arXiv subjects

Kaustav Dey

Publications and source records attributed to Kaustav Dey.

7 recordsLinked to original sources

arXiv2023

High-pressure crystal growth and investigation of the metal-to-metal transition of Ruddlesden-Popper trilayer nickelates La$_4$Ni$_3$O$_{10}$

Single crystals of Ruddlesden-Popper nickelates La$_4$Ni$_3$O$_{10}$ were grown by means of the floating-zone technique at oxygen pressure of 20~bar. Our results reveal the effects of the annealing process under pressure on the crystal structure. We present the requirements for crystal growth and show how a reported ferromagnetic impurity phase can be avoided. The different growth and post-annealing processes result in two distinct phases $P2_1/a$ and {\it Bmab} in which the metal-to-metal transitions occur at 152~K and 136~K, respectively.

cond-mat.str-el↗

arXiv2022

Passivity-Based Decentralized Criteria for Small-Signal Stability of Power Systems with Converter-Interfaced Generation

With the increasing penetration of converter-interfaced distributed generation systems, it would be advantageous to specify local compliance criteria for these devices to ensure the small-signal stability of the interconnected system. Passivity of the device admittance, which is an example of a local criterion, has been used previously to avoid resonances between these devices and the lightly damped oscillatory modes of the network. Typical active and reactive power control strategies like droop control and virtual synchronous generator control inherently violate the passivity constraints on admittance at low frequencies, although this does not necessarily mean that the interconnected system will be unstable. Therefore, passivity of the admittance is unsuitable as a stability criterion for devices that are represented by their wide-band models. To overcome this problem, this paper proposes the use of criteria based on admittance at higher frequencies and an alternative transfer function at lower frequencies. The alternative representation uses active and reactive power and the derivatives of the polar components of voltage as interface variables. To allow for the separate analysis at low and high frequencies, the device dynamics should exhibit a slow-fast separation; this is proposed as an additional constraint. Adherence to the proposed criteria is not onerous and is easily verifiable through frequency response analysis.

eess.SY↗

arXiv2022

Simplified Stability Assessment of Power Systems with Variable-Delay Wide-Area Damping Control

Power electronic devices such as HVDC and FACTS can be used to improve the damping of poorly damped inter-area modes in large power systems. This involves the use of wide-area feedback signals, which are transmitted via communication networks. The performance of the closed-loop system is strongly influenced by the delay associated with wide-area signals. The random nature of this delay introduces a switched linear system model. The stability assessment of such a system requires linear matrix inequality based approaches. This makes the stability analysis more complicated as the system size increases. To address this challenge, this paper proposes a delay-processing strategy that simplifies the modelling and analysis in discrete-domain. In contrast to the existing stability assessment techniques, the proposed approach is advantageous because the stability, as well as damping performance, can be accurately predicted by a simplified analysis. The proposed methodology is verified with a case study on the 2-area 4-machine power system with a series compensated tie-line. The results are found to be in accordance with the predictions of the proposed simplified analysis.

eess.SY↗

arXiv2021

Passivity of Electrical Transmission Networks modelled using Rectangular and Polar D-Q variables

The increasing penetration of converter-interfaced distributed energy resources has brought out the need to develop decentralized criteria that would ensure the small-signal stability of the inter-connected system. Passivity of the D-Q admittance or impedance is a promising candidate for such an approach. It is facilitated by the inherent passivity of the D-Q impedance of an electrical network. However, the passivity conditions are generally restrictive and cannot be complied with in the low frequency range by the D-Q admittance of devices that follow typical power control strategies. However, this does not imply that the system is unstable. Therefore, alternative formulations that use polar variables (magnitude/phase angle of voltages and real/reactive power injection instead of the D-Q components of voltages and currents) are investigated. Passivity properties of the electrical network using these different formulations are brought out in this paper through analytical results and illustrative examples.

eess.SY↗

arXiv2021

Magneto-structural coupling in ilmenite-type NiTiO$_3$: a combined diffraction and dilatometry study

We report the ground state magnetic structure and in-field magnetostrictive effects of NiTiO$_3$ studied by means of zero field and in-field single-crystal neutron diffraction, magnetization, and high-resolution dilatometry experiments. Zero-field neutron diffraction on NiTiO$_3$ single crystals reveals an easy-plane antiferromagnet with a multidomain ground state. Upon application of external magnetic fields, neutron diffraction shows the evolution of domains with spins perpendicular to the applied field. The rotation of spins in the multidomain state exhibits pronounced lattice changes in the magnetostriction measurements. We see magnetization and magnetostriction measurements sale with each other in the multidomain state revealing the strong coupling of spins to the lattice.

cond-mat.str-el↗

arXiv2020

Magnetoelastic coupling and Grüneisen scaling in NdB$_4$

We report high-resolution capacitance dilatometry studies on the uniaxial length changes in a NdB$_4$ single crystal. The evolution of magnetically ordered phases below $T_{\rm N}$= 17.2~K (commensurate antiferromagnetic phase, cAFM), $T_{\rm IT}$= 6.8~K (intermediate incommensurate phase, IT), and $T_{\rm LT}$= 4.8~K (low-temperature phase, LT) is associated with pronounced anomalies in the thermal expansion coefficients. The data imply significant magneto-elastic coupling and evidence of a structural phase transition at $T_{\rm LT}$ . While both cAFM and LT favor structural anisotropy $δ$ between in-plane and out-of-plane length changes, it competes with the IT-type of order, i.e., $δ$ is suppressed in that phase. Notably, finite anisotropy well above $T_{\rm N}$ indicates short-range correlations which are, however, of neither cAFM, IT, nor LT-type. Grüneisen analysis of the ratio of thermal expansion coefficient and specific heat enables the derivation of uniaxial as well as hydrostatic pressure dependencies. While $α$/$c_{\rm p}$ evidences a single dominant energy scale in LT, our data imply precursory fluctuations of a competing phase in IT and cAFM, respectively. Our results suggest the presence of orbital degrees of freedom competing with cAFM and successive evolution of a magnetically and orbitally ordered ground state.

cond-mat.str-el↗

arXiv2020

Magnetic phase diagram and magneto-elastic coupling of NiTiO3

We report high-resolution dilatometry on high-quality single crystals of NiTiO3 grown by means of the optical floating-zone technique. The anisotropic magnetic phase diagram is constructed from thermal expansion and magnetostriction studies up to B=15T and magnetization studies in static (15T) and pulsed (60T) magnetic fields. Our data allow to quantitatively study magneto-elastic coupling and to determine uniaxial pressure dependencies. While the entropy changes are found to be of magnetic nature, Grüneisen analysis implies only one relevant energy scale in the whole low-temperature regime. Thereby, our data suggest that the observed structural changes due to magneto-elastic coupling and previously reported magnetodielectric coupling[1] are driven by the same $magnetic$ degrees of freedom that lead to long-range magnetic order in NiTiO3, which in turn, establishes a linear magnetodielectric coupling in this compound.

cond-mat.str-el↗