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Latest articles for Journal of Physics: Condensed Matter

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  • Exploring the correlation between chemical bonding and structural distortions in TbCu0.33Te2
    The design of solid-state materials requests a thorough understanding of the structural preferences among plausible structure models. Since the bond energy contributes to the formation energy of a given structure model, it also is decisive to determine the nature of chemical bonding for a given material. In this context, we were motivated to explore the correlation between chemical bonding and structural distortions within the low-dimensional tellurium fragments in TbCu0.33Te2. The ternary telluride was obtained from high-temperature solid-state reactions, while structure determinations based on x-ray diffraction experiments did not point to the presence of any structural distortion above 100 K. However, the results of first-principles-based computations indicate that a potential structural distortion within the low-dimensional tellurium fragments also correlates to an optimization of overall bonding.

  • Intriguing magnetic and electronic behaviors in La and Ru doped Sr2IrO4
    We report a detailed experimental study of the structural, magnetic and electrical properties of La and Ru doped (Sr1−x Lax)2Ir1−xRuxO4 (x= 0.05, 0.15). X-ray diffraction analysis reveals that both samples crystallize in a tetragonal structure with a space group I41/acd without impurities. Substitution with La and Ru leads to an increase in the lattice parameter a and a decrease in c. With increasing doping concentration, the Ir–O–Ir bond angle increases while the Ir–O bond length decreases. X-ray photoelectron spectroscopy (XPS) shows that Ir has Ir3+ (5d6) and Ir4+ (5d5) charge states, where the Ir4+ charge state decreases with an increase in doping concentration. The dc magnetic susceptibility χ (T) of x= 0.05 reveals a transition from paramagnetic to weak ferromagnetic (wFM) at TC ∼ 229 K, arising from the canted antiferromagnetic (AFM) spin arrangement. The magnetic ordering temperature TC remains unaltered for higher doping, whereas the magnetic moment is significantly reduced. The analysis of real and imaginary components of ac susceptibility data, based on conventional critical slow model, frequency shift per decade and Vogel–Fulcher law, unanimously evidences the existence of reentrant spin-glass behavior (RSG), i.e. the coexistence of weak ferromagnetism and spin glass phases for the lowest doping of x= 0.05. On the other hand, for higher doping (x = 0.15) of hole and electron, the RSG phase vanishes, leaving only the wFM phase at the same temperature as observed in x = 0.05. This suggests the higher doping of La and Ru does not affect the magnetic order, but removes the disorder between FM and AFM phases. The electrical resistivity (ρ) measurement analysis reveals that both the samples show semiconducting/insulating behavior across the temperature range. The ρ of the x= 0.05 sample is lower than that of pure sample Sr2IrO4 (Bhatti et al 2015 J. Phys.:Condens. Matter27 016005), while ρ of x = 0.15 shows two orders of magnitude larger than the x = 0.05 sample at low temperatures. The conduction mechanism of both samples is described by the 2D Mott’s variable-range hopping model. Our results demonstrate that co-doping of two cation sites generates intriguing, competing hopping and magnetic processes.

  • Hybridization effects on the magnetic ground state of ruthenium in double perovskite La2ZnRu1−x Ti x O6
    An exotic quantum mechanical ground state, i.e. the non-magnetic Jeff = 0 state, has been predicted for higher transition metal systems, due to the influence of strong spin–orbit coupling (SOC) or in other words, due to unquenched orbital moment contribution. However, previous attempts to experimentally realize such a state in 5d4 systems had mostly been clouded by solid-state effects or the reduced strength of the renormalized SOC that might allow significant triplon condensation. Interestingly, a recent study on vacancy ordered double perovskite compound K2RuCl6 by Takahashi et al (2021 Phys. Rev. Lett.127 227201) concluded that even within LS coupling regime the Ru4+ 4d4 ions, within isolated RuCl6 octahedra, strongly accommodate J multiplets having Jeff = 0 as the ground state with weakly interacting Jeff = 1 excitation, due to large unquenced Ru orbital angular momentum in the system. In the present report, we show results from the double perovskite La2ZnRuO6, where Ru4+ ions form isolated RuO6 octahedra but unlike K2RuCl6, they remain chemically connected via corner-sharing with nonmagnetic ZnO6 octahedra. Next, we move on to separate out the RuO6 octahedra further by doping the Ru-site with Ti4+, in order to probe the character of the Ru4+ ions within a different structural background. We find that the system stabilizes in P2 space group with tilted octahedra without distortion as has been confirmed by the x-ray powder diffraction and x-ray absorption spectroscopic studies. Interestingly, the x-ray photoelectron spectroscopic valance band spectra indicated certain inhomogeneity around the half-doping region, while confirming insulating ground state for all. Moreover, unlike the vacancy ordered double perovskite cases, it is observed that here the Ru orbital angular momentum gets substantially quenched and only the Ru spin magnetic moments are realized.

  • Structural, dielectric and magnetic properties of Sb/Cr-doped CaCu3Ti4O12 quadruple perovskite oxides
    Driven by the miniaturization of microelectronic devices and their multifunctionalities, the development of new quadruple-perovskite oxides with high dielectric constants and high Curie temperature are highly required. Herein, we report on the structural, dielectric and magnetic properties of Sb/Cr-doped CaCu3Ti4O12 (CCTO) quadruple perovskite oxides, CaCu3Ti3.9Sb0.1O12 (CCTSO) and CaCu3Ti3CrO12 (CCTCO). Structural Rietveld refinements demonstrated that the CCTO, CCTSO, and CCTCO ceramics adopted a cubic crystal structure (Im space group). They exhibited spherical shapes with nearly uniform particle sizes. XPS spectra clarified Cu3+ ions in the CCTSO ceramics, while Cu2+ and Cu+ ions, and Cr3+ ions in the CCTCO ceramics. All the ceramic samples displayed nearly frequency independent dielectric behaviors at low temperatures but a relaxor dielectric behavior at high temperatures. Such relaxor dielectric behavior in the CCTO and CCTSO ceramics was ascribed to the movement of doubly ionized oxygen vacancies but in the CCTCO ceramics to the movement of singly ionized oxygen vacancies. Impedance and modulus spectra reveal the significance contributions of grains and grain boundaries with non-Debye behavior. At room temperature (RT) the dielectric constant (ϵr) and dielectric loss (tanδ) of CCTO ceramics at 1 kHz were 15 922 and 0.126, respectively. An order reduction of tanδ was achieved in the CCTSO ceramics. In the CCTCO ceramics, the ϵr and tanδ at RT and 1 kHz were 975 and 0.453, respectively. The CCTCO powders exhibited antiferroelectric behavior at 2 K with a saturation magnetization (MS) of 1.42 μB/f.u., while the MS for the CCTSO powders was only 0.027 μB/f.u. All ceramic samples exhibited semiconductor characteristics owing to their continuous decreases of resistivity from 2 K to 800 K. Our present work demonstrates an effective route to tuning the dielectric and magnetic properties of CCTO oxides via B-site non-magnetic/magnetic ion-doping.

  • Spectral heat flux redistribution upon interfacial transmission
    In nonmetallic crystals, heat is transported by phonons of different frequencies, each contributing differently to the overall heat flux spectrum. In this study, we demonstrate a significant redistribution of heat flux among phonon frequencies when phonons transmit across the interface between dissimilar solids. This redistribution arises from the natural tendency of phononic heat to re-establish the bulk distribution characteristic of the material through which it propagates. Remarkably, while the heat flux spectra of dissimilar solids are typically distinct in their bulk forms, they can become nearly identical in superlattices or sandwich structures where the layer thicknesses are smaller than the phonon mean free paths. This phenomenon reflects that the redistribution of heat among phonon frequencies to the bulk distribution does not occur instantaneously at the interface, rather it develops over a distance on the order of phonon mean-free-paths.