state of quantum matter where the magnetoelectric response couples strongly to its low-energy magnetic fluctuations. It is fundamentally different from an axion insulator with a static quantized magnetoelectric response. The dynamical axion field exhibits many exotic phenomena such as axionic polariton and axion instability. However, these effects have not been experimentally confirmed due to the lack of proper topological magnetic materials. Combining analytic models and first-principles calculations, here we predict a series of van der Waals layered Mn2Bi2Te5-related topological antiferromagnetic materials that could host the long-sought dynamical axion field with a topological origin. We also show that a large dynamical axion field can be achieved in antiferromagnetic insulating states close to the topological phase transition. We further propose the optical and transport experiments to detect such a dynamical axion field. Our results could directly aid and facilitate the search for topological-origin large dynamical axion field in realistic materials.

Temperature and polarization dependence of low-energy magnetic fluctuations in nearly optimally doped NaFe0.9785Co0.0215As

We use unpolarized and polarized neutron scattering to study the temperature and polarization dependence of low-energy magnetic fluctuations in nearly-optimal-doped NaFe$_{0.9785}$Co$_{0.0215}$As, with coexisting superconductivity ($T_{\rm c}\approx19$ K) and weak antiferromagnetic order ($T_{\rm N}\approx30$ K, ordered moment $\approx0.02$ $\mu_{\rm B}$/Fe). A single spin resonance mode with intensity tracking the superconducting order parameter is observed, although energy of the mode only softens slightly on approaching $T_{\rm c}$. Polarized neutron scattering reveals that the single resonance is mostly isotropic in spin space, similar to overdoped NaFe$_{0.935}$Co$_{0.045}$As but different from optimal electron-, hole-, and isovalent-doped BaFe$_2$As$_2$ compounds, all featuring an additional prominent anisotropic component. Spin anisotropy in NaFe$_{0.9785}$Co$_{0.0215}$As is instead present at energies below the resonance, which becomes partially gapped below $T_{\rm c}$, similar to the situation in optimal-doped YBa$_2$Cu$_3$O$_{6.9}$. Our results indicate that anisotropic spin fluctuations in NaFe$_{1-x}$Co$_x$As appear in the form of a resonance in the underdoped regime, become partially gapped below $T_{\rm c}$ near optimal doping and disappear in overdoped compounds.

NMR evidence for static local nematicity and its cooperative interplay with low-energy magnetic fluctuations in FeSe under pressure

We present $^{77}$Se-NMR measurements on single-crystalline FeSe under pressures up to 2 GPa. Based on the observation of the splitting and broadening of the NMR spectrum due to structural twin domains, we discovered that static, local nematic ordering exists well above the bulk nematic ordering temperature, $T_{\rm s}$. The static, local nematic order and the low-energy stripe-type antiferromagnetic spin fluctuations, as revealed by NMR spin-lattice relaxation rate measurements, are both insensitive to pressure application. These NMR results provide clear evidence for the microscopic cooperation between magnetism and local nematicity in FeSe.

Anomalous magnetic fluctuations in superconducting Sr2RuO4 revealed by Ru101 nuclear spin-spin relaxation

We carried out $^{101}\mathrm{Ru}$ nuclear quadrupole resonance (NQR) measurement on superconducting (SC) ${\mathrm{Sr}}_{2}{\mathrm{RuO}}_{4}$ under zero magnetic field $(H=0)$ and found that the nuclear spin-spin relaxation rate $1/{T}_{2}$ is enhanced in the SC state. The $1/{T}_{2}$ measurement in the SC state under $H=0$ is effective for detecting slow magnetic fluctuations parallel to the quantized axis of the nuclear spin. Our results indicate that low-energy magnetic fluctuations perpendicular to the ${\mathrm{RuO}}_{2}$ plane emerge when the superconductivity sets in, which is consistent with the previous $^{17}\mathrm{O}\text{-NQR}$ result that the nuclear spin-lattice relaxation rate $1/{T}_{1}$ of the in-plane O site exhibits anomalous behavior in the SC state. The enhancement of the magnetic fluctuations in the SC state is unusual and suggests that the fluctuations are related to the unconventional SC pairing. We suggest that this phenomenon is a consequence of the spin degrees of freedom of the spin-triplet pairing.

New superconductivity dome in LaFeAsO1−x F x far away from magnetism and accompanied by structural transition

We report on the discovery and novel physics of a new superconductivity dome in LaFeAsO1−xFx with high-doping rate (0.25 ≤x≤0.75) synthesized by using the high-pressure technique. The maximal critical temperature Tc = 30 K peaked at xopt = 0.5 ∼0.55, which is even higher than that at x≤ 0.2. By nuclear magnetic resonance (NMR), we find that the new superconducting dome is far away from a magnetically ordered phase without low-energy magnetic fluctuations. Instead, NMR and transmission electron microscopy measurements indicate that a C4 rotation symmetry-breaking structural transition takes place for x> 0.5 above Tc. The electrical resistivity shows a temperature-linear behavior around the doping level where the crystal transition temperature extrapolate to zero and Tc is the maximal, suggesting the importance of quantum fluctuations associated with the structural transition. Our results point to a new paradigm of high temperature superconductivity.

ChemInform Abstract: Spin Fluctuations in Iron Pnictides and Chalcogenides: From Antiferromagnetism to Superconductivity

AbstractReview: 396 refs.

New Superconductivity Dome in LaFeAsO1−xFx Accompanied by Structural Transition**Supported by the Strategic Priority Research Program of the Chinese Academy of Sciences under Grant No XDB07020200, the National Basic Research Program of China under Grant Nos 2012CB821402, 2011CBA00109 and 2011CBA00101, and the National Natural Science Foundation of China under Grant No 11204362.

High-temperature superconductivity is often found in the vicinity of antiferromagnetism. This is also true in LaFeAsO1−xFx (x ≤ 0.2) and many other iron-based superconductors, which leads to proposals that superconductivity is mediated by fluctuations associated with the nearby magnetism. Here we report the discovery of a new superconductivity dome without low-energy magnetic fluctuations in LaFeAsO1−xFx with 0.25 ≤ x ≤ 0.75, where the maximal critical temperature Tc at xopt = 0.5−0.55 is even higher than that at x ≤ 0.2. By nuclear magnetic resonance and transmission electron microscopy, we show that a C4 rotation symmetry-breaking structural transition takes place for x > 0.5 above Tc. Our results point to a new paradigm of high temperature superconductivity.

Spin-orbit transitions inα- andγ−CoV2O6

$\gamma$-triclinic and $\alpha$-monoclinic polymorphs of CoV$_{2}$O$_{6}$ are two of the few known transition metal ion based materials that display stepped $1/3$ magnetization plateaus at low temperatures. Neutron diffraction [M. Markkula et al. Phys. Rev. B 86, 134401 (2012)], x-ray dichroism [N. Hollmann et al. Phys. Rev. B 89, 201101(R) (2014)], and dielectric measurements [K. Singh et al. J. Mater. Chem. 22, 6436 (2012)] have shown a coupling between orbital, magnetic and structural orders in CoV$_{2}$O$_{6}$. We apply neutron inelastic scattering to investigate this coupling by measuring the spin-orbit transitions in both $\alpha$ and $\gamma$ polymorphs. We find the spin-exchange and anisotropy in monoclinic $\alpha$-CoV$_{2}$O$_{6}$ to be weak in comparison with the spin-orbit coupling $\lambda$ and estimate an upper limit of $|J/\lambda| \sim$ 0.05. However, the spin exchange is larger in the triclinic polymorph and we suggest the excitations are predominately two dimensional. The local compression of the octahedra surrounding the Co$^{2+}$ ion results in a direct coupling between higher energy orbital levels, the magnetic ground state, and elastic strain. CoV$_{2}$O$_{6}$ is therefore an example where the local distortion along with the spin-orbit coupling provides a means of intertwining structural and magnetic properties. We finish the paper by investigating the low-energy magnetic fluctuations within the ground state doublet and report a magnetic excitation that is independent of the local crystalline electric field. We characterize the temperature and momentum dependence of these excitations and discuss possible connections to the magnetization plateaus.

Spin fluctuations in iron pnictides and chalcogenides: From antiferromagnetism to superconductivity

The present article reviews recent experimental investigations of spin dynamics in iron-based superconductors and their parent compounds by means of inelastic neutron scattering. It mainly focuses on the most contemporary developments in this field, pertaining to the observations of magnetic resonant modes in new superconductors, spin anisotropy of low-energy magnetic fluctuations that has now been observed in a wide range of chemical compositions and doping levels, as well as their momentum-space anisotropy incurred by the spin-nematic order. The implications of these new findings for our understanding of the superconducting state, along with the remaining unsettled challenges for neutron spectroscopy, are discussed. Cet article fait une revue des expériences de diffusion inélastique des neutrons sur la dynamique de spin dans les supraconducteurs à base de fer et leurs composés parents. Il se concentre sur les développements les plus récents dans le domaine : l'observation des modes de résonance magnétique dans les nouveaux supraconducteurs, l'anisotropie de spin des fluctuations magnétiques de basse énergie qui est maintenant observée dans un large spectre de compositions chimiques et de dopage, ainsi que l'anisotropie dans l'espace réciproque engendrée par l'ordre nématique des spins. Les implications de ces nouvelles découvertes pour notre compréhension de la supraconductivité, ainsi que les défis dans ce domaine pour la spectroscopie de neutrons, sont discutés.

Glassy low-energy spin fluctuations and anisotropy gap in La1.88Sr0.12CuO4

We present high-resolution triple-axis neutron scattering studies of the high-temperature superconductor La1.88Sr0.12CuO4 (Tc=27 K). The temperature dependence of the low-energy incommensurate magnetic fluctuations reveals distinctly glassy features. The glassiness is confirmed by the difference between the ordering temperature TN ~ Tc inferred from elastic neutron scattering and the freezing temperature Tf ~ 11 K obtained from muon spin rotation studies. The magnetic field independence of the observed excitation spectrum as well as the observation of a partial suppression of magnetic spectral weight below 0.75 meV for temperatures smaller than Tf, indicate that the stripe frozen state is capable of supporting a spin anisotropy gap, of a magnitude similar to that observed in the spin and charge stripe ordered ground state of La1.875Ba0.125CuO4. The difference between TN and Tf implies that the significant enhancement in a magnetic field of nominally elastic incommensurate scattering is caused by strictly in-elastic scattering -- at least in the temperature range between Tf and Tc -- which is not resolved in the present experiment. Combining the results obtained from our study of La1.88Sr0.12CuO4 with a critical reappraisal of published neutron scattering work on samples with chemical composition close to p=0.12, where local probes indicate a sharp maximum in Tf(p), we arrive at the view that the low-energy fluctuations are strongly dependent on composition in this regime, with anisotropy gaps dominating only sufficiently close to p=0.12 and superconducting spin gaps dominating elsewhere.

Spin fluctuations and superconductivity in powders of Fe1+xTe0.7Se0.3as a function of interstitial iron concentration

Using neutron inelastic scattering, we investigate the role of interstitial iron on the low-energy spin fluctuations in powder samples of Fe_{1+x}Te_{0.7}Se_{0.3}. We demonstrate how combining the principle of detailed balance along with measurements at several temperatures allows us to subtract both temperature-independent and phonon backgrounds from S(Q,\omega) to obtain purely magnetic scattering. For small values of interstitial iron (x=0.009(3)), the sample is superconducting (T_{c}=14 K) and displays a spin gap of 7 meV peaked in momentum at wave vector q_{0}=(\pi,\pi) consistent with single crystal results. On populating the interstitial iron sites, the superconducting volume fraction decreases and we observe a filling in of the low-energy magnetic fluctuations and a decrease of the characteristic wave vector of the magnetic fluctuations. For large concentrations of interstitial iron (x=0.048(2)) where the superconducting volume fraction is minimal, we observe the presence of gapless spin fluctuations at a wave vector of q_{0}=(\pi,0). We estimate the absolute total moment for the various samples and find that the amount of interstitial iron does not change the total magnetic spectral weight significantly, but rather has the effect of shifting the spectral weight in Q and energy. These results show that the superconducting and magnetic properties can be tuned by doping small amounts of iron and are suggestive that interstitial iron concentration is also a controlling dopant in the Fe_{1+x}Te_{1-y}Se_{y} phase diagram in addition to the Te/Se ratio.

Anomalous dependence ofc-axis polarized FeB1gphonon mode with Fe and Se concentrations in Fe1+yTe1−xSex

We report on an investigation of the lattice dynamical properties in a range of Fe${}_{1+y}$Te${}_{1\ensuremath{-}x}$Se${}_{x}$ compounds, with special emphasis on the $c$-axis polarized vibration of Fe with ${\mathrm{B}}_{1g}$ symmetry, a Raman active mode common to all families of Fe-based superconductors. We have carried out a systematic study of the temperature dependence of this phonon mode as a function of Se $x$ and excess Fe $y$ concentrations. In parent compound Fe${}_{1+y}$Te, we observe an unconventional broadening of the phonon between room temperature and magnetic ordering temperature ${T}_{N}$. The situation smoothly evolves toward a regular anharmonic behavior as Te is substituted for Se and long-range magnetic order is replaced by superconductivity. Irrespective to Se contents, excess Fe is shown to provide an additional damping channel for the ${\mathrm{B}}_{1g}$ phonon at low temperatures. We performed density functional theory ab initio calculations within the local density approximation to calculate the phonon frequencies, including magnetic polarization and Fe nonstoichiometry in the virtual crystal approximation. We obtained a good agreement with the measured phonon frequencies in the Fe-deficient samples, while the effects of Fe excess are poorly reproduced. This may be due to excess Fe-induced local magnetism and low-energy magnetic fluctuations that cannot be treated accurately within these approaches. As recently revealed by neutron scattering and muon spin rotation studies, these phenomena occur in the temperature range where anomalous decay of the ${\mathrm{B}}_{1g}$ phonon is observed and suggests a peculiar coupling of this mode with local moments and spin fluctuations in Fe${}_{1+y}$Te${}_{1\ensuremath{-}x}$Se${}_{x}$.

Persistence of magnons in a site-diluted dimerized frustrated antiferromagnet

We present inelastic neutron scattering and thermodynamic measurements characterizing the magnetic excitations in a disordered spin-liquid antiferromagnet with non-magnetic substitution. The parent compound Ba3Mn2O8 is a dimerized, quasi-two-dimensional geometrically frustrated quantum disordered antiferromagnet. We substitute this compound with non-magnetic V 5+ for the S = 1 Mn5+ ions, Ba3(Mn1−xV x)2O8, and find that the singlet–triplet excitations which dominate the spectrum of the parent compound persist for the full range of substitution examined, up to x = 0.3. We also observe additional low-energy magnetic fluctuations which are enhanced at the greatest substitution values.

Orbital-ordering driven structural distortion in metallicSrCrO3

In contrast to the previous reports that the divalent perovskite ${\text{SrCrO}}_{3}$ was believed to be cubic structure and nonmagnetic metal, recent measurements suggest coexistence of majority tetragonally distorted weak antiferromagnetic phase and minority nonmagnetic cubic phase. Within the local (spin) density approximation [L(S)DA] our calculations confirm that a slightly tetragonally distorted phase indeed is energetically favored. Using the correlated band theory method $(\text{LDA}+\text{Hubbard}\text{ }U)$ as seems to be justified by the unusual behavior observed in ${\text{SrCrO}}_{3}$, above the critical value ${U}_{c}=4\text{ }\text{eV}$ only the distorted phase undergoes an orbital-ordering transition, resulting in ${t}_{2g}^{2}\ensuremath{\rightarrow}{d}_{xy}^{1}{({d}_{xz}{d}_{yz})}^{1}$ corresponding to the filling of the ${d}_{xy}$ orbital but leaving the other two degenerate. The Fermi surfaces of the cubic phase are simple with nesting features, although the nesting wave vectors do not correlate with known data. This is not uncommon in perovskites; the strongly directional $d\ensuremath{-}d$ bonding often leads to boxlike Fermi surfaces and either the nesting is not strong enough or the matrix elements are not large enough to promote instabilities. Fixed spin moment calculations indicate the cubic structure is just beyond a ferromagnetic Stoner instability $[IN(0)\ensuremath{\approx}1.1]$ in L(S)DA and that the energy is unusually weakly dependent on the moment out to $1.5{\ensuremath{\mu}}_{B}/\text{Cr}$ (varying only by 11 meV/Cr), reflecting low-energy long-wavelength magnetic fluctuations. We observe that this system shows strong magnetophonon coupling (change in Cr local moment is $\ensuremath{\sim}7.3{\ensuremath{\mu}}_{B}/\text{\AA{}}$) for breathing phonon modes.

Low-energy magnetic fluctuations in [formula omitted

Inelastic neutron scattering experiments were performed on U ( Ru 1 - x Rh x ) 2 Si 2 with x = 0 and 0.02 in order to investigate the variations of the antiferromagnetic (AF) fluctuations with doping Rh. We have found that the energy of the magnetic excitation observed at Q = ( 1 , 0 , 0 ) in the hidden order (HO) phase markedly decreases from ∼ 2.6 meV ( x = 0 ) to ∼ 0 ( x = 0.02 ) with increasing x . In addition, the staggered susceptibility χ Q estimated from the inelastic peak develops in the HO phase at x = 0.02 , while it is suppressed at x = 0 . These results indicate that the AF fluctuation in the HO phase is significantly enhanced with doping Rh.

Shadow bands at the Fermi surface of Bi2Sr2CaCu2O8+x superconductors: structural or antiferromagnetic origin?

In this paper, we report measurements on the Fermi surface (FS) topology of Bi2Sr2CaCu2O8+x (Bi-2212) superconductors by angle scanning photoemission spectroscopy using polarized synchrotron radiation. Our k-scanning approach allows us to record the full symmetry of the main features of the FS throughout large portions of the reciprocal space, as well as their dependence on the polarization and energy of the incident light. In the recorded two-dimensional images, we have clearly identified in addition to the principal FS and its “umklapps” replicas another bands of much smaller intensity, previously reported as “shadow FS”. The k-dependence of these bands is identical that to the principal FS, but displaced by a wave-vector (π, π). In the present study, we provide a qualitative analysis of this superstructure as well as its relationship with the structural order and the low-energy magnetic fluctuations of these compounds.

Exotic magnetic and superconducting phases in CeCu 2Si 2 close to magnetically ordered state : Cu-NQR study

We report Cu-NQR results under pressure on heavy-fermion (HF) superconductor CeCu 2Si 2 close to a magnetic phase, Ce 0.99Cu 2.02Si 2 and CeCu 2(Si 0.98Ge 0.02) 2. It is found that an application of a minute pressure exceeding 0.3 GPa markedly suppresses low-energy magnetic fluctuations observed at ambient pressure ( P=0) in Ce 0.99Cu 2.02Si 2. Correspondingly, anomalous superconducting (SC) characteristics evolve into conventional HF-SC ones with increasing pressure. In the latter, the Ge substitution works as a negative chemical pressure. CeCu 2(Si 0.98Ge 0.02) 2 reveals a magnetic phase transition at T N=0.75 K followed by the SC phase transition at T c∼0.4 K at P=0. It is suggested experimentally that the magnetic fluctuations seen in Ce 0.99Cu 2.02Si 2 at P=0 possesses a quantum critical character of both SC and magnetically ordered states.

63Cu NMR evidence for enhanced antiferromagnetic correlations around Zn impurities in YBa2Cu3O6.7

Doping the high- T(c) superconductor YBa2Cu3O6.7 with 1.5% of nonmagnetic Zn impurities in CuO2 planes is shown to produce a considerable broadening of 63Cu NMR spectra, as well as an increase of low-energy magnetic fluctuations detected in 63Cu spin-lattice relaxation measurements. A model-independent analysis demonstrates that these effects are due to the development of staggered magnetic moments on many Cu sites around each Zn and that the Zn-induced moment in the bulk susceptibility might be explained by this staggered magnetization. Several implications of these enhanced antiferromagnetic correlations are discussed.