Context of project
Nonreciprocity is a phenomenon that has intrigued scientists and technologist for decades. In terms of technology, it has made tremendous impact, one particular and common example is the electronic diode (rectifier). In terms of scientific research it is usually a wonder the microscopic origins of such nonreciprocal phenomena. In this project, we explore the nonreciprocity mechanism when surface acoustic waves are absorbed by a thin ferromagnetic layer in resonance conditions, when spin waves are excited. 
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Abstract of our recent work
A fundamental form of magnon-phonon interaction is an intrinsic property of magnetic materials, the “magnetoelastic coupling.” This form of interaction has been the basis for describing magnetostrictive materials and their applications, where strain induces changes of internal magnetic fields. Different from the magnetoelastic coupling, more than 40 years ago, it was proposed that surface acoustic waves may induce surface magnons via rotational motion of the lattice in anisotropic magnets. However, a signature of this magnon-phonon coupling mechanism, termed magneto-rotation coupling, has been elusive. Here, we report the first observation and theoretical framework of the magneto-rotation coupling in a perpendicularly anisotropic film Ta/CoFeB(1.6 nanometers)/MgO, which consequently induces nonreciprocal acoustic wave attenuation with an unprecedented ratio of up to 100% rectification at a theoretically predicted optimized condition. Our work not only experimentally demonstrates a fundamentally new path for investigating magnon-phonon coupling but also justifies the feasibility of the magneto-rotation coupling application.

Read more
Nonreciprocal surface acoustic wave propagation via magneto-rotation coupling
M. Xu, K. Yamamoto, J. Puebla, K. Baumgaertl, B. Rana, K. Miura, H. Takahashi, D. Grundler, S. Maekawa, Y. Otani
Science Advances 6 (32), eabb1724 (2020)
advances.sciencemag.org/content/6/32/eabb1724

Recent theoretical work of the inverse effect: 
Non-reciprocal Pumping of Surface Acoustic Waves by Spin Wave Resonance
K Yamamoto, W Yu, T Yu, J Puebla, M Xu, S Maekawa, G Bauer
Journal of the Physical Society of Japan 89 (11), 113702 (2020)
journals.jps.jp/doi/full/10.7566/JPSJ.89.113702

ontext of project
Recently, we demonstrated the spin current generation via magnon – phonon coupling, reaching a maximum spin current JS on the order of 10^8 A/m^2 [Physical Review B 97 (18), 180301(R) (2018)]. This encouraging value is three orders of magnitude smaller than JS,DW ~ 10^11 A/m^2 required for current-induced motion of domain walls (DWs) or spin torques, but already two orders of magnitude beyond the threshold current needed for the motion of Bloch-type Skyrmions in chiral magnets. Enhancement of magnon – phonon would deliver improvements in the generation of spin current of at least four times by minimizing energy losses by acoustic wave reflectors. When forming an optimized acoustic cavity, the improvement of spin current generation no longer depends only in the energy losses but the enhancement of the magnon – phonon coupling strength.
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Abstract of our recent work
Surface acoustic waves (SAWs) in the GHz frequency range can inject spin currents dynamically into adjacent non-magnetic layers via the spin pumping effect associated with ferromagnetic resonance. Here, we demonstrate an enhancement of acoustic ferromagnetic resonance and spin current generation by a pair of SAW reflector gratings, which form an acoustic analog of the distributed Bragg reflector cavity. In the experiment, we confirmed 2.04±0.02 times larger SAW power absorption in a device with cavity than in the case of no acoustic cavity. We confirmed up to 2.96±0.02 times larger spin current generation by measuring electric voltages generated by the inverse Edelstein effect at the interface between Cu and Bi2O3. The results suggest that acoustic cavities would be useful to enhance the conversion efficiency in SAW driven coupled magnon–phonon dynamics. 

Read more
Enhancement of acoustic spin pumping by acoustic distributed Bragg reflector cavity
Y. Hwang, J. Puebla, M. Xu, A. Lagarrigue, K. Kondou and Y. Otani
Appl. Phys. Lett. 116, 252404 (2020), Editor's pick
aip.scitation.org/doi/10.1063/5.0011799 

Context of project
Nowadays, our society has to deal with an enormous and ever increasing amount of data. Although, this data revolution has many positives to take for our lives, there are concerns regarding the sustainability of new technologies to fulfill the need of larger capabilities for data storage and rapid exchange of information.         ​​ 
We were invited to write a review/perspective article to cover this very topic, the efforts towards the development of energy-efficient storage devices. We recapitulated part of the main directions of the last years of research in condensed matter in storage mechanisms and energy harvesting.   
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Abstract of our review
The current data revolution has, in part, been enabled by decades of research into magnetism and spin phenomena. For example, milestones such as the observation of giant magnetoresistance, and the resulting development of the spin-valve read head, continue to motivate device research. However, the ever-growing need for higher data processing speeds and larger data storage capabilities has caused a significant increase in energy consumption and environmental concerns. Ongoing research and development in spintronics should therefore reduce energy consumption while increasing information processing capabilities. Here, we provide an overview of the current status of research and technology developments in data storage and spin-mediated energy harvesting in relation to energy-efficient technologies. We give our perspective on the advantages and outstanding issues for various data-storage concepts, and energy conversion mechanisms enabled by spin. 

Read more
Spintronic devices for energy-efficient data storage and energy harvesting
J. Puebla, J. Kim, K. Kondou and Y. Otani
Communications Materials, 1, 24 (2020) 
 www.nature.com/articles/s43246-020-0022-5 (open access)

Context of project
​Arguably, one of the most enlightening works by the late physicist Charles Kittel is the theoretical description of the ferromagnetic resonance absorption, published first in 1947 [1] and extended to shape anistropies in 1948 [2]. At these early works, Kittel described the magnetization dynamics exerted in a ferromagnetic specimen at resonance conditions. Nowadays, the FMR is a versatile tool that allows studying magnetization dynamics in thin films, spin waves, magnetization switching and spin pumping. Remarkably, 10 years after his description of FMR, it was the same Charles Kittel who first formulated the coupling of spin waves (magnons) and lattice vibrations (phonons) at resonance conditions, giving origin to the acoustic excited FMR [3]. 

​ Kittel's early works reference: 
[1] journals.aps.org/pr/abstract/10.1103/PhysRev.71.270.2
[2] https://journals.aps.org/pr/abstract/10.1103/PhysRev.73.155
[3] https://journals.aps.org/pr/abstract/10.1103/PhysRev.110.836 

Our contribution
In the context of an invited contribution to the special issue on voltage control of nanomagnetism in the Journal of Physics D of the Institute of Physics (IOP), we published an overview of the current state of acoustic ferromagnetic resonance, a field we have been contributing in the last years. Our overview gives first a description of the mechanism of acoustic ferromagnetic resonance excited by surface acoustic waves, then we describe the generation of spin currents by a mechanism called acoustic spin pumping, and we also described briefly a relatively new related subject, the magneto-rotation coupling, a coupling of the rotation of the lattice with magnetic anisotropies. In memory of Prof. Charles Kittel who passed away the last May 2019 at the age of 102 years old.    

Read more
Acoustic ferromagnetic resonance and spin pumping induced by surface acoustic waves
J. Puebla, M. Xu, B. Rana, K. Yamamoto, S. Maekawa and Y. Otani
Journal of Physics D: Applied Physics, Vol. 53, No. 26 (2020) 
https://iopscience.iop.org/article/10.1088/1361-6463/ab7efe  
arXiv: https://arxiv.org/abs/2001.09581 

Context of project
​Research on single quantum dots has delivered breakthrough studies taking advantage of these artificial atom-like systems. One of the main properties of single dots is the relatively good isolation of carriers from the surrounding environment. Nonetheless, such good isolation complicates the deterministic control of changes coming in and out of the dots. In particular, such charge control is essential for applications as single photon sources and robust spins control.    
    
Review on coherent optical control of spin states in semiconductor quantum dots ref:
[1] iopscience.iop.org/article/10.1088/0268-1242/25/10/103001/meta

Our work 
We demonstrate control by applied electric field of the charge states in single self-assembled InP quantum dots placed in GaInP Schottky structures grown by metalorganic vapor phase epitaxy. This has been enabled by growth optimization leading to suppression of formation of large dots uncontrollably accumulating charge. Using bias- and polarization-dependent micro-photoluminescence, we identify the exciton multiparticle states and carry out a systematic study of the neutral exciton state dipole moment and polarizability. This analysis allows for the characterization of the exciton wave-function properties at the single-dot level for this type of quantum dot. Photocurrent measurements allow further characterization of exciton properties by electrical means, opening new possibilities for resonant excitation studies for such systems. 

Read more:
​Charge control in InP/(Ga, In) P single quantum dots embedded in Schottky diodes
O.D.D. Couto Jr, J. Puebla, E.A. Chekhovich, I.J. Luxmoore, C.J. Elliott, N. Babazadeh, M.S. Skolnick, A.I. Tartakovskii, A.B. Krysa
Physical Review B 84 (12), 125301 (2011)
journals.aps.org/prb/abstract/10.1103/PhysRevB.84.125301
Personal contribution: Device fabrication, performed experiments and analyze data

Context of project
​The achievement of a highly coherent and robust qubit is perhaps the main milestone in the race for the development of quantum information technologies. Early on, it was recognized that single carrier spins in III-V semiconductor quantum dots offer a very attractive technological platform, as well as properties for robust operation and optical accessibility. Naturally, the electron spin emerged as the primary candidate, however, the decoherence induced by the nuclear spin environment limits drastically its capabilities. Interestingly, the interaction between hole spins and nuclear spins is about 10% of that with the electrons. The caveat here is to find the appropriate material system or conditions to isolate and optically control a single hole spin.        

Review on coherent optical control of spin states in semiconductor quantum dots ref:
[1] iopscience.iop.org/article/10.1088/0268-1242/25/10/103001/meta

Our work 
[1] We demonstrate coherent optical control of a single hole spin confined to an InAs/GaAs quantum dot. A superposition of hole-spin states is created by fast (10–100 ps) dissociation of a spin-polarized electron hole pair. Full control of the hole spin is achieved by combining coherent rotations about two axes: Larmor precession of the hole spin about an external Voigt geometry magnetic field, and rotation about the optical axis due to the geometric phase shift induced by a picosecond laser pulse resonant with the hole trion transition.  

Read more:
​Coherent optical control of the spin of a single hole in an InAs/GaAs quantum dot
T.M. Godden, J.H. Quilter, A.J. Ramsay, Y. Wu, P. Brereton, S.J. Boyle, I.J. Luxmoore, J. Puebla-Nunez, A.M. Fox, M.S. Skolnick
Physical review letters 108 (1), 017402 (2012)
journals.aps.org/prl/abstract/10.1103/PhysRevLett.108.017402
arXiv: arxiv.org/abs/1106.6282​
Personal contribution: Device fabrication and device operation advice during experiments

[2] The preparation of a coherent heavy-hole spin via ionization of a spin-polarized electron-hole pair in an InAs/GaAs quantum dot in a Voigt geometry magnetic field is experimentally investigated. For a dot with a typical bright-exciton fine-structure splitting of 17μeV, the fidelity of the spin preparation is limited to 0.75, with optimum preparation occurring when the effective fine structure of the bright exciton matches the in-plane hole Zeeman energy. In principle, higher fidelities can be achieved byminimizing the bright-exciton fine-structure splitting.

Fast preparation of a single-hole spin in an InAs/GaAs quantum dot in a Voigt-geometry magnetic field
T.M. Godden, J.H. Quilter, A.J. Ramsay, Y. Wu, P. Brereton, I.J. Luxmoore, J. Puebla, A.M. Fox, M.S. Skolnick
Physical Review B 85 (15), 155310 (2012)
https://journals.aps.org/prb/abstract/10.1103/PhysRevB.85.155310 
​Personal contribution: Device fabrication and device operation advice during experiments

Context of project
​In-depth structural analysis of individual quantum dots is highly desirable, however, most of the available techniques are destructive. Ideally, we would like to find a noninvasive characterisation technique with capabilities for single dot studies. Optical detected magnetic resonance of nuclear spins is a well developed noninvasive technique that brings valuable structural information. The spin precession induced by the magnetic resonance condition strongly depends on the type of nucleus element, hence delivering information of element contents. However, in self-ensemble quantum dots the developed strain induces quadrupole broadening of the nuclear magnetic resonance spectra. Finding a way around this experimental limitation would allow noninvasive structural analysis of single dots, and moreover, may find applications as path for developing quantum information operations with nuclear spins.     

Nuclear magnetic resonance of optically pumped GaAs quantum wells ref:
[1] journals.aps.org/prl/abstract/10.1103/PhysRevLett.72.1368

Our work 
We realize the growth of self-catalyzed core–shell GaAs/GaAsP nanowires (NWs) on Si substrates using molecular-beam epitaxy. Transmission electron microscopy of single GaAs/GaAsP NWs demonstrates their high crystal quality and shows domination of the GaAs zinc-blende phase. Using continuous-wave and time-resolved photoluminescence (PL), we make a detailed comparison with uncapped GaAs NWs to emphasize the effect of the GaAsP capping in suppressing the nonradiative surface states. Significant PL enhancement in the core–shell structures exceeding 3 orders of magnitude at 10 K is observed; in uncapped NWs PL is quenched at 60 K, whereas single core–shell GaAs/GaAsP structures exhibit bright emission even at room temperature. From analysis of the PL temperature dependence in both types of NW we are able to determine the main carrier escape mechanisms leading to the PL quench.

Read more:
​Structural analysis of strained quantum dots using nuclear magnetic resonance
E.A. Chekhovich, K.V. Kavokin, J. Puebla, A.B. Krysa, M. Hopkinson, A.D. Andreev, A.M. Sanchez, R. Beanland, M.S. Skolnick, A.I. Tartakovskii
Nature nanotechnology 7 (10), 646 (2012)
www.nature.com/articles/nnano.2012.142
arXiv: arxiv.org/abs/1112.4079 ​
News and Views: www.nature.com/articles/nnano.2012.171?draft=collection&proof=true&platform=oscar
Personal contribution: Sample preparation, single dot mapping and dynamic nuclear polarization study

Context of project
​A long standing technological challenge is the integration of high optical quality III-V semiconductor nanostructures with the mainstream of electronic industry based on Silicon. Such integration may allow the development of single photon sources, light mediated quantum information and novel photovoltaic modules.       

III-V nanowires on Silicon ref:
[1] www.nature.com/articles/s41467-019-08807-9

Our work 
We realize the growth of self-catalyzed core–shell GaAs/GaAsP nanowires (NWs) on Si substrates using molecular-beam epitaxy. Transmission electron microscopy of single GaAs/GaAsP NWs demonstrates their high crystal quality and shows domination of the GaAs zinc-blende phase. Using continuous-wave and time-resolved photoluminescence (PL), we make a detailed comparison with uncapped GaAs NWs to emphasize the effect of the GaAsP capping in suppressing the nonradiative surface states. Significant PL enhancement in the core–shell structures exceeding 3 orders of magnitude at 10 K is observed; in uncapped NWs PL is quenched at 60 K, whereas single core–shell GaAs/GaAsP structures exhibit bright emission even at room temperature. From analysis of the PL temperature dependence in both types of NW we are able to determine the main carrier escape mechanisms leading to the PL quench.

Read more:
​Effect of a GaAsP shell on the optical properties of self-catalyzed GaAs nanowires grown on silicon
O.D.D. Couto Jr, D. Sercombe, J. Puebla, L. Otubo, I.J. Luxmoore, M. Sich, T.J. Elliott, E.A. Chekhovich, L.R. Wilson, M.S. Skolnick, H.Y. Liu, A.I. Tartakovskii
Nano letters 12 (10), 5269-5274 (2012)
https://pubs.acs.org/doi/10.1021/nl302490y
arXiv: arxiv.org/abs/1206.4857 
Personal contribution: Nanowire sample mapping and high resolution microscopy

Context of project
​One of the most studied topics in solid state physics of semiconductor nanostructures is the control of single electron spins confined in 2-dimensions (nanowires) and 1-dimension (quantum dots). In particular, for self-ensemble quantum dots (QDs), one main source of electron spin decoherence are the nuclear spins. A single QD one single electron spin can be interacting with tens of thousands of incoherent nuclear spins. One popular approach to diminish the influence of the incoherent nuclear spin bath is known as dynamic nuclear polarisation (DNP). The DNP consists on transferring spin polarisation from the electrons to the nuclei. Ideally, an efficient and fast transfer of spin polarisation would result in a 100% polarisation of the nuclear spin bath, therefore fading this source of decoherence for electron spins.     

Dynamic nuclear polarisation with single electron spins ref:
[1] journals.aps.org/prl/abstract/10.1103/PhysRevLett.100.067601

Our work
We study experimentally the dependence of dynamic nuclear spin polarization on the power of nonresonant optical excitation in two types of individual neutral semiconductor quantum dots: InGaAs/GaAs and GaAs/AlGaAs. We show that the mechanism of nuclear spin pumping via second-order recombination of optically forbidden (“dark”) exciton states reported in InP/GaInP quantum dots [E. A. Chekhovich et al., Phys. Rev. B 83, 125318 (2011)] is relevant for material systems considered in this work. In the InGaAs/GaAs dots this nuclear spin polarization mechanism is particularly pronounced, resulting in Overhauser shifts up to ∼80 μeV achieved at ultralow optical excitation power, ∼1000 times smaller than the power required to saturate ground state excitons. The Overhauser shifts observed at ultralow power pumping in the interface GaAs/AlGaAs dots are generally found to be smaller (up to∼40 μeV). Furthermore in GaAs/AlGaAs we observe dot-to-dot variation and even sign reversal of the Overhauser shift which is attributed to the dark-bright exciton mixing originating from electron-hole exchange interaction in dots with reduced symmetry. Nuclear spin polarization degrees reported in this work under ultralow-power optical pumping are comparable to those achieved by techniques such as resonant optical pumping or above-gap pumping with high-power circularly polarized light. Dynamic nuclear polarization via second-order recombination of “dark” excitons may become a useful tool in single quantum dot applications, where manipulation of the nuclear spin environment or electron spin is required.

Read more:
​Dynamic nuclear polarization in InGaAs/GaAs and GaAs/AlGaAs quantum dots under non-resonant ultra-low power optical excitation
J. Puebla, E.A. Chekhovich, M. Hopkinson, P. Senellart, A. Lemaitre, M.S. Skolnick, A.I. Tartakovskii
Phys. Rev. B 88 (4), 9 (2013)
journals.aps.org/prb/abstract/10.1103/PhysRevB.88.045306
arXiv: arxiv.org/abs/1306.0469
Personal contribution: Device fabrication, spectroscopy experiments, data analysis and manuscript writing 

Context of project
​Liquid helium systems are the most used cryostats. However, the so-call wet systems are facing a critical period as a result of increasing cost and storage issues of liquid helium. Consequently, in many research labs and tech companies there are a rapidly growing demand for close cycle systems (so-called dry Cryo-systems). When it comes to applications such as SPM (Scanning Probe Microscopy), so far the residual vibrations created by the cryo-cooling mechanism in close cycle cryostats are a limiting factor. In this project we aim at reducing the spurious vibrations that limit cutting edge SPM characterization.  

Our work 
We report on state-of-the-art scanning probe microscopy measurements performed in a pulse tube based top- loading closed-cycle cryostat with a base temperature of 4K and a 9T magnet. We introduced measures to reduce the level of mechanical and acoustic noise coupling into the system to enable scanning probe experiments. The extremely low vibration amplitudes in our system enabled successful imaging of 0.39nm lattice steps on single crystalline SrTiO3 as well as magnetic vortices in Bi2Sr2CaCu2O8+x superconductor. Fine control over sample temperature and applied magnetic field further enabled us to probe the helimagnetic and the elusive skyrmion-lattice phases in Fe0:5Co0:5Si with unprecedented signal-to-noise ratio of 20:1. Finally, we demonstrate for the first time quartz-crystal tuning fork shear-force microscopy in a closed-cycle cryostat.​ 

Read more:
​Scanning probe microscopy in an ultra-low vibration closed-cycle cryostat: Skyrmion lattice detection and tuning fork implementation
F.P. Quacquarelli, J. Puebla, T. Scheler, D. Andres, C. Bödefeld, B. Sipos,  C. Dal Savio, A. Bauer, C. Pfleiderer, A. Erb, K. Karrai
Microscopy Today 23 (6), 12-17 (2015)
www.cambridge.org/core/journals/microscopy-today/article/scanning-probe-microscopy-in-an-ultralow-vibration-closedcycle-cryostat-skyrmion-lattice-detection-and-tuning-fork-implementation/3C35B165DBB2572559EA0F2C94E73B15#
arXiv: arxiv.org/abs/1404.2046 
​Personal contribution: Vibration analysis, decoupling of vibration from sample space, assistance on scanning probe microscopy