Deepam Maurya

Dark Matter Search Results from the PICO-2L C3F8 Bubble Chamber

New data are reported from the operation of a 2 liter C3F8 bubble chamber in the SNOLAB underground laboratory, with a total exposure of 211.5 kg days at four different energy thresholds below 10 keV. These data show that C3F8 provides excellent electron-recoil and alpha rejection capabilities at very low thresholds. The chamber exhibits an electron-recoil sensitivity of <3.5×10(-10) and an alpha rejection factor of >98.2%. These data also include the first observation of a dependence of acoustic signal on alpha energy. Twelve single nuclear recoil event candidates were observed during the run. The candidate events exhibit timing characteristics that are not consistent with the hypothesis of a uniform time distribution, and no evidence for a dark matter signal is claimed. These data provide the most sensitive direct detection constraints on WIMP-proton spin-dependent scattering to date, with significant sensitivity at low WIMP masses for spin-independent WIMP-nucleon scattering.

Tunable self-biased magnetoelectric response in homogenous laminates

In this study, we demonstrate self-biased magnetoelectric effect in homogenous two-phase magnetostrictive-piezoelectric laminates. Our results illustrate the method for tuning the magnitude of self-bias effect and provide understanding behind the hysteretic changes. We model this phenomenon by considering the magnetization hysteresis with shape-induced demagnetization effect. The self-biased response was found to be directly related to the nature of magnetization and can be tuned by variation in demagnetization state and the resultant differential magnetic flux distribution. These results present significant advancement toward development of AC magnetic field sensor and magnetoelectric composite based on-chip devices by eliminating the need for DC bias.

Synthesis mechanism of grain-oriented lead-free piezoelectric Na0.5Bi0.5TiO3–BaTiO3 ceramics with giant piezoelectric response

In this paper, we report the synthesis of [001]pc/[012]Rh (pc: pseudo cubic, Rh: rhombohedral) grain oriented lead-free piezoelectric 0.93(Na0.5Bi0.5TiO3)–0.07BaTiO3 (NBT–BT) ceramic with Na0.5Bi0.5TiO3 (NBT) as the seed template. The difference in surface energy along with the chemical potential gradient between the stable NBT seeds and the metastable liquid phase was the driving force for the growth of textured grain. Interfaces in the microstructure were found to be coherent at the atomic scale facilitating the domain wall motion with an applied electric field. The strong texturing in [001]pc/[012]Rh was found to result in extra structural distortions manifested by a decreasing lattice parameter and increasing rhombohedral angle (α). The textured specimen exhibited rather ordered domains with smaller size as compared to its randomly oriented counterpart. The piezoelectric response was found to increase monotonously with the increase in the degree of texturing and the optimized microstructure was found to provide 200% enhancement in the magnitude of piezoelectric coefficient (d33 ∼ 322 pC N−1) as compared to its randomly oriented form (d33 ∼ 160 pC N−1).

Origin of high piezoelectric response in A-site disordered morphotropic phase boundary composition of lead-free piezoelectric 0.93(Na0.5Bi0.5)TiO3-0.07BaTiO3

Perovskite piezoelectric compositions near the morphotropic phase boundary (MPB) are known to exhibit high piezoelectric response. In lead-based ABO3 compound with B-site disorder, the origin of this enhancement has been associated with the presence of an intermediate monoclinic/orthorhombic state that bridges the adjacent ferroelectric rhombohedral and tetragonal phases. However, the origin of high piezoelectric response in lead-free ABO3 compounds with A-site disorder has not been conclusively established. We describe a microscopic model derived from comparative analyses of high resolution transmission electron microscopy and neutron diffraction that explains the origin of high piezoelectric response in lead-free MPB compositions of 0.93(Na0.5Bi0.5)TiO3–0.07BaTiO3. Direct observation of nanotwins with monoclinic symmetry confirmed the presence of an intermediate bridging phase that facilitates a pathway for polarization reorientation. Monoclinic distortions of an average rhombohedral phase are attributed to localized displacements of atoms along the non-polar directions.

Enhanced piezoelectricity and nature of electric-field induced structural phase transformation in textured lead-free piezoelectric Na0.5Bi0.5TiO3-BaTiO3 ceramics

This letter provides a comparative description of the properties of textured and randomly oriented poly-crystalline lead-free piezoelectric 0.93(Na0.5Bi0.5TiO3)-0.07BaTiO3 (NBT-BT) ceramics. A high longitudinal piezoelectric constant of (d33) ∼ 322 pC/N was obtained in (001)PC textured NBT-7BT ceramics, which is almost ∼2× times the d33 coefficient reported for randomly oriented ceramics of the same composition. In situ neutron diffraction experiments revealed that characteristically different structural responses are induced in textured and randomly oriented NBT-BT ceramics upon application of electric fields (E), which are likely related to the varying coherence lengths of polar nanoregions and internal stresses induced by domain switching.

Improved dark matter search results from PICO-2L Run 2

New data are reported from a second run of the 2-liter PICO-2L C3F8 bubble chamber with a total exposure of 129 kg-days at a thermodynamic threshold energy of 3.3 keV. These data show that measures taken to control particulate contamination in the superheated fluid resulted in the absence of the anomalous background events observed in the first run of this bubble chamber. One single nuclear-recoil event was observed in the data, consistent both with the predicted background rate from neutrons and with the observed rate of unambiguous multiple-bubble neutron scattering events. The chamber exhibits the same excellent electron-recoil and alpha decay rejection as was previously reported. These data provide the most stringent direct detection constraints on weakly interacting massive particle (WIMP)-proton spin-dependent scattering to date for WIMP masses <50 GeV/c2.

Giant strain with ultra-low hysteresis and high temperature stability in grain oriented lead-free K0.5Bi0.5TiO3-BaTiO3-Na0.5Bi0.5TiO3 piezoelectric materials

We synthesized grain-oriented lead-free piezoelectric materials in (K0.5Bi0.5TiO3-BaTiO3-xNa0.5Bi0.5TiO3 (KBT-BT-NBT) system with high degree of texturing along the [001]c (c-cubic) crystallographic orientation. We demonstrate giant field induced strain (~0.48%) with an ultra-low hysteresis along with enhanced piezoelectric response (d33 ~ 190pC/N) and high temperature stability (~160°C). Transmission electron microscopy (TEM) and piezoresponse force microscopy (PFM) results demonstrate smaller size highly ordered domain structure in grain-oriented specimen relative to the conventional polycrystalline ceramics. The grain oriented specimens exhibited a high degree of non-180° domain switching, in comparison to the randomly axed ones. These results indicate the effective solution to the lead-free piezoelectric materials.

Giant energy density in [001]-textured Pb(Mg1/3Nb2/3)O3-PbZrO3-PbTiO3 piezoelectric ceramics

Pb(Zr,Ti)O3 (PZT) based compositions have been challenging to texture or grow in a single crystal form due to the incongruent melting point of ZrO2. Here we demonstrate the method for achieving 90% textured PZT-based ceramics and further show that it can provide highest known energy density in piezoelectric materials through enhancement of piezoelectric charge and voltage coefficients (d and g). Our method provides more than ∼5× increase in the ratio d(textured)/d(random). A giant magnitude of d·g coefficient with value of 59 000 × 10−15 m2 N−1 (comparable to that of the single crystal counterpart and 359% higher than that of the best commercial compositions) was obtained.

Effect of poling on nanodomains and nanoscale structure in A-site disordered lead-free piezoelectric Na0.5Bi0.5TiO3–BaTiO3

This paper establishes the nanoscale poling mechanism operating in A-site disordered lead-free piezoelectric ceramics. Nanoscale domain maps and quantitative structural changes were obtained by deploying high-resolution transmission electron microscopy, dielectric spectroscopy, Raman spectroscopy and neutron scattering. Based on these results a microscopic model is proposed that provides insight into the E-field induced structural transformation. The stripe-like nanodomains in the unpoled system transformed into lamellar tetragonal domains with a reduced degree of displacement disorder during poling. It is proposed that the synergic effect of change in octahedral tilt disorder and cation displacement disorder leads to this transformation under an E-field. The criterion for achieving superior functional response includes stabilization of the long range order and reduction in the tilt disorder through compositional adjustments. Understanding of the poling mechanism in lead-free piezoelectric materials has been mostly limited to the behavior of domains under an applied field. However, this work provides an in-depth understanding of the changes in the local structure along with domain morphology under an applied field.

Giant piezoelectric voltage coefficient in grain-oriented modified PbTiO3 material

A rapid surge in the research on piezoelectric sensors is occurring with the arrival of the Internet of Things. Single-phase oxide piezoelectric materials with giant piezoelectric voltage coefficient (g, induced voltage under applied stress) and high Curie temperature (Tc) are crucial towards providing desired performance for sensing, especially under harsh environmental conditions. Here, we report a grain-oriented (with 95% <001> texture) modified PbTiO3 ceramic that has a high Tc (364 °C) and an extremely large g33 (115 × 10−3 Vm N−1) in comparison with other known single-phase oxide materials. Our results reveal that self-polarization due to grain orientation along the spontaneous polarization direction plays an important role in achieving large piezoelectric response in a domain motion-confined material. The phase field simulations confirm that the large piezoelectric voltage coefficient g33 originates from maximized piezoelectric strain coefficient d33 and minimized dielectric permittivity ɛ33 in [001]-textured PbTiO3 ceramics where domain wall motions are absent.