Ferromagnetic Spin Fluctuations Research Articles

Electrical transport under extreme conditions in the spin-ladder antiferromagnetTaFe1.25Te3

We report the electrical transport of the layered, spin-ladder antiferromagnet ${\mathrm{TaFe}}_{1.25}{\mathrm{Te}}_{3}$ (${T}_{N}\ensuremath{\sim}167$ K) under intense magnetic field (up to 60 T) and high pressure (up to 50 GPa). In high magnetic field, ${\mathrm{TaFe}}_{1.25}{\mathrm{Te}}_{3}$ undergoes two metamagnetic transitions, manifested in both the magnetoresistance and the Hall resistivity. Synchrotron x-ray diffraction reveals no obvious structural transition up to $\ensuremath{\sim}50$ GPa. However, the application of a small pressure ($\ensuremath{\sim}0.7$ GPa) fosters a sizable anomalous Hall conductivity and a negative magnetoresistance, possibly arising from the competing ferromagnetic exchange interaction in this antiferromagnet. Both the anomalous Hall conductivity and the negative magnetoresistance culminate at a pressure of $\ensuremath{\sim}4$ GPa. With further increase of pressure, both physical quantities diminish. The Hall coefficient and the magnetoresistance change sign above $\ensuremath{\sim}20$ GPa, presumably due to the cessation of ferromagnetic spin fluctuations in this high pressure regime.

Topological states and competing magnetic fluctuations in iron germanides

The correlated multiorbital multiband electronic structures of iron-based superconductors harbor an abundance of exotic electronic states, such as spin density waves, superconductivity, topological surface states, and Majorana zero modes. In this paper, we carry out density functional theory combined with dynamical mean-field theory calculations of the electronic structures, spin dynamics, and topological properties in $\mathrm{Y}{\mathrm{Fe}}_{2}{\mathrm{Ge}}_{2}$ and MgFeGe and compare them with $\mathrm{Ba}{\mathrm{Fe}}_{2}{\mathrm{As}}_{2}$ and LiFeAs. We find a coexistence of ferromagnetic and antiferromagnetic spin fluctuations in $\mathrm{Y}{\mathrm{Fe}}_{2}{\mathrm{Ge}}_{2}$ and MgFeGe, which are attributed to the weakly dispersive bands just below the Fermi level. Moreover, we demonstrate that MgFeGe is in a strong topological phase and has two sets of Dirac-cone-like topological surface states on the (001) surface. The coexistence of superconductivity and nontrivial band topology in these two compounds demonstrates that it is interesting to explore topological superconductivity and Majorana zero modes in iron germanides.

Possible coexistence of antiferromagnetic and ferromagnetic spin fluctuations in the spin-triplet superconductor UTe2 revealed by Te125 NMR under pressure

A spin-triplet superconducting state mediated by ferromagnetic (FM) spin fluctuations has been suggested to occur in the newly discovered heavy-fermion superconductor ${\mathrm{UTe}}_{2}$. However, the recent neutron scattering measurements revealed the presence of antiferromagnetic (AFM) spin fluctuations in ${\mathrm{UTe}}_{2}$. Here, we report the $^{125}\mathrm{Te}$ nuclear magnetic resonance studies of a single-crystal ${\mathrm{UTe}}_{2}$, suggesting the coexistence of FM and AFM spin fluctuations in ${\mathrm{UTe}}_{2}$. Owing to the two different Te sites in the compound, we conclude that the FM spin fluctuations are dominant within ladders and the AFM spin fluctuations originate from the interladder magnetic coupling. Although AFM spin fluctuations exist in the system, the FM spin fluctuations in the ladders may play an important role in the appearance of the spin-triplet superconducting state of ${\mathrm{UTe}}_{2}$.

Monte Carlo study of the pseudogap and superconductivity emerging from quantum magnetic fluctuations

The origin of the pseudogap behavior, found in many high-Tc superconductors, remains one of the greatest puzzles in condensed matter physics. One possible mechanism is fermionic incoherence, which near a quantum critical point allows pair formation but suppresses superconductivity. Employing quantum Monte Carlo simulations of a model of itinerant fermions coupled to ferromagnetic spin fluctuations, represented by a quantum rotor, we report numerical evidence of pseudogap behavior, emerging from pairing fluctuations in a quantum-critical non-Fermi liquid. Specifically, we observe enhanced pairing fluctuations and a partial gap opening in the fermionic spectrum. However, the system remains non-superconducting until reaching a much lower temperature. In the pseudogap regime the system displays a “gap-filling" rather than “gap-closing" behavior, similar to the one observed in cuprate superconductors. Our results present direct evidence of the pseudogap state, driven by superconducting fluctuations.

Electronic structure and magnetism of the Hund's insulator CrI3

${\mathrm{CrI}}_{3}$ is a two-dimensional ferromagnetic (FM) van der Waals material with a charge gap of 1.1--1.2 eV. In this paper, the electronic structure and magnetism of ${\mathrm{CrI}}_{3}$ are investigated by using density functional theory and dynamical mean-field theory. Our calculations successfully reproduce a charge gap of $\ensuremath{\sim}1.1\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ in the paramagnetic (PM) state when a Hund's coupling ${J}_{\mathrm{H}}=0.7\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ is included with an onsite Hubbard $U=5\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$. In contrast, with a large $U$ value of 8 eV and negligible Hund's coupling ${J}_{\mathrm{H}}, {\mathrm{CrI}}_{3}$ is predicted to be a moderately correlated metal in the PM state. We conclude that ${\mathrm{CrI}}_{3}$ is a Mott-Hund's insulator due to the half-filled configuration of the Cr $3d {t}_{2g}$ orbitals. The Cr $3d {e}_{g}$ orbitals are occupied by $\ensuremath{\sim}1$ electron, which leads to strong valence fluctuations so that the Cr $3d$ orbitals cannot be described by a single state. Moreover, at finite temperature, the calculated ordered static magnetic moment in the FM state is significantly larger in the $R\overline{3}$ phase than in the $C2/m$ phase. This observation indicates that the structural phase transition from the $C2/m$ phase to the $R\overline{3}$ phase with decreasing temperature is driven by FM spin fluctuations.

Spin-triplet superconductivity driven by finite-momentum spin fluctuations

A small number of superconductors are believed to exhibit intrinsic spin-triplet pairing, and they are often discussed in terms of a simple, $^{3}\mathrm{He}$-like picture where ferromagnetic spin fluctuations provide the ``glue.'' However, in some cases in which reliable inelastic neutron scattering measurements are available, spin excitations are found to be peaked at finite momentum $\mathbf{q}$ rather than $\mathbf{q}=\mathbit{0}$. Here we investigate some simple models that exhibit triplet pairing arising from antiferromagnetic spin fluctuations. We show that a strong peak at larger $\mathbf{q}$ in the magnetic susceptibility can drive such states and can give rise to pairing states with nodes in the ${k}_{z}$ plane even in the presence of a pure two-dimensional Fermi surface. In these situations, dominant pair scattering processes occur between Fermi surface segments with like signs of the superconducting order parameter, yet they are consistent with an overall odd-parity state. We examine the applicability of these scenarios to the putative triplet superconductor $\mathrm{U}{\mathrm{Te}}_{2}$.

Spin-triplet superconductivity in K 2 Cr 3 As 3

A spin-triplet superconductor with ferromagnetic spin fluctuation superconducts at a critical temperature of 6.5 K.

Suppression of antiferromagnetic order and strong ferromagnetic spin fluctuations in Ca(Co1−xNix)2−yAs2 single crystals

${\mathrm{CaCo}}_{2\ensuremath{-}y}{\mathrm{As}}_{2}$ is a unique itinerant system having strong magnetic frustration. Here, we report the effect of electron doping on the physical properties resulting from Ni substitutions for Co. The single crystals of $\mathrm{Ca}{({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2\ensuremath{-}y}{\mathrm{As}}_{2}$ were characterized by single-crystal x-ray diffraction, energy-dispersive x-ray spectroscopy, magnetization $M$ versus temperature $T$, magnetic field $H$, time $t$, and heat capacity ${C}_{\mathrm{p}}(H,T)$ measurements. The A-type antiferromagnetic (AFM) transition temperature ${T}_{\mathrm{N}}=52$ K for $x=0$ decreases to 22 K with only 3% Ni substitution and is completely suppressed for $x>0.16$. For $0.11\ensuremath{\le}x\ensuremath{\le}0.52$ strong ferromagnetic (FM) fluctuations develop as revealed by magnetic susceptibility $\ensuremath{\chi}(T)=M(T)/H$ measurements. For $x=0.11$ and 0.16 competing AFM and FM interactions result in a reentrant spin-glass behavior below ${T}_{\mathrm{N}}$, as evidenced by the observations of thermomagnetic hysteresis and magnetic relaxation. Enhanced FM fluctuations are also found for the $x=0.21$ and 0.31 crystals, where ${\ensuremath{\chi}}_{c}$ increases significantly at low $T$. A large $\ensuremath{\chi}$ anisotropy in these compositions where ${\ensuremath{\chi}}_{c}$ is up to a factor of two larger than ${\ensuremath{\chi}}_{ab}$ suggests that the FM spin fluctuations are quasi-1D in nature. Weak ferromagnetic contributions to the magnetization are found at $T=2$ K for $x=0.11$--0.31. Heat-capacity ${C}_{\mathrm{p}}(T)$ measurements reveal the presence of FM quantum spin fluctuations for $0.11\ensuremath{\le}x\ensuremath{\le}0.52$, where a logarithmic $T$ dependence of ${C}_{\mathrm{p}}(T)/T$ is observed at low $T$. The suppression of AFM order by the development of strong FM fluctuations in $\mathrm{Ca}{({\mathrm{Co}}_{1\ensuremath{-}x}{\mathrm{Ni}}_{x})}_{2\ensuremath{-}y}{\mathrm{As}}_{2}$ crystals suggests the presence of a FM quantum-critical point at $x\ensuremath{\approx}0.20$. Our density-functional theory (DFT) calculations confirm that FM fluctuations are enhanced by Ni substitutions for Co in ${\mathrm{CaCo}}_{2\ensuremath{-}y}{\mathrm{As}}_{2}$. The Sommerfeld electronic heat-capacity coefficient is enhanced for $x=0$, 0.21, and 0.42 by about a factor of two compared to DFT calculations of the density of states (DOS) at the Fermi energy, suggesting an enhancement of the DOS from electron-phonon and/or electron-electron interactions. The crystals with $x>0.52$ do not exhibit FM spin fluctuations or magnetic order at $T\ensuremath{\ge}1.8$ K, which was found from the DFT calculations to arise from a Stoner transition. Superconductivity is not observed above 1.8 K for any of the compositions. Neutron-diffraction studies of crystals with $x=0.11$ and 0.16 in the crossover regime $(0.1\ensuremath{\lesssim}x\ensuremath{\lesssim}0.2)$ show no evidence of A-type ordering as observed in the parent compound with $x=0$. Furthermore, no other common magnetic structures, such as ferromagnetic (FM), helical stacking of in-plane FM layers, or in-plane AFM structure, are found with an ordered moment greater than the uncertainty of $0.05\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}$ per transition-metal atom.

Metal-insulator transition in Cr-doped hollandite vanadate K2V8O16

The hollandite K2V8O16 is a metal with ferromagnetic spin fluctuations in the high temperature (tetragonal I4/m) phase and encounters metal-insulator transition (MIT) at 170 K. The mechanism behind the MIT is still controversial due to inadequate information on the electronic structure. Furthermore, substitution of Rb for K or Ti for V increases the transition temperature. We have investigated whether break in the mirror symmetry is responsible for the insulating ground state of K2V8O16 and since the Cr counterpart is a robust ferromagnet, whether Cr doping which leads to break in mirror symmetry can help in driving a gap. We find that both 25% and 75% Cr doping break the mirror symmetry but only the former leads to an insulating ground state. Ferromagnetism (FM) remains intact in the insulating phase of K2V6Cr2O16. The structural, electronic and magnetic properties of pure and doped K2V8O16 were investigated within first-principles calculations using density functional theory (DFT). Electron correlation suppresses orbital fluctuations between the partially occupied Cr and V-3dt2g states. Consequently, transfer of charge (electron) from V-3d to Cr-3dt2g states is observed which facilitates Cr3+-V4+ charge ordering. Furthermore, Peierls like structural distortion is associated to the breaking of mirror symmetry in the Cr-V rectangular four chain columns within the crystal. Therefore, the simultaneous effect of Peierls instability, charge ordering and Coulomb correlation is responsible for the MIT in K2V6Cr2O16. Besides, Cr-3d O-2p hybridization and Cr-O coupling increases with Cr doping. These two effects are cooperatively responsible for the observed FM in insulating K2V6Cr2O16. More interestingly, the strength of FM is augmented with Cr concentration in K2V8O16.

Intrachain collinear magnetism and interchain magnetic phases in Cr3As3−K -based materials

We perform a comparative study of the KCr3As3 and the K2Cr3As3 quasi 1D compounds, and show that the strong interplay between the lattice and the spin degrees of freedom promotes a new collinear ferrimagnetic ground state within the chains in presence of intrachain antiferromagnetic couplings. We propose that the interchain antiferromagnetic coupling in KCr3As3 plays a crucial role for the experimentally observed spin-glass phase with low critical temperature. In the same region of the parameter space, we predict K2Cr3As3 to be non-magnetic but on the verge of the magnetism, sustaining interchain ferromagnetic spin fluctuations while the intrachain spin fluctuations are antiferromagnetic.

On Sharp Enhancement of Effective Mass of Quasiparticles and Coefficient of T2 Term of Resistivity around First-Order Metamagnetic Transition Observed in UTe2

The mechanism underlying the enhancement of the Sommerfeld coefficient of quasiparticles at the first-order metamagnetic transition in UTe_2, reported by Miyake et al. in J. Phys. Soc. Jpn. 88, 063706 (2019), is discussed theoretically by taking into account the ferromagnetic order-parameter fluctuations on the basis of the Landau theory of phase transition. We find that the enhanced ferromagnetic spin fluctuation gives rise to the enhancement of the effective mass of the quasiparticles or the Sommerfeld coefficient \gamma,which is consistent with the experimental observations. At the same time, the Kadowaki-Woods type scaling around the metamagnetic transition, reported by Imajo et al.in J. Phys. Soc. Jpn. 88, 083705 (2019) and Knafo et al. in J. Phys. Soc. Jpn. 88, 063705 (2019), is also understood semi-quantitatively by assuming reasonable values of parameters of Landau-type free energy reproducing key quantities characterizing the metamagnetic transition.

Pairing symmetry in monolayer of orthorhombic CoSb

Ferromagnetism and superconductivity are generally considered to be antagonistic phenomena in condensed matter physics. Here, we theoretically study the interplay between the ferromagnetic and superconducting orders in a recent discovered monolayered CoSb superconductor with an orthorhombic symmetry and net magnetization, and demonstrate the pairing symmetry of CoSb as a candidate of non-unitary superconductor with time-reversal symmetry breaking. By performing the group theory analysis and the first-principles calculations, the superconducting order parameter is suggested to be a triplet pairing with the irreducible representation of 3B2u, which displays intriguing nodal points and non-zero periodic modulation of Cooper pair spin polarization on the Fermi surface topologies. These findings not only provide a significant theoretical insight into the coexistence of superconductivity and ferromagnetism, but also reveal the exotic spin polarized Cooper pairing driven by ferromagnetic spin fluctuations in a triplet superconductor.

Magnetic order and fluctuations in the quasi-two-dimensional planar magnet .

We use neutron scattering to investigate spin excitations in , which has a -axis incommensurate helical structure of the two-dimensional (2D) in-plane ferromagnetic (FM) ordered layers for . By comparing the wave vector and energy dependent spin excitations in helical ordered and paramagnetic , we find that Ni doping, while increasing lattice disorder in , enhances quasi-2D FM spin fluctuations. However, our band structure calculations within the combined density functional theory and dynamic mean field theory (DFT+DMFT) failed to generate a correct incommensurate wave vector for the observed helical order from nested Fermi surfaces. Since transport measurements reveal increased in-plane and -axis electrical resistivity with increasing Ni doping and associated lattice disorder, we conclude that the helical magnetic order in may arise from a quantum order-by-disorder mechanism through the itinerant electron mediated Ruderman-Kittel-Kasuya-Yosida (RKKY) interactions.

Incommensurate Spin Fluctuations in the Spin-Triplet Superconductor Candidate UTe_{2}.

Spin-triplet superconductors are of extensive current interest because they can host topological state and Majorana fermions important for quantum computation. The uranium-based heavy-fermion superconductor UTe_{2} has been argued as a spin-triplet superconductor similar to UGe_{2}, URhGe, and UCoGe, where the superconducting phase is near (or coexists with) a ferromagnetic (FM) instability and spin-triplet electron pairing is driven by FM spin fluctuations. Here we use neutron scattering to show that, although UTe_{2} exhibits no static magnetic order down to 0.3K, its magnetism in the [0,K,L] plane is dominated by incommensurate spin fluctuations near an antiferromagnetic ordering wave vector and extends to at least 2.6meV. We are able to understand the dominant incommensurate spin fluctuations of UTe_{2} in terms of its electronic structure calculated using a combined density-functional and dynamic mean-field theory.

Anomalous magnetoresistance in centrosymmetric skyrmion-lattice magnet Gd2PdSi3

We performed a systematic study of the temperature- and field-dependence of magnetization and resistivity of Gd2PdSi3, which is a centrosymmetric skyrmion crystal. We show that a phase diagram can be constructed based on the anomalous magnetoresistance with one-to-one correspondence among all the feature, while the DC magnetization behavior is consistent with the reported phase diagram based on AC susceptibility. In addition, the crossover boundary from a paramagnetic state into the field-induced ferromagnetic state is also identified. Our results suggest that the ferromagnetic spin fluctuations above the Néel temperature play a key role in the high sensitivity of the resistivity anomalies to magnetic field, pointing to the rich interplay of different magnetic correlations at zero and finite wave vectors underlying the skyrmion lattice in this frustrated itinerant magnet.

Evidence for ferromagnetic order in the CoSb layer ofLaCoSb2

The emergence of unconventional superconductivity is generally considered to be related to spin fluctuations. Unveiling the intriguing behaviors of spin fluctuations in parent compounds with layered transition-metal ions may shed light on the search for exotic unconventional superconductors. Here, based on the framework of the first-principles calculations, we theoretically propose that LaCoSb$_2$ is a weak antiferromagnetic layered metal with an in-plane ferromagnetic moment of 0.88 $\mu_B$ at the Co sites, as a candidate parent compound of the cobalt-based superconductors. Importantly, this theoretical finding is experimentally supported by our magnetization measurements on polycrystalline samples of LaCo$_{0.78}$Sb$_2$. Following the symmetry analysis, we suggest a possible $p$-wave superconductivity hosted in doped LaCoSb$_2$ emerging at the verge of ferromagnetic spin fluctuations, which implies potential applications in topological quantum computing in future.

Chiral superconductivity in heavy-fermion metal UTe2.

Spin-triplet superconductors are condensates of electron pairs with spin 1 and an odd-parity wavefunction1. An interesting manifestation of triplet pairing is the chiral p-wave state, which is topologically non-trivial and provides a natural platform for realizing Majorana edge modes2,3. However, triplet pairing is rare in solid-state systems and has not been unambiguously identified in any bulk compound so far. Given that pairing is usually mediated by ferromagnetic spin fluctuations, uranium-based heavy-fermion systems containing f-electron elements, which can harbour both strong correlations and magnetism, are considered ideal candidates for realizing spin-triplet superconductivity4. Here we present scanning tunnelling microscopy studies of the recently discovered heavy-fermion superconductor UTe2, which has a superconducting transition temperature of 1.6 kelvin5. We find signatures of coexisting Kondo effect and superconductivity that show competing spatial modulations within one unit cell. Scanning tunnelling spectroscopy at step edges reveals signatures of chiral in-gap states, which have been predicted to exist at the boundaries of topological superconductors. Combined with existing data that indicate triplet pairing in UTe2, the presence of chiral states suggests that UTe2 is a strong candidate for chiral-triplet topological superconductivity.

Competing magnetic correlations across the ferromagnetic quantum critical point in the Kondo system CeTi1−xVxGe3 : V51 NMR as a local probe

$^{51}$V nuclear magnetic resonance (NMR) and magnetization studies on CeTi$_{1-x}$V$_x$Ge$_3$ have been performed to explore the evolution from the ferromagnetic ($x = 0.113$) to the antiferromagnetic Kondo lattice state ($x = 1$), with focus on the emergence of a possible ferromagnetic quantum critical point (FMQCP) at $x_c \approx 0.4$. From the temperature dependence of the nuclear spin-lattice relaxation rate, $1/T_1T$, and the Knight shift, \textit{K}, for $x=0.113$ and $x=1$ a considerable competition between ferro- and antiferromagnetic correlations is found. Around the critical concentration ($x = 0.35, 0.405$) quantum-critical spin fluctuations entail weak antiferromagnetic spin fluctuations admixed with ferromagnetic spin fluctuations. The FMQCP in CeTi$_{1-x}$V$_x$Ge$_3$ therefore is not purely ferromagnetic in nature.

Lifshitz transition and nontrivial H-doping effect in the Cr-based superconductor KCr3As3Hx

We report the first-principles study on the H-intercalated Cr-based superconductor KCr$_3$As$_3$H$_x$. Our results show a paramagnetic ground state for KCr$_3$As$_3$H. The electronic structure consists of two quasi-one-dimensional (Q1D) Fermi-surfaces and one 3D Fermi-surface which are mainly contributed by Cr-d$_{z^2}$, d$_{x^2-y^2}$ and d$_{xy}$ orbitals. The bare electron susceptibility shows a $\Gamma$-centered imaginary peak, indicating possible ferromagnetic spin fluctuations. Upon moderate hole doping, the system undergoes a Lifshitz transition, which may enhance the Q1D feature of the system. The Bader charge analysis and electron localization functions reveal a strong bonding nature of hydrogen in KCr$_3$As$_3$H, which results in a nontrivial electron doping in KCr$_3$As$_3$H.

Suppression of ferromagnetic spin fluctuations in the filled skutterudite superconductor SrOs4As12 revealed by As75 NMR-NQR measurements

Motivated by the recent observation of ferromagnetic spin correlations in the filled skutterudite SrFe$_4$As$_{12}$ [Ding et al., Phys. Rev. B 98, 155149 (2018)], we have carried out $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the role of magnetic fluctuations in a newly discovered isostructural superconductor SrOs$_4$As$_{12}$ with a superconducting transition temperature of $T_{\rm c}$ $\sim$ 4.8 K. Knight shift $K$ determined by the NQR spectrum under a small magnetic field ($\le$ 0.5 T) is nearly independent of temperature, consistent with the temperature dependence of the magnetic susceptibility. The nuclear spin-lattice relaxation rate divided by temperature, 1/$T_1T$, is nearly independent of temperature above $\sim$ 50 K and increases slightly with decreasing temperature below the temperature. The temperature dependence is reasonably explained by a simple model where a flat band structure with a small ledge near the Fermi energy is assumed. By comparing the present NMR data with those in SrFe$_4$As$_{12}$, we found that the values of $|K|$ and $1/T_1T$ in SrOs$_4$As$_{12}$ are smaller than those in SrFe$_4$As$_{12}$, indicating no obvious ferromagnetic spin correlations in SrOs$_4$As$_{12}$. From the temperature dependence of 1/$T_1$ in the superconducting state, an $s$-wave superconductivity is realized.