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- ItemSaturation of the anomalous Hall effect at high magnetic fields in altermagnetic RuO2(Melville, NY : AIP Publ., 2023) Tschirner, Teresa; Keßler, Philipp; Gonzalez Betancourt, Ruben Dario; Kotte, Tommy; Kriegner, Dominik; Büchner, Bernd; Dufouleur, Joseph; Kamp, Martin; Jovic, Vedran; Smejkal, Libor; Sinova, Jairo; Claessen, Ralph; Jungwirth, Tomas; Moser, Simon; Reichlova, Helena; Veyrat, LouisObservations of the anomalous Hall effect in RuO2 and MnTe have demonstrated unconventional time-reversal symmetry breaking in the electronic structure of a recently identified new class of compensated collinear magnets, dubbed altermagnets. While in MnTe, the unconventional anomalous Hall signal accompanied by a vanishing magnetization is observable at remanence, the anomalous Hall effect in RuO2 is excluded by symmetry for the Néel vector pointing along the zero-field [001] easy-axis. Guided by a symmetry analysis and ab initio calculations, a field-induced reorientation of the Néel vector from the easy-axis toward the [110] hard-axis was used to demonstrate the anomalous Hall signal in this altermagnet. We confirm the existence of an anomalous Hall effect in our RuO2 thin-film samples, whose set of magnetic and magneto-transport characteristics is consistent with the earlier report. By performing our measurements at extreme magnetic fields up to 68 T, we reach saturation of the anomalous Hall signal at a field Hc ≃ 55 T that was inaccessible in earlier studies but is consistent with the expected Néel-vector reorientation field.
- ItemFlux Growth and Characterization of Bulk InVO4 Crystals(Basel : MDPI, 2023) Voloshyna, Olesia; Gorbunov, Mikhail V.; Mikhailova, Daria; Maljuk, Andrey; Seiro, Silvia; Büchner, BerndThe flux growth of InVO4 bulk single crystals has been explored for the first time. The reported eutectic composition at a ratio of V2O5:InVO4 = 1:1 could not be used as a self-flux since no sign of melting was observed up to 1100 °C. Crystals of InVO4 of typical size 0.5 × 1 × 7 mm3 were obtained using copper pyrovanadate (Cu2V2O7) as a flux, using Pt crucibles. X-ray powder diffraction confirmed the orthorhombic Cmcm structure. Rests of the flux material were observed on the sample surface, with occasional traces of Pt indicating some level of reaction with the crucible. X-ray absorption spectroscopy showed that oxidation states of indium and vanadium ions are +3 and +5, respectively. The size and high quality of the obtained InVO4 crystals makes them excellent candidates for further study of their physical properties.
- ItemFloating Zone Growth of Pure and Pb-Doped Bi-2201 Crystals(Basel : MDPI, 2024) Roslova, Maria; Büchner, Bernd; Maljuk, AndreyIn this review, we summarize recent progress in crystal growth and understanding of the influence of crystal structure on superconductivity in pure and Pb-doped Bi2Sr2CuOy (Bi-2201) materials belonging to the overdoped region of high-temperature cuprate superconductors. The crystal growth of Bi-2201 superconductors faces challenges due to intricate materials chemistry and the lack of knowledge of corresponding phase diagrams. Historically, a crucible-free floating zone method emerged as the most promising growth approach for these materials, resulting in high-quality single crystals. This review outlines the described methods in the literature and the authors’ synthesis endeavors encompassing Pb-doped Bi-2201 crystals, provides a detailed structural characterization of as-grown and post-growth annealed samples, and highlights optimal growth conditions that yield large-size, single-phase, and compositionally homogeneous Bi-2201 single crystals.
- ItemCovalency versus magnetic axiality in Nd molecular magnets: Nd-photoluminescence, strong ligand-field, and unprecedented nephelauxetic effect in fullerenes NdM2N@C80 (M = Sc, Lu, Y)(Cambridge : RSC, 2023) Yang, Wei; Rosenkranz, Marco; Velkos, Georgios; Ziegs, Frank; Dubrovin, Vasilii; Schiemenz, Sandra; Spree, Lukas; de Souza Barbosa, Matheus Felipe; Guillemard, Charles; Valvidares, Manuel; Büchner, Bernd; Liu, Fupin; Avdoshenko, Stanislav M.; Popov, Alexey A.Nd-based nitride clusterfullerenes NdM2N@C80 with rare-earth metals of different sizes (M = Sc, Y, Lu) were synthesized to elucidate the influence of the cluster composition, shape and internal strain on the structural and magnetic properties. Single crystal X-ray diffraction revealed a very short Nd-N bond length in NdSc2N@C80. For Lu and Y analogs, the further shortening of the Nd-N bond and pyramidalization of the NdM2N cluster are predicted by DFT calculations as a result of the increased cluster size and a strain caused by the limited size of the fullerene cage. The short distance between Nd and nitride ions leads to a very large ligand-field splitting of Nd3+ of 1100-1200 cm−1, while the variation of the NdM2N cluster composition and concomitant internal strain results in the noticeable modulation of the splitting, which could be directly assessed from the well-resolved fine structure in the Nd-based photoluminescence spectra of NdM2N@C80 clusterfullerenes. Photoluminescence measurements also revealed an unprecedentedly strong nephelauxetic effect, pointing to a high degree of covalency. The latter appears detrimental to the magnetic axiality despite the strong ligand field. As a result, the ground magnetic state has considerable transversal components of the pseudospin g-tensor, and the slow magnetic relaxation of NdSc2N@C80 could be observed by AC magnetometry only in the presence of a magnetic field. A combination of the well-resolved magneto-optical states and slow relaxation of magnetization suggests that Nd clusterfullerenes can be useful building blocks for magneto-photonic quantum technologies.
- ItemSuppression of nematicity by tensile strain in multilayer FeSe/SrTiO3 films(College Park, MD : APS, 2023) Lou, Rui; Suvorov, Oleksandr; Grafe, Hans-Joachim; Kuibarov, Andrii; Krivenkov, Maxim; Rader, Oliver; Büchner, Bernd; Borisenko, Sergey; Fedorov, AlexanderThe nematicity in multilayer FeSe/SrTiO3 films has been previously suggested to be enhanced with decreasing film thickness. Motivated by this, there have been many discussions about the competing relation between nematicity and superconductivity. However, the criterion for determining the nematicity strength in FeSe remains highly debated. The understanding of nematicity as well as its relation to superconductivity in FeSe films is therefore still controversial. Here, we fabricate multilayer FeSe/SrTiO3 films using molecular beam epitaxy and study the nematic properties by combining angle-resolved photoemission spectroscopy, Se77 nuclear magnetic resonance, and scanning tunneling microscopy experiments. We unambiguously demonstrate that, near the interface, the nematic order is suppressed by the SrTiO3-induced tensile strain; in the bulk region further away from the interface, the strength of nematicity recovers to the bulk value. Our results not only solve the recent controversy about the nematicity in multilayer FeSe films, but also offer valuable insights into the relationship between nematicity and superconductivity.
- ItemTunable positions of Weyl nodes via magnetism and pressure in the ferromagnetic Weyl semimetal CeAlSi([London] : Nature Publishing Group UK, 2024) Cheng, Erjian; Yan, Limin; Shi, Xianbiao; Lou, Rui; Fedorov, Alexander; Behnami, Mahdi; Yuan, Jian; Yang, Pengtao; Wang, Bosen; Cheng, Jin-Guang; Xu, Yuanji; Xu, Yang; Xia, Wei; Pavlovskii, Nikolai; Peets, Darren C.; Zhao, Weiwei; Wan, Yimin; Burkhardt, Ulrich; Guo, Yanfeng; Li, Shiyan; Felser, Claudia; Yang, Wenge; Büchner, BerndThe noncentrosymmetric ferromagnetic Weyl semimetal CeAlSi with simultaneous space-inversion and time-reversal symmetry breaking provides a unique platform for exploring novel topological states. Here, by employing multiple experimental techniques, we demonstrate that ferromagnetism and pressure can serve as efficient parameters to tune the positions of Weyl nodes in CeAlSi. At ambient pressure, a magnetism-facilitated anomalous Hall/Nernst effect (AHE/ANE) is uncovered. Angle-resolved photoemission spectroscopy (ARPES) measurements demonstrated that the Weyl nodes with opposite chirality are moving away from each other upon entering the ferromagnetic phase. Under pressure, by tracing the pressure evolution of AHE and band structure, we demonstrate that pressure could also serve as a pivotal knob to tune the positions of Weyl nodes. Moreover, multiple pressure-induced phase transitions are also revealed. These findings indicate that CeAlSi provides a unique and tunable platform for exploring exotic topological physics and electron correlations, as well as catering to potential applications, such as spintronics.
- ItemCoupled mechanical oscillator enables precise detection of nanowire flexural vibrations(London : Springer Nature, 2023) Sharma, Maneesha; Sathyadharma Prasad, Aniruddha; Freitag, Norbert H.; Büchner, Bernd; Mühl, ThomasThe field of nanowire (NW) technology represents an exciting and steadily growing research area with applications in ultra-sensitive mass and force sensing. Existing detection methods for NW deflection and oscillation include optical and field emission approaches. However, they are challenging for detecting small diameter NWs because of the heating produced by the laser beam and the impact of the high electric field. Alternatively, the deflection of a NW can be detected indirectly by co-resonantly coupling the NW to a cantilever and measuring it using a scanning probe microscope. Here, we prove experimentally that co-resonantly coupled devices are sensitive to small force derivatives similar to standalone NWs. We detect force derivatives as small as 10−9 N/m with a bandwidth of 1 Hz at room temperature. Furthermore, the measured hybrid vibration modes show clear signatures of avoided crossing. The detection technique presented in this work verifies a major step in boosting NW-based force and mass sensing.
- ItemPhonon thermal transport shaped by strong spin-phonon scattering in a Kitaev material Na2Co2TeO6([London] : Nature Publishing Group, 2024) Hong, Xiaochen; Gillig, Matthias; Yao, Weiliang; Janssen, Lukas; Kocsis, Vilmos; Gass, Sebastian; Li, Yuan; Wolter, Anja U. B.; Büchner, Bernd; Hess, ChristianThe report of a half-quantized thermal Hall effect and oscillatory structures in the magnetothermal conductivity in the Kitaev material α-RuCl3 have sparked a strong debate on whether it is generated by Majorana fermion edge currents, spinon Fermi surface, or whether other more conventional mechanisms are at its origin. Here, we report low temperature thermal conductivity (κ) of another candidate Kitaev material, Na2Co2TeO6. The application of a magnetic field (B) along different principal axes of the crystal reveals a strong directional-dependent B impact on κ, while no evidence for mobile quasiparticles except phonons can be concluded at any field. Instead, severely scattered phonon transport prevails across the B−T phase diagram, revealing cascades of phase transitions for all B directions. Our results thus cast doubt on recent proposals for significant itinerant magnetic excitations in Na2Co2TeO6, and emphasize the importance of discriminating true spin liquid transport properties from scattered phonons in candidate materials.
- ItemPhononic-magnetic dichotomy of the thermal Hall effect in the Kitaev material Na2 Co2 TeO6(College Park, MD : APS, 2023) Gillig, Matthias; Hong, Xiaochen; Wellm, Christoph; Kataev, Vladislav; Yao, Weiliang; Li, Yuan; Büchner, Bernd; Hess, ChristianThe quest for a half-quantized thermal Hall effect of a Kitaev system represents an important tool to probe topological edge currents of emergent Majorana fermions. Pertinent experimental findings for α-RuCl3 are, however, strongly debated, and it has been argued that the thermal Hall signal stems from phonons or magnons rather than from Majorana fermions. Here, we investigate the thermal Hall effect of the Kitaev candidate material Na2Co2TeO6, and we show that the measured signal emerges from at least two components, phonons and magnetic excitations. This dichotomy results from our discovery that the longitudinal and transversal heat conductivities share clear phononic signatures, while the transversal signal changes sign upon entering the low-temperature, magnetically ordered phase. Our results demonstrate that uncovering a genuinely quantized magnetic thermal Hall effect in Kitaev topological quantum spin liquids such as α-RuCl3 and Na2Co2TeO6 requires disentangling phonon vs magnetic contributions, including potentially fractionalized excitations such as the expected Majorana fermions.
- ItemFacile one-pot hydrothermal synthesis of a zinc oxide/curcumin nanocomposite with enhanced toxic activity against breast cancer cells(London : RSC Publishing, 2023) Madeo, Lorenzo Francesco; Schirmer, Christine; Cirillo, Giuseppe; Froeschke, Samuel; Hantusch, Martin; Curcio, Manuela; Nicoletta, Fiore Pasquale; Büchner, Bernd; Mertig, Michael; Hampel, SilkeZinc oxide/Curcumin (Zn(CUR)O) nanocomposites were prepared via hydrothermal treatment of Zn(NO3)2 in the presence of hexamethylenetetramine as a stabilizing agent and CUR as a bioactive element. Three ZnO : CUR ratios were investigated, namely 57 : 43 (Zn(CUR)O-A), 60 : 40 (Zn(CUR)O-B) and 81 : 19 (Zn(CUR)O-C), as assessed by thermogravimetric analyses, with an average hydrodynamic diameter of nanoaggregates in the range of 223 to 361 nm. The interaction of CUR with ZnO via hydroxyl and ketoenol groups (as proved by X-ray photoelectron spectroscopy analyses) was found to significantly modify the key properties of ZnO nanoparticles with the obtainment of a bilobed shape (as shown by scanning electron microscopy), and influenced the growth process of the composite nanoparticles as indicated by the varying particle sizes determined by powder X-ray diffraction. The efficacy of Zn(CUR)O as anticancer agents was evaluated on MCF-7 and MDA-MB-231 cancer cells, obtaining a synergistic activity with a cell viability depending on the CUR amount within the nanocomposite. Finally, the determination of reactive oxygen species production in the presence of Zn(CUR)O was used as a preliminary evaluation of the mechanism of action of the nanocomposites.