Session S42: Stress and Strain in Quantum Materials

Enhanced superconductivity and ferroelectric quantum criticality in plastically deformed strontium titanate

The properties of quantum materials are typically tuned via application of pressure, magnetic field or chemical substitution. In this talk, I will describe a novel approach using irreversible, plastic deformation of single crystals, which we apply to the well-known dilute superconductor SrTiO₃ (STO). We find that plastically deformed electron-doped STO exhibits low-dimensional superconductivity with a significant enhancement of the superconducting transition temperature, T_c. Furthermore, we find evidence of possible superconducting correlations already at temperatures two orders of magnitude higher than the bulk T_c. Using neutron and x-ray scattering, we demonstrate that the primary effect of compressive plastic deformation is the creation and self-organization of dislocations into wall-like structures. The strong strain fields near such walls induce local ferroelectricity and enhanced quantum critical ferroelectric fluctuations, evidenced by Raman scattering measurements. The T_c enhancement is consistent with a theoretical proposal in which superconductivity in STO is mediated by soft polar fluctuations. I will also briefly describe more recent results on the structural, electronic, and magnetic properties of deformed STO as well as our efforts to extend this novel approach to other systems. These emerging results demonstrate the promise of plastic deformation and dislocation engineering as tools for the manipulation of electronic properties of quantum materials [1-4].

[1] S. Hameed, D. Pelc et al., Nature Materials 21, 54 (2022)

[2] M. Li and Y. Wang, Nature Materials 21, 3 (2022)

[3] X. Wang, A. Kundu, B. Xu et al., arXiv:2308.14801v1 (2023)

[4] I. Khayr et al., in preparation   

The stress-strain relationship of Sr2RuO4 across a Lifshitz transition.

The stress-strain relationship is a standard characterisation of engineering materials, but not of quantum materials. This is chiefly because of challenges in measurement: typical samples of quantum materials are small, and large strains are typically required to achieve interesting changes in electronic structure. Here, we report stress-strain data on Sr2RuO4 as it is tuned through a stress-driven Lifshitz transition. At 4 K, the Young's modulus of Sr2RuO4 drops by ≈10% at the Lifshitz transition, then increases by ≈20% beyond it, solely due to changes in electronic structure. Most of this change can be accounted for with a non-interacting model. We also show that stress-strain data can be used to obtain the condensation energy of a stress-induced magnetic phase in Sr2RuO4 that occurs beyond this Lifshitz transition. These results show that stress-strain is a promising technique for obtaining thermodynamic data on electronic phase transitions in quantum materials.   

Combining In-situ Uniaxial Strain and Single Crystal X-ray Scattering to Study Condensed Matter Challenges from Nematicity to Bicritical Behavior.

In this talk I will review how deploying single crystal in-situ uniaxial strain using low temperature x-ray scattering with simultaneous electrical measurement capabilities at the Advanced Photon Source deepens our understanding of complex quantum phenomena. X-ray techniques including high resolution diffraction, resonant magnetic, linear / circular and inelastic scattering have facilitated the exploration of fundamental characteristics within material systems displaying emergent quantum behaviors. 

 

To illustrate this ongoing instrument development we will discuss i) nematic behavior in Fe based superconductors (1,-3) and ii) the bicritical behavior in rare earth(RE) tri-tellurides. (4) Here, electronic or structural nematicity is coupled to both the lattice and conducting electrons making structural and transport measurements sensitive to the underlying fluctuations. This, in principle makes x-ray scattering coupled with electrical measurements indispensable tools in this field. In the RE tri-tellurides, structural instabilities in the form of charge density waves (CDW) emerge. Uniaxial strain materializes as a key mechanism for controlling the orientation of these modulations The coexistence of orthogonal CDWs further creates a strain-variable bicritical phase space, offering a rich terrain for investigation and discovery. Lastly, we will provide insights into our ongoing studies in strain-based quantum devices (5) and offer a glimpse into our future plans, which encompass multi-modal instrument developments harnessing the power of combined x-ray techniques.   

Strain engineering of layered square-planar nickelate thin films

The discovery of superconductivity in infinite-layer Nd_0.8Sr_0.2NiO₂ thin films reignited an interest in the nickelates as cuprate analogues. More broadly, the infinite-layer nickelates are the n = ∞ member of a homologous series of ‘layered square-planar nickelates’, R_n+1Ni_nO_2n+2 or (RNiO₂)_n(RO₂), where R = trivalent rare-earth cation and n > 1.   These compounds host n quasi-two-dimensional NiO₂ planes separated by (RO₂)⁻ spacer layers, making the layered square-planar nickelates, Nd_n+1Ni_nO_2n+2, are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality.  Our work investigates the role of epitaxial strain in the competing requirements for the synthesis of the Ruddlesden-Popper compounds, R_n+1Ni_nO_3n+1, and subsequent reduction to the square-planar phase, R_n+1Ni_nO_2n+2. We synthesize our highest quality Ruddlesden-Popper compounds films under compressive strain and observe a high density of extended defects forms under tensile strain. Films reduced on compressive substrates become insulating and form compressive strain-induced c-axis canting defects, while films on the intermediate strain state are metallic and, for optimal layering, superconducting. This work provides a pathway to the synthesis of Nd_n+1Ni_nO_2n+2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy.   

Crystallization of heavy fermions via epitaxial strain in spinel LiV2O4 thin film+

The mixed-valent spinel LiV₂O₄ is known as the first oxide heavy-fermion system with an enhanced specific heat coefficient γ ~400 mJ/molK². There is a general consensus that a subtle interplay of charge, spin, and orbital degrees of freedom of correlated electrons plays a crucial role in the enhancement of quasi-particle mass, but the specific mechanism has remained yet elusive. A charge-ordering (CO) instability of V3+ and V4+ ions that is geometrically frustrated by the V pyrochlore sublattice from forming a long-range CO down to T =0 K has been proposed as a prime candidate for the mechanism. We uncover the hidden CO instability by applying epitaxial strain on single-crystalline LiV2O4 thin films [1]. We find a crystallization of heavy fermions in a LiV2O4 film on MgO, where a charge-ordered insulator comprising of a stack of V3+ and V4+ layers along [001], the historical Verwey-type ordering, is stabilized by the in-plane tensile and out-of-plane compressive strains from the substrate. Our discovery of the [001] Verwey-type CO, together with previous realizations of a distinct [111] CO in bulk single crystals under hydrostatic pressures, evidence the close proximity of the heavy-fermion state to degenerate CO states mirroring the geometrical frustration of the V pyrochlore lattice, which supports the CO instability scenario for the mechanism behind the heavy fermion formation.

[1] U.Nieman et al., PNAS 120, e2215722120 (2023).

+work done in collaboration with U.Nieman, Y.M.Wu, R.Oka, D.Hirai, Y.Wanga, Y.E.Suyolocua, M.Kim, P.v.Aken.