Fermi Surface Nesting Research Articles

Charge correlations suppress unconventional pairing in the Holstein model

In a recent work by Schrodi $\textit{et al}$. [Phys. Rev. B. $\textbf{104}$, L140506 (2021)], the authors find an unconventional superconducting state with a sign-changing order parameter using the Migdal-Eliashberg theory, including the first vertex correction. This unconventional solution arises despite using an isotropic bare electron-phonon coupling in the Hamiltonian. We examine this claim using hybrid quantum Monte Carlo for a single-band Holstein model with a cuprate-like noninteracting band structure and identical parameters to Schrodi $\textit{et al}$.. Our Monte Carlo results for these parameters suggest that unconventional pairing correlations do not exceed their noninteracting values at any carrier concentration we have checked. Instead, strong charge-density-wave correlations persist at the lowest accessible temperatures for dilute and nearly half-filled bands. Lastly, we present arguments for how vertex-corrected Migdal-Eliashberg calculation schemes can lead to uncontrolled results in the presence of Fermi surface nesting.

Hole doping dependent electronic instability and electron-phonon coupling in infinite-layer nickelates

Recently, charge density waves (CDWs) have been observed in $\mathrm{CaCu}{\mathrm{O}}_{2}\text{\ensuremath{-}}\mathrm{analogous}$ infinite-layer nickelates $R\mathrm{Ni}{\mathrm{O}}_{2}$ ($R=\mathrm{La}$, Nd) but exhibit very different hole doping dependent behaviors compared to that in cuprates, raising a challenging question on its origin. In this paper, employing density functional theory, many-body dynamic mean field theory, and determinant quantum Monte Carlo calculations, we propose a synergetic contribution from both electronic instability (EI) and moment-dependent electron-phonon coupling (MEPC). Unexpectedly, the EI and MEPC are mainly contributed by Ni $3{d}_{x2\ensuremath{-}y2}$ and $R 5{d}_{z2}$, highlighting the unique multiorbital feature. Interestingly, a strong Fermi surface nesting (FSN) induced by the unique feature of van Hove singularity (VHS) across the Fermi level exists in $R\mathrm{Ni}{\mathrm{O}}_{2}$, which is sensitive to hole doping. The hole doping can rapidly reduce FSN of Ni $3{d}_{x2\ensuremath{-}y2}$ by shifting VHS and decrease the occupation of $R 5{d}_{z2}$, which can largely weaken EI and MEPC in $R\mathrm{Ni}{\mathrm{O}}_{2}$. Remarkably, the temperature-insensitive feature of EI and MEPC could be a hint for rather high-temperature CDWs observed in undoped $R\mathrm{Ni}{\mathrm{O}}_{2}$. Our theory may offer one possible explanation to the experimentally observed CDW formation and its hole-doping dependence in nickelates, and also establishes a unified understanding of the hole doping dependent EI and MEPC in nickelates and cuprates.

Density-wave tendency from a topological nodal-line perspective

The understanding of density waves is a vital component of our insight into electronic quantum matters. Here, we propose an additional mosaic to the existing mechanisms such as Fermi-surface nesting, electron–phonon coupling, and exciton condensation. In particular, we find that certain two-dimensional (2D) spin density-wave systems are equivalent to three-dimensional (3D) Dirac nodal-line systems in the presence of a magnetic field, whose electronic structure takes the form of Dirac-fermion Landau levels and allows a straightforward analysis of its optimal filling. The subsequent minimum-energy wave vector varies over a continuous range and shows no direct connection to the original Fermi surfaces in 2D. Also, we carry out numerical calculations where the results on model examples support our theory. Our study points out that we have yet to attain a complete understanding of the emergent density wave formalism.

Novel Ultrahard Extended Hexagonal C10, C14 and C18 Allotropes with Mixed sp2/sp3 Hybridizations: Crystal Chemistry and Ab Initio Investigations

Based on 4H, 6H and 8H diamond polytypes, novel extended lattice allotropes C10, C14 and C18 characterized by mixed sp3/sp2 carbon hybridizations were devised based on crystal chemistry rationale and first-principles calculations of the ground state structures and energy derived properties: mechanical, dynamic (phonons), and electronic band structure. The novel allotropes were found increasingly cohesive along the series, with cohesive energy values approaching those of diamond polytypes. Regarding mechanical properties, C10, C14, and C18 were found ultrahard with Vickers hardness slightly below that of diamond. All of them are dynamically stable, with positive phonon frequencies reaching maxima higher than in diamond due to the stretching modes of C=C=C linear units. The electronic band structures expectedly reveal the insulating character of all three diamond polytypes and the conductive character of the hybrid allotropes. From the analysis of the bands crossing the Fermi level, a nesting Fermi surface was identified, allowing us to predict potential superconductive properties.

Evolution of static charge density wave order, amplitude mode dynamics, and suppression of Kohn anomalies at the hysteretic transition in EuTe4

Charge density wave (CDW) induces periodic spatial modulation of the charge density that is commensurate or incommensurate with the host lattice periodicity, and leads to partial or complete electronic band-gap opening at the Fermi level (${E}_{\mathrm{F}}$). The recent finding of unconventional hysteresis within the CDW phase of ${\mathrm{EuTe}}_{4}$, not observable in other rare-earth tellurides $R{\mathrm{Te}}_{n}$ ($n=2$, 3), has highlighted the role of the relative phase of CDW distortion in weakly coupled Te layers. However, detailed structural and dynamical characterization of CDW distortion on the hysteretic transition is lacking. Here we report on the static CDW order, dynamics of the amplitude mode, and their evolution on the hysteretic transition using meV resolution elastic and inelastic x-ray scattering. We discover previously unidentified multiple commensurate and incommensurate CDW wave vectors ${\mathbf{q}}_{\mathrm{CDW}}$ along all three crystallographic axes. Importantly, we find that the previously reported $b$-axis CDW peak is coupled with the interlayer CDW phase and consequently co-occurs with the doubling of the unit cell along the $c$ axis. We confirm the presence of the competing $a$-axis CDW order but found it to be four orders of magnitude weaker than the $b$-axis CDW. Furthermore, we observe multiple Kohn anomalies at ${\mathbf{q}}_{\mathrm{CDW}}$ driven by Fermi surface nesting and hidden nesting, confirming earlier reports based on electronic and lattice susceptibility simulations. The amplitude mode and Kohn anomalies are found to suppress on unconventional hysteretic transition, suggesting the presence of nondegenerate metastable states, which we identify from the x-ray scattering measurements and simulations.

Orbital-Hybridization-Driven Charge Density Wave Transition in CsV3 Sb5 Kagome Superconductor.

Owing to its inherent non-trivial geometry, the unique structural motif of the recently discovered kagome topological superconductor AV3 Sb5 (A = K, Rb, Cs) is an ideal host of diverse topologically non-trivial phenomena, including giant anomalous Hall conductivity, topological charge order, charge density wave (CDW), and unconventional superconductivity. Despite possessing a normal-state CDW order in the form of topological chiral charge order and diverse superconducting gaps structures, it remains unclear how fundamental atomic-level properties and many-body effects including Fermi surface nesting, electron-phonon coupling, and orbital hybridization contribute to these symmetry-breaking phenomena. Here, the direct participation of the V3d-Sb5p orbital hybridization in mediating the CDW phase transition in CsV3 Sb5 is reported. The combination of temperature-dependent X-ray absorption and first-principles studies clearly indicates the inverse Star-of-David structure as the preferred reconstruction in the low-temperature CDW phase. The results highlight the critical role that Sb orbitals play and establish orbital hybridization as the direct mediator of the CDW states and structural transition dynamics in kagome unconventional superconductors. This is a significant step toward the fundamental understanding and control of the emerging correlated phases from the kagome lattice through the orbital interactions and provides promising approaches to novel regimes in unconventional orders and topology.

Metavalent Bonding Origins of Unusual Properties of Group IV Chalcogenides.

A distinct type of metavalent bonding (MVB) is recently proposed to explain an unusual combination of anomalous functional properties of group IV chalcogenide crystals, whose electronic mechanisms and origin remain controversial. Through theoretical analysis of evolution of bonding along continuous paths in structural and chemical composition space, emergence of MVB in rocksalt chalcogenides is demonstrated as a consequence of weakly broken symmetry of parent simple-cubic crystals of Group V metalloids. High electronic degeneracy at the nested Fermi surface of parent metal drives spontaneous breaking of its translational symmetry with structural and chemical fields, which open up a small energy gap and mediate strong coupling between conduction and valence bands making metavalent crystals highly polarizable, conductive, and sensitive to bond-lengths. Stronger symmetry-breaking structural and chemical fields, however, transform them discontinuously to covalent and ionic semiconducting states. MVB involves bonding-antibonding pairwise interactions alternating along linear chains of at least five atoms, which facilitate long-range electron transfer in response to polar fields causing unusual properties. The precise picture of MVB predicts anomalous second-order Raman scattering as an addition to set off their unusual properties, and will guide in design of new metavalent materials with improved thermoelectric, ferroelectric and nontrivial electronic topological properties.

Soft-Phonon and Charge-Density-Wave Formation in Nematic BaNi2As2

We use diffuse and inelastic x-ray scattering to study the formation of an incommensurate charge-density-wave (I-CDW) in BaNi_{2}As_{2}, a candidate system for charge-driven electronic nematicity. Intense diffuse scattering is observed around the modulation vector of the I-CDW, Q_{I-CDW}. It is already visible at room temperature and collapses into superstructure reflections in the long-range ordered state where a small orthorhombic distortion occurs. A clear dip in the dispersion of a low-energy transverse optical phonon mode is observed around Q_{I-CDW}. The phonon continuously softens upon cooling, ultimately driving the transition to the I-CDW state. The transverse character of the soft-phonon branch elucidates the complex pattern of the I-CDW satellites observed in the current and earlier studies and settles the debated unidirectional nature of the I-CDW. The phonon instability and its reciprocal space position are well captured by our abinitio calculations. These, however, indicate that neither Fermi surface nesting, nor enhanced momentum-dependent electron-phonon coupling can account for the I-CDW formation, demonstrating its unconventional nature.

Recent Progress in Phase Stability and Elastic Anomalies of Group VB Transition Metals

Recently discovered phase transition and elastic anomaly of compression-induced softening and heating-induced hardening (CISHIH) in group VB transition metals at high-pressure and high-temperature (HPHT) conditions are unique and interesting among typical metals. This article reviews recent progress in the understanding of the structural and elastic properties of these important metals under HPHT conditions. Previous investigations unveiled the close connection of the remarkable structural stability and elastic anomalies to the Fermi surface nesting (FSN), Jahn–Teller effect, and electronic topological transition (ETT) in vanadium, niobium, and tantalum. We elaborate that two competing scenarios are emerging from these advancements. The first one focuses on phase transition and phase diagram, in which a soft-mode driven structural transformation of BCC→RH1→RH2→BCC under compression and an RH→BCC reverse transition under heating in vanadium were established by experiments and theories. Similar phase transitions in niobium and tantalum were also proposed. The concomitant elastic anomalies were considered to be due to the phase transition. However, we also showed that there exist some experimental and theoretical facts that are incompatible with this scenario. A second scenario is required to accomplish a physically consistent interpretation. In this alternative scenario, the electronic structure and associated elastic anomaly are fundamental, whereas phase transition is just an outcome of the mechanical instability. We note that this second scenario is promising to reconcile all known discrepancies but caution that the phase transition in group VB metals is elusive and is still an open question. A general consensus on the relationship between the possible phase transitions and the mechanical elasticity (especially the resultant CISHIH dual anomaly, which has a much wider impact), is still unreached.

The carriers doping effect on electronic and optical behaviors of newly layered Sr1- xHfxFBiS2 alloying materials for light-modulator devices

BiS2-layered superconductors have captivated the colossal attention of the scientific community owing to their structural analogies to other unconventional superconducting materials, termed cuprates and Fe-based superconducting materials. Herein, we inspect how the physical characteristics will be altered in the SrFBiS2 mother compound upon the compositional doping of the hafnium element. The electronic structure characteristics of Sr1−xHfxFBiS2 (x = 0 to 1) are simulated with the virtue of the full-potential linearized augmented plane-wave technique. Accordingly, the evaluated band gap of the parent compound (SrFBiS2) is roughly about 0.88 eV, which vanishes under the substitutional impurity impact of tetravalent (Hf+4). However, a metallic character occurs when Hf is substituted in the SrFBiS2 mother compound, although the bottom conduction band is relocated downward to the Fermi level with the emergence of a viable superconducting state. The band structures and Fermi surface (FS) topology vary obviously with the augmentation of Hf concentration and FS nesting is acquired at the wave vector (π, π, 0). Moreover, the optical characteristics of the explored compounds are probed, while the optical anisotropy in the optical absorption spectra is well marked in the mother material and diminishes through the substitutional Hf impurity effect. The metallic character with Drude-model arises when the incident photon energies span from zero photon energy to infrared spectrum when Hf substitutes Sr in the SrFBiS2 parent compound. The spin orbit coupling (SOC) significantly influences the band structures, Fermi surface topologies, and optical features of these doped systems based on the heavy elements, like Hf or Bi. Hence, the induced nesting FS outcomes may turn Sr1−xHfxFBiS2 into striking materials for newly layered superconductors under the doping effect of hafnium element at low compositional content. Eventually, it is inferred that the SrFBiS2 parent compound would be a potential contender for new superconductivity and may be useful for optical communications and laser devices.

Charge filling induced quasi-two dimensional charge/spin density wave channel in a ballistic transport device

Charge filling controlled mean field metal–insulator phase transition is examined in the context of two dimensional Fermi surface nesting and van Hove singularity induced charge density wave (CDW), spin density wave (SDW) condensates. In the framework of a coherent ballistic transport model utilizing the Non-Equilibrium Green Function approach (NEGF), a three terminal device with metallic gate, source, drain and CDW/SDW channel is simulated and studied. Within the validity of mean field approximation, we exposit the commensurability and boundary effects. The efficacy of the Hubbard model for (quasi) two dimensional Charge and Spin Density Wave materials is discussed. A two orbital generalization of the effective Hamiltonian is proposed for transport calculations in rare earth Tellurides RTe 3.

Interplay between Pair Density Wave and a Nested Fermi Surface.

We show that spontaneous time-reversal-symmetry (TRS) breaking can naturally arise from the interplay between pair density wave (PDW) ordering at multiple momenta and nesting of Fermi surfaces (FSs). Concretely, we consider the PDW superconductivity on a hexagonal lattice with nested FS at 3/4 electron filling, which is related to a recently discovered superconductor CsV_{3}Sb_{5}. Because of nesting of the FSs, each momentum k on the FS has at least two counterparts -k±Q_{α} (α=1, 2, 3) on the FS to form finite momentum (±Q_{α}) Cooper pairs, resulting in a TRS and inversion broken PDW state with stable Bogoliubov Fermi pockets. Various spectra, including (local) density of states, electron spectral function, and the effect of quasiparticle interference, have been investigated. The partial melting of the PDW will give rise to 4×4 and (4/sqrt[3])×(4/sqrt[3]) charge density wave (CDW) orders, in addition to the 2×2 CDW. Possible implications to real materials such as CsV_{3}Sb_{5} and future experiments have been discussed further.

Linear-in-frequency optical conductivity over a broad range in the three-dimensional Dirac semimetal candidate Ir2In8Se

The optical conductivity of the new Dirac semimetal candidate ${\mathrm{Ir}}_{2}{\mathrm{In}}_{8}\mathrm{Se}$ is measured in a frequency range from 40 to 30 000 ${\mathrm{cm}}^{\ensuremath{-}1}$ at temperatures from 300 K down to 10 K. The measurement reveals that the compound is a low carrier density metal. We find that the real part of the conductivity ${\ensuremath{\sigma}}_{1}(\ensuremath{\omega})$ is linear in frequency over a broad range from 500 to 4000 ${\mathrm{cm}}^{\ensuremath{-}1}$ at 300 K and varies slightly with cooling. This linearity strongly suggests the presence of three-dimensional linear electronic bands with band crossings near the Fermi level. Band-structure calculations indicate the presence of type-II Dirac points. By comparing our data with the optical conductivity computed from the band structure, we conclude that the observed linear dependence mainly originates from the Dirac cones and the transition between the Dirac cones and the next lower bands. In addition, a weak energy gap feature is resolved below the charge density wave phase transition temperature in the reflectivity spectra. An enhanced structure arising from imperfect Fermi surface nesting is identified in the electronic susceptibility function, suggesting a Fermi surface nesting driven instability.

Direct observation of multiband charge density waves in NbTe2

The electronic structure of periodic lattice distortion (PLD) in ${\mathrm{NbTe}}_{2}$ was examined using low-temperature scanning tunneling spectroscopy and microscopy. The striped PLDs with $3\ifmmode\times\else\texttimes\fi{}1$ and $1\ifmmode\times\else\texttimes\fi{}\frac{9}{2}$ superstructures were characterized in real and reciprocal space. The simultaneous formation of momentum-specific suppressions in the spectral weight and phase shifts of the wavefront related to the superstructures were observed at multiple energies. These unusual energy dependencies are well agreed with the charge density waves (CDWs) formed in the multiband electronic structure of ${\mathrm{NbTe}}_{2}$. Fermi-surface nesting and reconstruction of the reciprocal lattice were suggested in developing the multiband CDWs in ${\mathrm{NbTe}}_{2}$.

Disentangling the role of bond lengths and orbital symmetries in controlling Tc of optimally doped YBa2Cu3O7

Optimally doped YBCO (YBa$_{2}$Cu$_{3}$O$_{7}$) has a high critical temperature, at 92 K. It is largely believed that Cooper pairs form in YBCO and other cuprates because of spin fluctuations, the issue and the detailed mechanism is far from settled. In the present work, we employ a state-of-the-art \emph{ab initio} ability to compute both the low and high energy spin fluctuations in optimally doped YBCO. We benchmark our results against recent inelastic neutron scattering and resonant inelastic X-ray scattering measurements. Further, we use strain as an external parameter to modulate the spin fluctuations and superconductivity. We disentangle the roles of Barium-apical Oxygen hybridization, the interlayer coupling and orbital symmetries by applying an idealized strain, and also a strain with a fully relaxed structure. We show that shortening the distance between Cu layers is conducive for enhanced Fermi surface nesting, that increases spin fluctuations and drives up $T_{c}$. However, when the structure is fully relaxed electrons flow to the d$_{z^2}$ orbital as a consequence of a shortened Ba-O bond which is detrimental for superconductivity

Role of quantum well in Pd(111) thin film magnetism

The face-centered cubic palladium (Pd) is paramagnetic but exhibits ferromagnetism in special environments, such as nanostructures and thin films. In this study, first-principles density functional calculations are performed to investigate the magnetism of the Pd(111) films. Results show that a ferromagnetic state is more stable than a paramagnetic state with a period of ∼2.5 monolayers. A Fermi surface nesting wave vector of 1.15 Å−1 in bulk Pd is observed along the [111] direction, which matches well with the periodicity of the thin film ferromagnetism. Quantum well states cross the Fermi level periodically in tune on Fermi surface nesting, which is the main cause of the ferromagnetism, satisfying the Stoner criterion. Strain effect is discussed, and tensile strain may strengthen the ferromagnetism. In addition, the surface energy of the Pd(111) film is affected by the quantum well states, showing the oscillatory film stability with thickness.

Pressure-induced high−Tc superconductivity in the ternary clathrate system Y-Ca-H

The hydrogen-rich ternary compounds under high pressure which rely on the synergistic effect of crystal structure and electronic property changes have a good prospect of realizing room temperature superconductivity. Here, we study the structure and superconductivity of the ternary Y-Ca-H system under high pressure using $ab\phantom{\rule{0.16em}{0ex}}\phantom{\rule{0.16em}{0ex}}initio$ calculations. We found four unprecedented ternary hydrides with clathrate hydrogen sublattice: $P\text{\ensuremath{-}}3m1\text{\ensuremath{-}}\mathrm{Y}{\mathrm{Ca}}_{2}{\mathrm{H}}_{18}, Fm\text{\ensuremath{-}}3m\text{\ensuremath{-}}{\mathrm{Y}}_{3}\mathrm{Ca}{\mathrm{H}}_{24}, P4/mmm\text{\ensuremath{-}}\mathrm{YCa}{\mathrm{H}}_{20}$, and $Fm\text{\ensuremath{-}}3m\text{\ensuremath{-}}\mathrm{Y}{\mathrm{Ca}}_{3}{\mathrm{H}}_{24}$ which are potential high-temperature superconductors. Strikingly, the superconducting transition temperature of ${\mathrm{Y}}_{3}\mathrm{Ca}{\mathrm{H}}_{24}$ was calculated to be 242--258 K at 150 GPa. The strong electron-phonon coupling and the high electron density of states of hydrogen at the Fermi level play important roles in high-temperature superconductivity. Further analysis suggests that the phonon softening in the mid-frequency region induced by Fermi surface nesting effectively enhances the electron-phonon coupling. We found that the appropriate stoichiometric ratio is crucial for the discovery of new hydrides in the Y-Ca-H system, which would provide ideas for exploring ternary hydrides.

Chiral SO(4) spin-valley density wave and degenerate topological superconductivity in magic-angle twisted bilayer graphene

Starting from a realistic extended Hubbard model for a ${p}_{x,y}$-orbital tight-binding model on the Honeycomb lattice, we perform a thorough investigation of the possible electron instabilities in magic-angle twisted bilayer graphene near the van Hove (VH) dopings. Here we focus on the interplay between the two symmetries of the system. One is the approximate $\mathrm{SU}(2)\ifmmode\times\else\texttimes\fi{}\mathrm{SU}(2)$ symmetry which leads to the degeneracy between the intervalley spin density wave (SDW) and valley density wave (VDW) as well as that between the intervalley singlet and triplet superconductivities (SCs). The other is the ${D}_{3}$ symmetry which leads to the degeneracy among the three symmetry-related wave vectors of the density-wave (DW) orders, originating from the Fermi-surface nesting. The interplay between these two degeneracies leads to intriguing quantum states relevant to recent experiments, as revealed by our systematic random-phase-approximation based calculations followed by a succeeding mean-field energy minimization for the groundstate energy. At the $\mathrm{SU}(2)\ifmmode\times\else\texttimes\fi{}\mathrm{SU}(2)$ symmetric point, the degenerate intervalley SDW and VDW are mixed into a new state of matter dubbed as the chiral SO(4) spin-valley DW. This state simultaneously hosts three four-component vectorial spin-valley DW orders with each adopting one wave vector, and the polarization directions of the three DW orders are mutually perpendicular to one another. In the presence of a tiny intervalley exchange interaction with coefficient ${J}_{H}\ensuremath{\rightarrow}{0}^{\ensuremath{-}}$ which breaks the $\mathrm{SU}(2)\ifmmode\times\else\texttimes\fi{}\mathrm{SU}(2)$ symmetry, a pure chiral SDW state is obtained. In the case of ${J}_{H}\ensuremath{\rightarrow}{0}^{+}$, although a nematic VDW order is favored, the two SDW orders with equal amplitudes are accompanied simultaneously. This nematic $\mathrm{VDW}+\mathrm{SDW}$ state possesses a stripy distribution of the charge density, consistent with the recent STM observations. On the aspect of SC, while the triplet $p+ip$ and singlet $d+id$ topological SCs are degenerate at ${J}_{H}=0$ near the VH dopings, the former (latter) is favored for ${J}_{H}\ensuremath{\rightarrow}{0}^{\ensuremath{-}}$ (${J}_{H}\ensuremath{\rightarrow}{0}^{+}$). In addition, the two asymmetric doping-dependent behaviors of the obtained pairing phase diagram are well consistent with experiments.

A broken translational symmetry state in an infinite-layer nickelate

A defining signature of strongly correlated electronic systems is the existence of competing phases with similar ground state energies, resulting in a rich phase diagram. While in the recently discovered nickelate superconductors, a high antiferromagnetic exchange energy has been reported, which implies the existence of strong electronic correlations, signatures of competing phases have not yet been observed. Here, we uncover a charge order (CO) in infinite-layer nickelates La1-xSrxNiO2 using resonant x-ray scattering across the Ni L-edge. In the parent compound, the CO arranges along the Ni-O bond direction with an incommensurate wave vector (0.344+/-0.002, 0) r.l.u., distinct from the stripe order in other nickelates which propagates along a direction 45 degree to the Ni-O bond. The CO resonance profile indicates that CO originates from the Ni 3d states and induces a parasitic charge modulation of La electrons. Upon doping, the CO diminishes and the ordering wave vector shifts toward a commensurate value of 1/3 r.l.u., indicating that the CO likely arises from strong correlation effects and not from Fermi surface nesting.

First-principles study on the magnetic and electronic properties of the high-pressure orthorhombic phase of MnSe

Exploring the mechanism of unconventional superconductivity has been one of the most important topics in condensed matter physics, while studying the magnetic and electronic properties of the parent compounds of unconventional superconductors can provide helpful clues. Recently, superconductivity in the orthorhombic-phase MnSe was successfully induced by applying high pressure, which makes MnSe the second Mn-based superconductor after MnP. Based on the spin-polarized density functional theory calculations, we have studied the magnetic and electronic properties of the orthorhombic-phase MnSe under high pressure. We show that there may exist strong antiferromagnetic (AFM) fluctuations in a narrow energy window (less than 3 meV/Mn) among an AFM state dubbed AFM3 and a series of staggered $n$-mer AFM states. Here the $n$-mer means that a set of $n$ adjacent spins on a line are parallelly aligned. In the AFM3 state and the $n$-mer AFM states, the Mn spins show AFM coupling along the $x$ axis and ferromagnetic (FM) coupling along the $y$ axis, but respectively host FM and staggered $n$-mer AFM correlations along the $z$ axis. Our calculations indicate that the orthorhombic-phase MnSe exhibits a metallic behavior in the low-energy magnetic states, in good accordance with the previous experimental observations. We also map the calculated energies onto an effective Heisenberg model and obtain the exchange couplings $J$, whose values can serve as a reference for analyzing the data from magnetic measurements. Two usual mechanisms like Fermi surface nesting and electron-phonon coupling can be ruled out as the origin of superconductivity. The magnetic properties in the orthorhombic-phase MnSe under high pressure need future in-depth experimental examination.