arXiv subjects

Qing-Bo Liu

Publications and source records attributed to Qing-Bo Liu.

8 recordsLinked to original sources

arXiv2026

Realizing Two-Dimensional Altermagnets with 100% Spin Polarization along the [110] Direction in Ambidentate-Ligand Based Square Organometallic Lattices

Altermagnetism, as a novel magnetic paradigm, not only possesses a zero net magnetic moment, but also exhibits a substantial nonrelativistic spin-splitting effect. Currently, developing a simple and experimentally feasible method for designing altermagnets is eagerly pursued. Moreover, achieving 100% spin polarization along certain directions in valence or conduction bands for altermagnets remains challenging. To address these issues, based on a tight-binding model, we propose a viable chemical strategy for constructing organometallic altermagnets by using ambidentate ligands. Furthermore, in these organometallic altermagnets, direction-dependent 100% spin polarization along the [110] direction in band edges can be induced via the coexistence of altermagnetically spin-split dispersive and flat bands, where the flat band suppresses electron transport in one spin channel and the dispersive band enables conduction in the other spin channel. Then, via first-principles calculations, a screening of 90 two-dimensional ambidentate-ligand based square organometallic lattices leads to the identification of three altermagnetic materials, namely Fe(taz)2Cl2 (taz = 1,2,4- triazine), Co(app)2Cl2, and Fe(app)2Cl2 (app = 1,4-azaphosphinine), all of which possess 100% spin polarization along the [110] direction in valence bands, as well as notable piezomagnetic behavior. This study paves the way for achieving direction-dependent fully spin-polarization in altermagnetic materials.

cond-mat.mtrl-sci↗

arXiv2026

High-Throughput Discovery of Chiral Carbon-Boron Semimetals with Weyl Topology and Phonon-Mediated Superconductivity

Chiral crystals provide a natural platform for unconventional topological fermions, while light-element carbon--boron frameworks are promising for phonon-mediated superconductivity owing to strong covalent bonding and high phonon frequencies. Here, we combine Wyckoff-position-based structure generation with high-throughput first-principles calculations to search for C--B crystals in selected hexagonal chiral space groups. Starting from 972 generated $\mathrm{C}_m\mathrm{B}_n$ structures ($m,n=1\text{--}4$), successive filters based on energetic stability, dynamical stability, electronic metallicity, topological band features, and thermal stability identify six stable candidates: $(\mathrm{C}_3\mathrm{B}_2)_8$, $(\mathrm{C}\mathrm{B}_2)_3$, $(\mathrm{C}\mathrm{B}_4)_6$, $(\mathrm{C}_3\mathrm{B}_2)_6$, $(\mathrm{C}_2\mathrm{B}_3)_6$, and $(\mathrm{C}\mathrm{B}_2)_6$. These materials host diverse topological semimetal states near the Fermi level, including charge-one, charge-two, and charge-three Weyl points, nodal rings, and nodal-surface states. Notably, $(\mathrm{C}_3\mathrm{B}_2)_8$ exhibits screw-symmetry-protected charge-three Weyl points and strong electron--phonon coupling, yielding a McMillan transition temperature of $31.1\,\mathrm{K}$ for the Coulomb pseudopotential parameter $μ^* = 0.10$. In addition, $(\mathrm{C}\mathrm{B}_2)_3$ hosts multiple charge-one Weyl points and pronounced anisotropic multiband superconductivity revealed by fully anisotropic Eliashberg calculations. These results extend the materials scope of topological semimetals with phonon-mediated superconductivity to chemically designed chiral light-element structures.

cond-mat.mtrl-sci↗

arXiv2023

Catalogue of topological electrons and phonons in all allotropes of carbon

Carbon, as one of the most common element in the earth, constructs hundreds of allotropic phases to present rich physical nature. In this work, by combining the ab inito calculations and symmetry analyses method, we systematically study a large number of allotropes of carbon (703), and discovered 315 ideal topological phononic materials and 32 topological electronic materials. The ideal topological phononic nature includes single, charge-two, three, four Weyl honons, the Dirac or Weyl nodal lines phonons, and nodal surfaces phonons. And the topological electron nature ncludes topological insulator, (Type-II) Dirac points, triple nodal points, the Dirac (Weyl) nodal lines, quadratic nodal lines and so on. For convenience, we take the uni in SG 178 and pbg in SG 230 as the examples to describe the topological features in the main. We find that it is the coexistence of single pair Weyl phonons and one-nodal surfaces phonons in the uni in SG 178, which can form the single surface arc in the (100) surface BZ and isolated double-helix surface states (IDHSSs)in the (110) surface BZ. In topological semimetal pbg in SG 230, we find that the perfect triple degenerate nodal point can be found in the near Fermi level, and it can form the clear surface states in the (001) and (110) surface BZ. Our work not only greatly expands the topological features in all allotropes of carbon, but also provide many ideal platforms to study the topological electrons and phonons.

cond-mat.mtrl-sci↗

arXiv2022

Two-Dimensional Semiconducting Metal Organic Frameworks with Auxetic Effect, Room Temperature Ferrimagnetism, Chiral Ferroelectricity, Bipolar Spin Polarization and Topological Nodal Lines/Points

Two-dimensional (2D) semiconductors integrated with two or more functions are the cornerstone for constructing multifunctional nanodevices, but remain largely limited. Here, by tuning the spin state of organic linkers and the symmetry/topology of crystal lattice, we predict a class of unprecedented multifunctional semiconductors in 2D Cr(II) five-membered heterocyclic metal organic frameworks that simultaneously possess auxetic effect, room temperature ferrimagnetism, chiral ferroe-lectricity, electrically reversible spin polarization and topological nodal lines/points. Taking 2D Cr(TDZ)$_2$ (TDZ=1.2.5-thiadiazole) as an exemplification, the auxetic effect is produced by the anti-tetra-chiral lattice structure. The high temperature ferrimagnetism originates from the strong d-p direct magnetic exchange interaction between Cr cations and TDZ doublet radical anions. Meanwhile, the clockwise-counterclockwise alignment of TDZ' dipoles results in unique 2D chiral ferroelectricity with atomic-scale vortex-antivortex states. 2D Cr(TDZ)$_2$ is an intrinsic bipolar magnetic semiconductor where half-metallic conduction with switchable spin-polarization direction can be induced by applying a gate voltage. Besides, the symmetry of the little group C$_4$ of lattice structure endows 2D Cr(TDZ)$_2$ with topological nodal lines and a quadratic nodal point in the Brillouin zone near the Fermi level.

cond-mat.mtrl-sci↗

arXiv2022

Realization of the Single-pair-Weyl Phonons with the Maximum Charge Number in Acoustic Crystals

To observe the Weyl phonon (WP) with the maximum charge and to design a realistic material structure containing only single-pair-WPs have long been considered two challenges in the field of topology physics. Here we have successfully designed an acoustic crystal to realize the single-pair-WPs with the maximum charge for the first time. Our theoretical simulations on acoustic band dispersions demonstrate that protected by the time-reversal symmetry ($\cal T$) and the point group symmetries, a WP with the charge -4 ($\mathcal{C}=-4$) and another WP with $\mathcal{C}=+4$ are located at the high-symmetry point $Γ$ and R, respectively, with the absence of any other kinds of WPs. Moreover, the singe-pair-WPs obtained here are designed by the simplest two-band mode, and the related quadruple-helicoid Fermi acrs can be observed clearly in experiments, since they aren't covered by any bulk bands and hybridized by other kinds of WPs. Our theoretical results provide a reliable acoustic crystal to study the topological properties of the single-pair-WPs with the maximum charge for experimentalists in this field.

cond-mat.mtrl-sci↗

arXiv2022

Beyond no-go theorem' Weyl phonons

By using \emph{ab initio} calculations and symmetry analysis, we define a new class of Weyl phonons, i.e., isolated Weyl phonons (IWPs), which are characterized by Chern number $\pm$2 or $\pm$4 in their acoustic phononic spectra and protected by the time inversion symmetry and point group symmetries. More importantly, their particular topological feature make them circumvent from the no-go theorem. Some high-symmetry points, behaving as isolated Weyl points in the space groups (SGs) of the related phononic systems, tend to form IWPs. As enumerated in Table I, the IWPs are located at the center of three-dimensional Brillouin zone (BZ), and protected by the time-reversal symmetry ($\cal T$) and the corresponding point group symmetries. Moreover, a realistic chiral crystal material example of K$_2$Mg$_2$O$_3$ in SG 96, a monopole IWP with Chern number -2 is found at the high-symmetry point $Γ$, and in another material example of Nb$_3$Al$_2$N in SG 213, a monopole IWP with Chern number +4 is confirmed at the point $Γ$. It is interesting that that IWPs can not form the surface arcs in the surface BZ, which has not been reported in the phononic systems to present. Our theoretical results not only uncover a new class of Weyl phonons (IWPs), but also put forwards an effective way to search the IWPs in spinless systems.

cond-mat.mes-hall↗

arXiv2020

Symmetry-Enforced Weyl Phonons

In spinful electronic systems, time-reversal symmetry makes that all Kramers pairs at the time-reversal-invariant momenta are Weyl points (WPs) in chiral crystals. Here, we find that such symmetry-enforced WPs can also emerge in bosonic systems ($e.g.$ phonons and photons) due to nonsymmorphic symmetries. We demonstrate that for some nonsymmorphic chiral space groups, several high-symmetry $k$-points can host $only$ WPs in the phononic systems, dubbed symmetry-enforced Weyl phonons (SEWPs). The SEWPs, enumerated in Table I, are pinned at the boundary of the three-dimensional (3D) Brillouin zone (BZ) and protected by nonsymmorphic crystal symmetries. By performing first-principles calculations and symmetry analysis, we propose that as an example of SEWPs, the two-fold degeneracies at P are monopole WPs in K$_2$Sn$_2$O$_3$ with space group 199. The two WPs of the same chirality at two nonequivalent P points are related by time-reversal symmetry. In particular, at $\sim 17.5$ THz, a spin-1 Weyl phonon is also found at H, since two Weyl phonons at P carrying a non-zero net Chern number cannot exist alone in the 3D BZ. The significant separation between P and H points makes the surface arcs long and clearly visible. Our findings not only present an effective way to search for WPs in bosonic systems, but also offer some promising candidates for studying monopole Weyl and spin-1 Weyl phonons in realistic materials.

cond-mat.mtrl-sci↗

arXiv2018

Magnetic nanotubes: A new material platform to realize robust spin-Seebeck effect and perfect thermal spin-filtering effect

To construct reliable material platforms and to uncover new rules to realize spin-Seebeck effect (SSE) and thermal spin-filtering effect (SFE) are core topics in spin caloritronics. Here we design several single-layer boron-nitrogen nanotubes (BNNTs) with n boron (nitrogen) atoms substituted by carbons in every unit cell. We find that for n = 1, the magnetic BNNTs generate a good SSE with nearly symmetric spin-up and spin-down currents; while as the carbon dopant concentration increases (c.f. n $\geq$ 2), a high rotational symmetry of the carbons contributes to generate the SSE with more symmetric thermal spin-up and spin-down currents, otherwise towards the thermal SFE. Moreover, some metallic BNNTs can generate the SSE or the SFE with finite threshold temperatures, due to the compensation effect around the Fermi level. More importantly, we find that the compression strain engineering is an effective route to improve these effects and to realize the transition between them. These theoretical results about the SSE in nanotubes enrich the spin caloritronics, and put forwards new material candidates to realize the SSE and other inspiring thermospin phenomena

cond-mat.mes-hall↗