Research Highlights FML

Around 50 years ago, LiTi2O4 was reported to be the only spinel oxide to exhibit a superconducting transition with the highest Tc ≈ 13.7 K. Recently, MgTi2O4 has been found to be another spinel oxide to reveal a superconducting transition with Tc ≈ 3 K, however, its superconducting state is realized only in thin film superlattices involving SrTiO3.We find that a V-doped Mg1-xTi2O4 phase, which gets stabilized as a thin surface layer on top of a nearly stoichiometric and insulating V-doped MgTi2O4 bulk sample, exhibits high-temperature superconductivity with Tc ≈ 16 K. The superconducting transition is also confirmed through a concomitant sharp diamagnetic transition immediately below Tc. Thus, V-doped Mg1-xTi2O4 exhibits the maximum Tc among spinel superconductors and also possesses a very high critical field.

(A. Rahaman et al. Phys. Rev. B 107, 245124 (2023))

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.107.245124

Using spark-plasma sintering protocol, we have been able to engineer room-temperature ferroelectricity (FE) in two otherwise nonferroelectric RCrO3 (R= rare-earth) compounds, namely, DyCrO3 (which is reported as a quantum paraelectric) and LaCrO3 (which is already known to be paraelectric). Out of these two emergent room-temperature FE materials, SPS-LaCrO3 also undergoes a high-temperature antiferromagnetic ordering at 290 K, thus coming very close to becoming the first roomtemperature multiferroic material in this promising family of RCrO3 compounds.

(S. Mishra et al. Phys. Rev. B 107, 214104 (2023))

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.107.214104 

A spark-plasma sintered GdCrO3 (SPS-GCO) is found to stabilize in its ferroelectric phase beyond room temperature. The SPS-GCO undergoes antiferromagnetic ordering at much lower temperatures, only below ≈170 K. Thus, any role of magnetism to the observed room temperature ferroelectricity in SPS-GCO can be ruled out. Using detailed Rietveld refinements of room-temperature x-ray diffraction patterns, SPS-GCO is found to stabilize in the noncentrosymmetric orthorhombic Pna21 space group (the reported low-temperature ferroelectric phase in GCO). The ferroelectric Pna21 phase of SPS-GCO (stabilized at room temperature using the high-pressure and high-temperature spark-plasma sintering process) undergoes transition to the paraelectric centrosymmetric phase upon heating beyond ≈450 K (as confirmed using dielectric and calorimetric measurements), which on subsequent cooling to room temperature does not undergo a transition back to the ferroelectric phase and remains in the centrosymmetric Pbnm phase.

(S. Mishra et al. Phys. Rev. B (Letters) 104, L180101 (2021))

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.104.L180101

Materials in which a Mott-insulator to metal transition (Mott breakdown) can be induced using a moderate d.c. current (with an electric field that is much smaller than the corresponding Zener breakdown voltage) is promising for electronic device applications and offer a unique platform for investigation of the underlying emergent mechanism. Here we report a novel Mott breakdown phenomenon in a spinel compound, namely MgTi1.2V0.8O4 with extremely small threshold electric fields. Using first-principles calculation and several experimental probes, we further elucidated the origin of this unique effect.

(A. Rahaman et al. Phys. Rev. B 103, 245145 (2021))

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.103.245145

Ferroelectric (FE) materials usually possess very high band gap (∼3−4 eV) and extremely poor electrical conductivity, which renders them unsuitable forphotovoltaic applications. Here, we demonstrate that a carefully designed Bi−Fe codoped BaTiO3 (BTO) system (Ba1−xBixTi0.9Fe0.1O3−δ, 0≤x≤0.10) provides a unique platform with the simultaneous optimization of low band gap, high FE polarization, and reasonable conductivity. Subsequently, we report significantly enhanced photovoltaic response within the series. Such an approach of optimizing the desired physical properties in a closely related mixed phase material where the ferroelectricity is engineered in the majority tetragonal BTO phase, while the minority hexagonal BTO phase aids in the reasonable conductivity (a combination that is not realizable in usual single phase FE materials), along with an optimum band gap, is promising in the realization of many more potential FE-based photovoltaic materials.  

(P. Pal et al. J. Phys. Chem. C. 125, 5315 (2021))

https://pubs.acs.org/doi/pdf/10.1021/acs.jpcc.0c10655

We show that simultaneous doping of Bi along with Fe into BaTiO3 effectively enhances the magnetoelectric (ME) multiferroic response (both ferromagnetism and ferroelectricity) at room temperature. We also report a systematic increase in large dielectric constant values as well as reduction in loss tangent values with relatively moderate temperature variation of the dielectric constant around room temperature with increasing Bi doping content, which makes these compounds promising as high-k dielectric materials. 

(P. Pal et al. Appl. Phys. Lett. 117, 012901 (2020))

https://aip.scitation.org/doi/suppl/10.1063/5.0004785

https://arxiv.org/abs/2008.06301 8.06301

Simultaneous coexistence of room-temperature ferromagnetism and ferroelectricity in Fe-doped BaTiO3 (BTO) is intriguing. Here, we investigate its origin and show that Fe-doped BTO has a mixed-phase room-temperature multiferroicity. We further identify different parameters which control the paraelectric hexagonal phase stability over the ferroelectric tetragonal one and control them to achieve a suitable codoped BTO compound with enhanced room-temperature multiferroic properties. 

(P. Pal et al. Phys. Rev. B 101, 064409 (2020))

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.101.064409

The presence of orbital degree of freedom in strongly correlated systems leads to unusual orderings. Here, using a combination of density-functional theory calculations and various experimental investigations, we reveal a unique tetramer orbital-ordered, chiral and polar ground state for a Ti3+containing MnTi2O4 spinel oxide. 

(A. Rahaman et al. Phys. Rev. B 100, 115162 (2019))

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.100.115162

Transport and magnetoresistance measurements, performed on metallic, high-carrier density YTiO3-CaTiO3 superlattices, reveal an unusual antiferromagnetic order in the ultrathin, epitaxially strained YTiO3 layers, in excellent agreement with recent theoretical predictions.

(P. Pal et al. Phys. Rev. B 98, 045420 (2018))

https://journals.aps.org/prb/abstract/10.1103/PhysRevB.98.045420