In-plane Uniaxial Stress Research Articles

Dependence of Tc of YBa2Cu3O6.67 on in-plane uniaxial stress

We probe the effect on ${T}_{\text{c}}$ of in-plane uniaxial stress applied to the high-temperature superconductor ${\mathrm{YBa}}_{2}{\mathrm{Cu}}_{3}{\mathrm{O}}_{6.67}$. We find a highly anisotropic response. Under compression along the $b$ axis, which reduces the orthorhombicity of the ${\mathrm{CuO}}_{2}$ planes, ${T}_{\text{c}}$ is broadly flat for stresses up to at least 1.7 GPa. Under compression along the $a$ axis, ${T}_{\text{c}}$ decreases steeply. For stresses beyond $\ensuremath{\approx}1$ GPa the decrease is quasilinear. We hypothesize that superconductivity is suppressed by competition with uniaxial charge density wave order, which has been found to onset at $\ensuremath{\approx}1$ GPa.

The superconductivity of Sr2RuO4 under c-axis uniaxial stress

Applying in-plane uniaxial pressure to strongly correlated low-dimensional systems has been shown to tune the electronic structure dramatically. For example, the unconventional superconductor Sr2RuO4 can be tuned through a single Van Hove point, resulting in strong enhancement of both Tc and Hc2. Out-of-plane (c axis) uniaxial pressure is expected to tune the quasi-two-dimensional structure even more strongly, by pushing it towards two Van Hove points simultaneously. Here, we achieve a record uniaxial stress of 3.2 GPa along the c axis of Sr2RuO4. Hc2 increases, as expected for increasing density of states, but unexpectedly Tc falls. As a first attempt to explain this result, we present three-dimensional calculations in the weak interaction limit. We find that within the weak-coupling framework there is no single order parameter that can account for the contrasting effects of in-plane versus c-axis uniaxial stress, which makes this new result a strong constraint on theories of the superconductivity of Sr2RuO4.

Evidence for Realignment of the Charge Density Wave State in ErTe3 and TmTe3 under Uniaxial Stress via Elastocaloric and Elastoresistivity Measurements

We report the evolution of the charge density wave (CDW) states in the quasi-two-dimensional rare-earth tritellurides (RTe3 for R=Er, Tm) under the influence of in-plane uniaxial stress. Measurements of the elastocaloric effect, resistivity, and elastoresistivity allow us to demonstrate the importance of in-plane antisymmetric strain on the CDW and to establish a phase diagram. We show that modest tensile stress parallel to the in-plane a axis can reversibly switch the direction of the ordering wave vector between the two in-plane directions, and present a free-energy expansion which reproduces the general structure of the observed phenomena. This work opens a new avenue in the study of RTe3 in its own right, and more generally establishes RTe3 as a promising model system for the study of strain-CDW interactions in a quasi-two-dimensional square lattice.5 MoreReceived 7 January 2022Revised 13 March 2022Accepted 15 April 2022DOI:https://doi.org/10.1103/PhysRevX.12.021046Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI.Published by the American Physical SocietyPhysics Subject Headings (PhySH)Research AreasCharge density wavesStrainTechniquesCalorimetryResistivity measurementsCondensed Matter, Materials & Applied Physics

Crystal reorientation and plastic deformation of single-layer MoS2 and MoSe2 under uniaxial stress

We investigate theoretically, through of first-principles calculations, the effect of the application of large in-plane uniaxial stress on single-layer of MoS2, MoSe2, and MoSSe alloys. For stress applied along the zigzag (zz) direction, we predict an anomalous behavior near the point fracture. This behavior is characterized by the reorientation of the MoS2 structure along the applied stress from zz to armchair due to the formation of transient square-lattice regions in the crystal, with an apparent crystal rotation of 30 degrees. After reorientation, a large plastic deformation remains after the stress is removed. This behavior is also observed in MoSe2 and in MoSSe alloys. This phenomenon is observed both in stress-constrained geometry optimizations and in ab initio molecular dynamics simulations at finite temperature and applied stress.

Magnetic frustration and spontaneous rotational symmetry breaking in PdCrO2

In the triangular layered magnet PdCrO2 the intralayer magnetic interactions are strong, however the lattice structure frustrates interlayer interactions. In spite of this, long-range, 120$^\circ$ antiferromagnetic order condenses at $T_N = 38$~K. We show here through neutron scattering measurements under in-plane uniaxial stress and in-plane magnetic field that this occurs through a spontaneous lifting of the three-fold rotational symmetry of the nonmagnetic lattice, which relieves the interlayer frustration. We also show through resistivity measurements that uniaxial stress can suppress thermal magnetic disorder within the antiferromagnetic phase.

Symmetric and antisymmetric strain as continuous tuning parameters for electronic nematic order

We report the separate response of the critical temperature of the nematic phase transition ${T}_{\mathrm{S}}$ to symmetric and antisymmetric strains for the prototypical underdoped iron pnictide $\mathrm{Ba}{({\mathrm{Fe}}_{0.975}{\mathrm{Co}}_{0.025})}_{2}{\mathrm{As}}_{2}$. This decomposition is achieved by comparing the response of ${T}_{\mathrm{S}}$ to in-plane uniaxial stress and hydrostatic pressure. In addition to quantifying the two distinct linear responses to symmetric strains, we find a quadratic variation of ${T}_{\mathrm{S}}$ as a response to antisymmetric strains ${\ensuremath{\varepsilon}}_{{\mathrm{B}}_{1\mathrm{g}}}=\frac{1}{2}({\ensuremath{\varepsilon}}_{\mathrm{xx}}\ensuremath{-}{\ensuremath{\varepsilon}}_{\mathrm{yy}})$, exceeding the nonlinear response to symmetric strains by at least two orders of magnitude. These observations establish orthogonal antisymmetric strain as a powerful tuning parameter for nematic order.

Uniaxial stress flips the natural quantization axis of a quantum dot for integrated quantum photonics

The optical selection rules in epitaxial quantum dots are strongly influenced by the orientation of their natural quantization axis, which is usually parallel to the growth direction. This configuration is well suited for vertically emitting devices, but not for planar photonic circuits because of the poorly controlled orientation of the transition dipoles in the growth plane. Here we show that the quantization axis of gallium arsenide dots can be flipped into the growth plane via moderate in-plane uniaxial stress. By using piezoelectric strain-actuators featuring strain amplification, we study the evolution of the selection rules and excitonic fine structure in a regime, in which quantum confinement can be regarded as a perturbation compared to strain in determining the symmetry-properties of the system. The experimental and computational results suggest that uniaxial stress may be the right tool to obtain quantum-light sources with ideally oriented transition dipoles and enhanced oscillator strengths for integrated quantum photonics.

Effect of uniaxial stress on the magnetic phases of CeAuSb2

We present results of measurements of resistivity of \CAS{} under the combination of $c$-axis magnetic field and in-plane uniaxial stress. In unstressed \CAS{} there are two magnetic phases. The low-field A phase is a single-component spin-density wave (SDW), with $\mathbf{q} = (\eta, \pm \eta, 1/2)$, and the high-field B phase consists of microscopically coexisting $(\eta, \eta, 1/2)$ and $(\eta, -\eta, 1/2)$ spin-density waves. Pressure along a $\langle 100 \rangle$ lattice direction is a transverse field to both of these phases, and so initially has little effect, however eventually induces new low- and high-field phases in which the principal axes of the SDW components appear to have rotated to the $\langle 100 \rangle$ directions. Under this strong $\langle 100 \rangle$ compression, the field evolution of the resistivity is much smoother than at zero strain: In zero strain, there is a strong first-order transition, while under strong $\langle 100 \rangle$ it becomes much broader. We hypothesize that this is a consequence of the uniaxial stress lifting the degeneracy between the (100) and (010) directions.

An atomistic investigation of the impact of in-plane uniaxial stress during solid phase epitaxial regrowth

We propose an atomistic comprehensive model based on a lattice kinetic Monte Carlo approach to analyse the impact of in-plane uniaxial stress during solid phase epitaxial regrowth. We observed no influence of tensile stress on the regrowth kinetics. In contrast, compressive stress leads to (i) a reduction of the macroscopic regrowth velocity, (ii) an enhancement of the amorphous/crystalline interface roughness, and (iii) defective Si formation. Our observations are in good agreement with experimental data from the literature. Our atomistic approach also clarifies the interpretation of the interface morphological instability based on the kinetics of microscopic events.

Design and Development of a Biaxial Tensile Test Device for a Thin Film Specimen

In this article, the design and development of a biaxial tensile test device and its specimen are described. The device, which was designed for evaluating the mechanical characteristics of a thin film specimen under in-plane uniaxial and biaxial tensile stress states, consists of four sets of a piezoelectric actuator, a load cell, a linear variable differential transformer (LVDT), and an actuator case including lever structures with displacement amplification function. The structures fabricated by wire electrical discharge machining are able to amplify the actuator’s displacement by a factor of 3.8 along the tensile direction. The biaxial test specimen prepared using conventional micromachining processes is composed of a cross-shaped film section and chucking parts supported by silicon springs. After square holes in four chuck parts are respectively hooked with four loading poles, the film section is tensioned to the directions where the poles get away from the center of the specimen. Tensile strain rate can be individually controlled for each tensile direction. Raman spectroscopic stress analyses demonstrated that the developed biaxial tensile test device was able to accurately apply not only uniaxial but also biaxial tensile stress to a single-crystal silicon (SCS) film specimen.

Polarized light from excitonic recombination in selectively etched GaN/AlN quantum dot ensembles on Si(111)

Multiple layers of GaN/AlN quantum dot (QD) ensembles were grown by the Stranski–Krastanov method on Si(111) using molecular beam epitaxy. During the subsequent cooling from growth temperature, the thermal expansion coefficient mismatch between the Si substrate and GaN/AlN film containing the vertically stacked QDs leads to an additional biaxial tensile stress of 20–30 kbar in the III-nitride film. We have selectively modified the thermal stress in the QD layers by etching a cross-hatched pattern into the as-grown sample using inductively coupled Cl2/Ar plasma reactive ion etching. The results show that a suitable choice of stripe width from ∼2 to 10 µm and orientation along [11−20] and [1−100] can create regions of in-plane uniaxial stress that enable a selective and local control of the polarized luminescence from ensembles of QDs which were probed with cathodoluminescence. Experimental results indicate that the polarization anisotropy vanishes at high temperatures (∼300 K) with an increasing e–h pair excitation for the QDs, while the anisotropy decreases more slowly with excitation at low temperatures (∼46 K). A theoretical modelling of the effect of carrier filling on the polarization anisotropy and the excitonic transition energy was performed, as based on three-dimensional self-consistent solutions of the Schrödinger and Poisson equations using the and effective-mass methods for calculations of the e–h wavefunctions and electron and hole quasi-Fermi levels for varying levels of state filling. We attribute carrier filling and a thermal excitation of holes into higher energy QD hole states during e–h pair excitation to account for the observed gradual decrease in the polarization anisotropy with an increasing e–h pair excitation density at T = 300 K.

Symmetry-breaking orbital anisotropy observed for detwinned Ba(Fe 1- x Co x ) 2 As 2 above the spin density wave transition

Nematicity, defined as broken rotational symmetry, has recently been observed in competing phases proximate to the superconducting phase in the cuprate high-temperature superconductors. Similarly, the new iron-based high-temperature superconductors exhibit a tetragonal-to-orthorhombic structural transition (i.e., a broken C 4 symmetry) that either precedes or is coincident with a collinear spin density wave (SDW) transition in undoped parent compounds, and superconductivity arises when both transitions are suppressed via doping. Evidence for strong in-plane anisotropy in the SDW state in this family of compounds has been reported by neutron scattering, scanning tunneling microscopy, and transport measurements. Here, we present an angle-resolved photoemission spectroscopy study of detwinned single crystals of a representative family of electron-doped iron-arsenide superconductors, Ba(Fe 1- x Co x ) 2 As 2 in the underdoped region. The crystals were detwinned via application of in-plane uniaxial stress, enabling measurements of single domain electronic structure in the orthorhombic state. At low temperatures, our results clearly demonstrate an in-plane electronic anisotropy characterized by a large energy splitting of two orthogonal bands with dominant d xz and d yz character, which is consistent with anisotropy observed by other probes. For compositions x > 0, for which the structural transition ( T S ) precedes the magnetic transition ( T SDW ), an anisotropic splitting is observed to develop above T SDW , indicating that it is specifically associated with T S . For unstressed crystals, the band splitting is observed close to T S , whereas for stressed crystals, the splitting is observed to considerably higher temperatures, revealing the presence of a surprisingly large in-plane nematic susceptibility in the electronic structure.

Selective control of polarized emission from patterned GaN/AlN quantum dot ensembles on Si(111)

GaN/AlN quantum dots (QDs) were grown by the Stranski–Krastanov method on Si(111). The thermal expansion coefficient mismatch between the Si substrate and GaN/AlN film leads to an additional biaxial tensile stress of 20–30 kbar in the III-nitride film, which we have selectively modified by etching a cross-hatched pattern into the as-grown sample. The results show that a suitable choice of stripe orientation and width from ∼2 to 10 μm can create regions of in-plane uniaxial stress that enable a selective and local control of the polarized luminescence from ensembles of QDs which were probed with cathodoluminescence.

Electronic and optical properties of GaN/AlN quantum dots on Si(111) subject to in-plane uniaxial stresses and variable excitation

We have studied the excitation- and polarization-dependent optical properties of GaN/AlN self-assembled quantum dots (QDs) grown on Si(111) substrates. Ensembles of QDs were subject to various external stress configurations that resulted from the thermal expansion coefficient mismatch between the GaN/AlN layers and the Si(111) substrate and ranged from in-plane uniaxial stress, primarily along the ⟨112¯0⟩ directions, to in-plane biaxial stress, having magnitudes ranging from 20–30 kbar. Limited regions of uniaxial stress were obtained by exploiting naturally occurring microcracks that form during the postgrowth cooling. These microcracks act as stressors in order to create the highly localized regions of uniaxial stress. The local strain tensors for such QDs, which are subject to an interfacial stress perturbation, have been determined by modeling the dependence of the QD excitonic transition energy on the interfacial stress. Cathodoluminescence (CL) measurements of the excitonic transitions exhibit an in-plane linear polarization anisotropy in close proximity to microcracks. The polarization anisotropy is strongly dependent on the sample temperature and the electron beam excitation conditions used to excite the QD ensemble. Localized CL spectroscopy of the QDs exhibits emissions from both the ground and excited states, whose relative contributions depend on the level of excitation and temperature. Experimental results indicate that the polarization anisotropy vanishes at high temperatures (∼300 K) with an increasing excitation of the QDs, while the anisotropy decreases more slowly with excitation at low temperatures (∼60 K). A theoretical modeling of the effect of carrier filling on the polarization anisotropy and the excitonic transition energy was performed, as based on three-dimensional self-consistent solutions of the Schrödinger and Poisson equations using the 6×6 k⋅p and effective mass methods for calculations of the e-h wave functions and electron and hole quasi-Fermi levels for varying levels of state filling. We attribute carrier filling and a thermal excitation of holes into higher energy QD hole states during excitation to account for the observed gradual decrease in the polarization anisotropy with an increasing electron-hole pair excitation density at T=300 K.

Polarized emission from GaN/AlN quantum dots subject to uniaxial thermal interfacial stresses

The authors have studied the excitation-dependent optical properties of GaN/AlN self-assembled quantum dots (QDs) grown on Si(111) substrates. Ensembles of QDs were subject to various external stress configurations that resulted from the thermal expansion coefficient mismatch between the GaN/AlN layers and the Si(111) substrate and ranged from in-plane uniaxial stress, primarily along the ⟨11-20⟩ directions, to in-plane biaxial stress, having magnitudes ranging from 15 to 30 kbars. The authors have exploited microcracks that form during the postgrowth cooling as stressors in order to create the highly localized regions of uniaxial stress over limited regions of the samples. Cathodoluminescence (CL) measurements of the excitonic transitions exhibit an in-plane linear polarization anisotropy in close proximity to microcracks that is strongly dependent on the e-beam current used to excite the QD ensemble. The excitonic transition energy in varying proximity to the microcracks was studied with CL wavelength imaging. Some aspects of the carrier recombination are inferred from the temperature dependence of the integrated CL intensity. CL activation energy (AE) imaging is used to study changes in the AE for the thermal quenching of the luminescence in close proximity to the microcracks, and correlations are obtained with changes in the QD excitonic transition energy and changes in the average AlN band edge energies relative to the ground electron and hole states in the QD. Localized CL spectroscopy of the QDs exhibits emissions from both the ground and excited states, whose relative contributions depend on the level of excitation and temperature. Experimental results indicate that the polarization anisotropy vanishes at high temperatures (∼300 K) with an increasing excitation of the QDs, while the anisotropy decreases more slowly with excitation at low temperatures (∼60 K). A theoretical modeling of the effect of carrier filling on the polarization anisotropy and the excitonic transition energy was performed, as based on three-dimensional self-consistent solutions of the Schrödinger and Poisson equations using the 6×6 k⋅p and effective mass methods for the calculation of the e-h wave functions. The authors attribute carrier filling and a thermal excitation of holes into higher energy QD hole states during excitation to account for the observed gradual decrease in the polarization anisotropy with an increasing electron-hole pair excitation density at T=300 K.

Stressed solid-phase epitaxial growth of (011) Si

The solid-phase epitaxial growth kinetics of amorphized (011) Si with application of in-plane uniaxial stress to magnitude of 0.9 ± 0.1 GPa were studied. Tensile stresses did not appreciably change the growth velocity compared with the stress-free case, whereas compression tended to retard the growth velocity to approximately one-half the stress-free value. The results are explained using a prior generalized atomistic model of stressed solid-solid phase transformations. In conjunction with prior observations of stressed solid-phase epitaxial growth of (001) Si, it is advanced that the activation volume tensor associated with ledge migration may be substrate orientation-dependent.

Stressed solid-phase epitaxial growth of ion-implanted amorphous silicon

The kinetics of stressed solid-phase epitaxial growth (SPEG), also referred to as solid-phase epitaxy, solid-phase epitaxial regrowth, solid-phase epitaxial recrystallization, and solid-phase epitaxial crystallization, of amorphous (α) silicon (Si) created via ion-implantation are reviewed. The effects of hydrostatic, in-plane uniaxial, and normal uniaxial compressive stress on SPEG kinetics are examined in intrinsic (0 0 1)Si. Particular emphasis is placed on unifying the results of different experiments in a single-stress-dependent SPEG model. SPEG kinetics are observed to suffer similar exponentially enhanced growth rates in hydrostatic and normal uniaxial compressive stress. However, there are discrepancies between researchers in terms of the influence of in-plane stress on growth rates. Two different stress-dependent SPEG models are thus advanced, each with different physical bases. The model advanced by Aziz et al. proposes SPEG can be modeled as a single-atomistic process while the model advanced by Rudawski et al. suggests that stress influences the nucleation and migration processes of growth differently and that SPEG cannot be modeled as a single step. The basis for the Rudawski et al. model is based on the crystal island and ledge migration model of SPEG advanced by others. Morphological instabilities of the growing α/crystalline interface with in-plane compression are also addressed within the context of both the Aziz et al. and Rudawski et al. models. Finally, using the Rudawski et al. model, it is possible to examine, characterize, and isolate the different atomistic processes during growth. Calculation of the activation energies for nucleation and migrations processes suggests that the activation energy of 2.7 eV observed for the growth rate in stress-free SPEG by Olsen and Roth is representative of the activation energy for the single-atomistic process of crystal island nucleation. Thus, the study of stressed SPEG provides a new atomistic picture of the nature of growth.

Tuning Photoluminescence Energy and Fine Structure Splitting in Single Quantum Dots by Uniaxial Stress

We report a photoluminescence (PL) energy red-shift of single quantum dots (QDs) by applying an in-plane compressive uniaxial stress along the [110] direction at a liquid nitrogen temperature. Uniaxial stress has an effect not only on the confinement potential in the growth direction which results in the PL shift, but also on the cylindrical symmetry of QDs which can be reflected by the change of the full width at half maximum of PL peak. This implies that uniaxial stress has an important role in tuning PL energy and fine structure splitting of QDs.

Physics of strain effects in semiconductors and metal-oxide-semiconductor field-effect transistors

A detailed theoretical picture is given for the physics of strain effects in bulk semiconductors and surface Si, Ge, and III–V channel metal-oxide-semiconductor field-effect transistors. For the technologically important in-plane biaxial and longitudinal uniaxial stress, changes in energy band splitting and warping, effective mass, and scattering are investigated by symmetry, tight-binding, and k⋅p methods. The results show both types of stress split the Si conduction band while only longitudinal uniaxial stress along ⟨110⟩ splits the Ge conduction band. The longitudinal uniaxial stress warps the conduction band in all semiconductors. The physics of the strain altered valence bands for Si, Ge, and III–V semiconductors are shown to be similar although the strain enhancement of hole mobility is largest for longitudinal uniaxial compression in ⟨110⟩ channel devices and channel materials with substantial differences between heavy and light hole masses such as Ge and GaAs. Furthermore, for all these materials, uniaxial is shown to offer advantages over biaxial stress: additive strain and confinement splitting, larger two dimensional in-plane density of states, smaller conductivity mass, and less band gap narrowing.

Effects of hydrostatic and uniaxial stress on the Schottky barrier heights of Ga-polarity and N-polarity n-GaN

We report measurements of the Schottky barrier heights of Ni/Au contacts on Ga-polarity and N-polarity n-GaN under hydrostatic pressure and applied in-plane uniaxial stress. Under hydrostatic pressure the two different polarities of GaN yield significantly different rates of Schottky barrier height increase with increasing pressure. Uniaxial stress parallel to the surface affects the Schottky barrier height only minimally. The observed changes in barrier height under stress are attributed to a combination of band structure and piezoelectric effects.