Rojewski, S.; Borkenhagen, R.; Li, R.; Ovalle, D. G.; Tsirkin, S. S.; Kumar, H.; Raju, K.; Sankar, R.; Lei, S.; Schoop, L. M.; Vergniory, M. G.; Kuntscher, C. A. Optical response of the Dirac semimetals GdSb0.56Te1.35 and GdSb0.45Te1.50: Influence of charge density wave distortion and magnetic order Journal Article Phys. Rev. B 113, 155141 (2026). @article{rojewski_optical_2026,
title = {Optical response of the Dirac semimetals GdSb_{0.56}Te_{1.35} and GdSb_{0.45}Te_{1.50}: Influence of charge density wave distortion and magnetic order},
author = {S. Rojewski and R. Borkenhagen and R. Li and D. G. Ovalle and S. S. Tsirkin and H. Kumar and K. Raju and R. Sankar and S. Lei and L. M. Schoop and M. G. Vergniory and C. A. Kuntscher},
url = {https://link.aps.org/doi/10.1103/nygm-n4fv},
doi = {10.1103/nygm-n4fv},
year = {2026},
date = {2026-04-20},
urldate = {2026-04-01},
journal = {Phys. Rev. B},
volume = {113},
number = {15},
pages = {155141},
abstract = {The square-net materials GdSb𝑥Te2−𝑥−𝛿 provide a platform for exploring the effects of charge density wave (CDW) distortion and band filling on the topological electronic band structure. In this work we use infrared spectroscopy to probe the temperature-dependent electronic excitations in GdSb0.56Te1.35 and GdSb0.45Te1.50 single crystals for different polarization directions parallel to the square-net layers. First-principles calculations were also performed to study the electronic band structure and interband optical conductivity of stoichiometric GdSbTe. For GdSb0.56Te1.35 the in-plane optical conductivity is independent of the polarization direction and contains a small Drude term, indicating a weak metallic character, and a pronounced near-infrared absorption band, which may be due to excitations across the CDW gap. Upon cooling, the optical response shows no significant temperature dependence within the experimental uncertainty, and no signatures associated with magnetic ordering are detected. In comparison, for GdSb0.45Te1.50 we observe an in-plane anisotropy and strongly reduced low-frequency optical conductivity, with small temperature dependence. Possible signatures of Dirac fermion excitations in the optical conductivity spectra are also analyzed and discussed. Our results demonstrate that small differences in the stoichiometry have a significant impact on the optical response of GdSb𝑥Te2−𝑥−𝛿 materials.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
The square-net materials GdSb𝑥Te2−𝑥−𝛿 provide a platform for exploring the effects of charge density wave (CDW) distortion and band filling on the topological electronic band structure. In this work we use infrared spectroscopy to probe the temperature-dependent electronic excitations in GdSb0.56Te1.35 and GdSb0.45Te1.50 single crystals for different polarization directions parallel to the square-net layers. First-principles calculations were also performed to study the electronic band structure and interband optical conductivity of stoichiometric GdSbTe. For GdSb0.56Te1.35 the in-plane optical conductivity is independent of the polarization direction and contains a small Drude term, indicating a weak metallic character, and a pronounced near-infrared absorption band, which may be due to excitations across the CDW gap. Upon cooling, the optical response shows no significant temperature dependence within the experimental uncertainty, and no signatures associated with magnetic ordering are detected. In comparison, for GdSb0.45Te1.50 we observe an in-plane anisotropy and strongly reduced low-frequency optical conductivity, with small temperature dependence. Possible signatures of Dirac fermion excitations in the optical conductivity spectra are also analyzed and discussed. Our results demonstrate that small differences in the stoichiometry have a significant impact on the optical response of GdSb𝑥Te2−𝑥−𝛿 materials. |
Mukharjee, P. K.; Erdmann, S.; Wang, L.; Kaiser, J.; Jesche, A.; Puphal, P.; Isobe, M.; Hepting, M.; Keimer, Bernhard; Gegenwart, P.; Tsirlin, A. A. Anisotropic magnetoelastic coupling in the honeycomb magnet Na3Co2SbO6 Unpublished (2026), arXiv:2603.03214. @unpublished{mukharjee2026anisotropicmagnetoelasticcouplinghoneycomb,
title = {Anisotropic magnetoelastic coupling in the honeycomb magnet Na_{3}Co_{2}SbO_{6}},
author = {P. K. Mukharjee and S. Erdmann and L. Wang and J. Kaiser and A. Jesche and P. Puphal and M. Isobe and M. Hepting and Bernhard Keimer and P. Gegenwart and A. A. Tsirlin},
url = {https://doi.org/10.48550/arXiv.2603.03214},
doi = {10.48550/arXiv.2603.03214},
year = {2026},
date = {2026-03-03},
urldate = {2026-03-03},
abstract = {We present magnetization and dilatometry measurements on the honeycomb cobaltate Na3Co2SbO6 and map out its detailed field-temperature phase diagram down to sub-Kelvin temperatures. Our data for in-plane magnetic fields show a strongly anisotropic c*-axis lattice response, which is dominated by the variation of Co–O–Co bond angles according to textbackslashtextitab initio calculations. At T=0.4K, the magnetization M(B) exhibits step-like features that are also highly anisotropic. In the case of Btextbartextbarb, a small hysteresis observed around the second field-induced magnetic transition (B_c2) indicates its first-order character, whereas divergence of the magnetic Grüneisen parameter at B_c2 is suppressed upon cooling and signals the absence of quantum critical behavior upon entering the field-polarized state. None of our thermodynamic measurements provide evidence for a field-induced quantum spin liquid state near or above B_c2.},
note = {arXiv:2603.03214},
keywords = {},
pubstate = {published},
tppubtype = {unpublished}
}
We present magnetization and dilatometry measurements on the honeycomb cobaltate Na3Co2SbO6 and map out its detailed field-temperature phase diagram down to sub-Kelvin temperatures. Our data for in-plane magnetic fields show a strongly anisotropic c*-axis lattice response, which is dominated by the variation of Co–O–Co bond angles according to textbackslashtextitab initio calculations. At T=0.4K, the magnetization M(B) exhibits step-like features that are also highly anisotropic. In the case of Btextbartextbarb, a small hysteresis observed around the second field-induced magnetic transition (B_c2) indicates its first-order character, whereas divergence of the magnetic Grüneisen parameter at B_c2 is suppressed upon cooling and signals the absence of quantum critical behavior upon entering the field-polarized state. None of our thermodynamic measurements provide evidence for a field-induced quantum spin liquid state near or above B_c2. |
Schilberth, F.; Kondákor, M.; Ukolov, D.; Pawbake, A.; Vasin, K.; Ercem, O.; Prodan, L.; Tsurkan, V.; Tsirlin, A. A.; Faugeras, C.; Lemmens, P.; Penc, K.; Kézsmárki, I.; Bordács, S.; Deisenhofer, J. Optical phonons as a testing ground for spin group symmetries Journal Article npj Quantum Mater. 11, 26 (2026). @article{schilberthOpticalPhononsTesting2025,
title = {Optical phonons as a testing ground for spin group symmetries},
author = {F. Schilberth and M. Kondákor and D. Ukolov and A. Pawbake and K. Vasin and O. Ercem and L. Prodan and V. Tsurkan and A. A. Tsirlin and C. Faugeras and P. Lemmens and K. Penc and I. Kézsmárki and S. Bordács and J. Deisenhofer},
url = {https://doi.org/10.1038/s41535-026-00857-9},
doi = {10.1038/s41535-026-00857-9},
year = {2026},
date = {2026-02-17},
urldate = {2026-02-17},
journal = {npj Quantum Mater.},
volume = {11},
pages = {26},
abstract = {Lattice vibrations are highly sensitive to crystal symmetries and their changes across phase transitions. The latter can modify irreducible (co)representations and corresponding infrared and Raman selection rules of phonons. This concept is established for relativistic magnetic point groups, simultaneously transforming spatial and spin coordinates. However, in altermagnets described by non-relativistic spin groups with disjunct symmetry operations for both vector spaces, the phonon selection rules have remained unexplored. Here, we present a detailed study of the infrared- and Raman-active modes in the collinear antiferromagnet and altermagnet candidate Co2Mo3O8. Comparing to ab initio calculations accurately capturing the eigenfrequencies, we identify all expected phonon modes at room temperature and deduce their selection rules using both symmetry approaches. Importantly, we observe the change of selection rules upon antiferromagnetic ordering, agreeing with the relativistic symmetry approach, while the spin group formalism predicts no changes. Therefore, optical phonons sensing the symmetry of the magnetic order can reveal if relevant magnon-phonon coupling is compatible with the spin-group approach or not.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Lattice vibrations are highly sensitive to crystal symmetries and their changes across phase transitions. The latter can modify irreducible (co)representations and corresponding infrared and Raman selection rules of phonons. This concept is established for relativistic magnetic point groups, simultaneously transforming spatial and spin coordinates. However, in altermagnets described by non-relativistic spin groups with disjunct symmetry operations for both vector spaces, the phonon selection rules have remained unexplored. Here, we present a detailed study of the infrared- and Raman-active modes in the collinear antiferromagnet and altermagnet candidate Co2Mo3O8. Comparing to ab initio calculations accurately capturing the eigenfrequencies, we identify all expected phonon modes at room temperature and deduce their selection rules using both symmetry approaches. Importantly, we observe the change of selection rules upon antiferromagnetic ordering, agreeing with the relativistic symmetry approach, while the spin group formalism predicts no changes. Therefore, optical phonons sensing the symmetry of the magnetic order can reveal if relevant magnon-phonon coupling is compatible with the spin-group approach or not. |
Gaggl, T.; Misawa, R.; Kriener, M.; Tanaka, Y.; Yamada, R.; Hirschberger, M. Bulk superconductivity in the kagome metal YRu3B2 Journal Article Phys. Scr. 101, 055912 (2026). @article{gaggl_bulk_2026,
title = {Bulk superconductivity in the kagome metal YRu_{3}B_{2}},
author = {T. Gaggl and R. Misawa and M. Kriener and Y. Tanaka and R. Yamada and M. Hirschberger},
url = {https://doi.org/10.1088/1402-4896/ae3c65},
doi = {10.1088/1402-4896/ae3c65},
year = {2026},
date = {2026-02-05},
urldate = {2026-02-01},
journal = {Phys. Scr.},
volume = {101},
number = {5},
pages = {055912},
abstract = {Materials with a kagome lattice have been heavily studied for their exotic electronic band structure, geometrical frustration, high-temperature charge-order transitions, and unconventional electron-phonon coupling. In LaRu3Si2, it was proposed that electronic flat bands conspire with the characteristic phonon spectrum of the kagome lattice to drive enhanced superconductivity at Tc = 6.8 K. Here, we report bulk superconductivity in the structural analogue YRu3B2, which hosts a pristine kagome lattice. We observe a superconducting transition at Tc = 0.7 K through magnetization, resistivity, and heat-capacity measurements in this novel kagome metal.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
Materials with a kagome lattice have been heavily studied for their exotic electronic band structure, geometrical frustration, high-temperature charge-order transitions, and unconventional electron-phonon coupling. In LaRu3Si2, it was proposed that electronic flat bands conspire with the characteristic phonon spectrum of the kagome lattice to drive enhanced superconductivity at Tc = 6.8 K. Here, we report bulk superconductivity in the structural analogue YRu3B2, which hosts a pristine kagome lattice. We observe a superconducting transition at Tc = 0.7 K through magnetization, resistivity, and heat-capacity measurements in this novel kagome metal. |
Magar, A.; Somesh, K.; Saravanan, M. P.; Sichelschmidt, J.; Skourski, Y.; Telling, M. T. F.; Ginga, V. A.; Tsirlin, A. A.; Nath, R. Proximate spin-liquid behavior in the double trillium lattice antiferromagnet K2Co2(SO4)3 Journal Article Phys. Rev. B 113, L020409 (2026). @article{magarProximateSpinliquidBehavior2025,
title = {Proximate spin-liquid behavior in the double trillium lattice antiferromagnet K_{2}Co_{2}(SO_{4})_{3}},
author = {A. Magar and K. Somesh and M. P. Saravanan and J. Sichelschmidt and Y. Skourski and M. T. F. Telling and V. A. Ginga and A. A. Tsirlin and R. Nath},
url = {https://doi.org/10.1103/m8mj-slvl},
doi = {10.1103/m8mj-slvl},
year = {2026},
date = {2026-01-16},
urldate = {2026-01-01},
journal = {Phys. Rev. B},
volume = {113},
number = {2},
pages = {L020409},
abstract = {We report proximate quantum spin liquid behavior in K2Co2 (SO4)3 with the magnetic Co2+ ions embedded on a highly frustrated three-dimensional double trillium lattice. Single-crystal and high-resolution synchrotron powder x-ray diffraction experiments reveal a structural phase transition at 𝑇_t≃125K from high-temperature cubic to low-temperature monoclinic phase with the threefold superstructure. Magnetization and heat capacity consistently show the formation of the 𝐽eff=1/2 state of Co2+ below 50 K. In zero field, K2Co2(SO4)3 shows signatures of static magnetic order formed below 𝑇*≃0.6K, but muon spin relaxation experiments reveal a large fluctuating component that persists down to at least 50 mK, reminiscent of quantum spin liquid (QSL). Static order is completely suppressed in the small magnetic field of ∼1T, and low-temperature heat capacity demonstrates the 𝑇2 behavior above this field, another fingerprint of QSL. Ab initio calculations show a competition of several antiferromagnetic couplings that render K2Co2(SO4)3 a promising pseudospin-1/2 material for studying quantum magnetism in the double trillium lattice geometry.},
keywords = {},
pubstate = {published},
tppubtype = {article}
}
We report proximate quantum spin liquid behavior in K2Co2 (SO4)3 with the magnetic Co2+ ions embedded on a highly frustrated three-dimensional double trillium lattice. Single-crystal and high-resolution synchrotron powder x-ray diffraction experiments reveal a structural phase transition at 𝑇_t≃125K from high-temperature cubic to low-temperature monoclinic phase with the threefold superstructure. Magnetization and heat capacity consistently show the formation of the 𝐽eff=1/2 state of Co2+ below 50 K. In zero field, K2Co2(SO4)3 shows signatures of static magnetic order formed below 𝑇*≃0.6K, but muon spin relaxation experiments reveal a large fluctuating component that persists down to at least 50 mK, reminiscent of quantum spin liquid (QSL). Static order is completely suppressed in the small magnetic field of ∼1T, and low-temperature heat capacity demonstrates the 𝑇2 behavior above this field, another fingerprint of QSL. Ab initio calculations show a competition of several antiferromagnetic couplings that render K2Co2(SO4)3 a promising pseudospin-1/2 material for studying quantum magnetism in the double trillium lattice geometry. |
