arXiv subjects

Andrea Morales

Publications and source records attributed to Andrea Morales.

11 recordsLinked to original sources

arXiv2026

Quantitative imaging of Abrikosov vortices by scanning quantum magnetometry

Understanding vortex matter in type-II superconductors is central to controlling dissipation and flux pinning in superconducting materials and devices. Here, we use cryogenic scanning nitrogen vacancy magnetometry (NVM) to image Abrikosov vortices in the cuprate superconductors BSCCO-2212 and YBCO under controlled field-cooled conditions. Measurements, which are performed using continuous-wave optically detected magnetic resonance (cw-ODMR) in a closed-cycle cryostat, yield quantitative magnetic-field maps with nanoscale spatial resolution. In BSCCO-2212 at 71 K, we resolve a well-ordered triangular vortex lattice, whose symmetry and spacing are confirmed through 2D Fourier analysis and are consistent with flux quantization. YBCO thin films imaged at 3 K exhibit a more disordered vortex arrangement reflecting stronger pinning, while maintaining quantitative agreement between measured vortex density and the applied magnetic field. These results render our cryogenic scanning NVM a reliable quantitative tool for real-space studies of vortices in high-$T_c$ superconductors, in particular since such a remarkable magnetic resolution has been achieved within relatively short acquisition times of 2 to 4 h.

cond-mat.supr-con↗

arXiv2025

Nanoscale magnetometry of a synthetic three-dimensional spin texture

Multilayered synthetic antiferromagnets (SAFs) are artificial three-dimensional (3D) architectures engineered to create novel, complex, and stable spin textures. Non-invasive and quantitative nanoscale magnetic imaging of the two-dimensional stray field profile at the sample surface is essential for understanding the fundamental properties of the spin-structure and being able to tailor them to achieve new functionalities. However, the deterministic detection of spin textures and their quantitative characterization at the nanoscale remain challenging. Here, we use nitrogen-vacancy scanning probe microscopy (NV-SPM) under ambient conditions to perform the first quantitative vector-field magnetometry measurements in the multilayered SAF [(Co/Pt)$_5$/Co/Ru]$_3$/(Co/Pt)$_6$. We investigate nanoscale static and dynamic properties of antiferromagnetic domains with boundaries hosting ``one-dimensional'' ferromagnetic stripes with ~ 100 nm of width and periodic modulation of the magnetization. By employing NV-SPM measurements in different imaging modes and involving NV-probes with various crystallographic orientations, we demonstrated distinct fingerprints emerging from GHz-range spin noise and constant stray fields on the order of several mT. This provides quantitative insights into the structure of domains and domain walls, as well as, into magnetic noise associated with thermal spin-waves. Our work opens up new opportunities for quantitative vector-field magnetometry of modern magnetic materials with tailored 3D spin textures and stray field profiles, and potentially novel spin-wave dispersions--in a quantitative and non-invasive manner, with exceptional magnetic sensitivity and nanometer scale spatial resolution.

cond-mat.mes-hall↗

arXiv2025

Minimizing sensor-sample distances in scanning nitrogen-vacancy magnetometry

Scanning magnetometry with nitrogen-vacancy (NV) centers in diamond has led to significant advances in the sensitive imaging of magnetic systems. The spatial resolution of the technique, however, remains limited to tens to hundreds of nanometers, even for probes where NV centers are engineered within 10 nm from the tip apex. Here, we present a correlated investigation of the crucial parameters that determine the spatial resolution: the mechanical and magnetic stand-off distances, as well as the sub-surface NV center depth in diamond. We study their contributions using mechanical approach curves, photoluminescence measurements, magnetometry scans, and nuclear magnetic resonance (NMR) spectroscopy of surface adsorbates. We first show that the stand-off distance is mainly limited by features on the surface of the diamond tip, hindering mechanical access. Next, we demonstrate that frequency-modulated atomic force microscopy (FM-AFM) feedback partially overcomes this issue, leading to closer and more consistent magnetic stand-off distances (26-87 nm) compared to the more common amplitude-modulated (AM-AFM) feedback (43-128 nm). FM operation thus permits improved magnetic imaging of sub-100-nm spin textures, shown for the spin cycloid in BFO and domain walls in a CoFeB synthetic antiferromagnet. Finally, by examining 1H and 19F NMR signals in soft contact with a polytetrafluoroethylene surface, we demonstrate a minimum NV-to-sample distance of 7.9+/-0.4 nm.

cond-mat.mes-hall↗

arXiv2025

Commercial scanning nitrogen vacancy magnetometer in a closed-cycle cryostat

The ability to measure magnetic fields on the nanometre scale at cryogenic temperatures is key to understand magnetism on the quantum level and to develop materials for new storage devices or quantum computers. Nitrogen vacancy (NV) centres in diamond have proven to be a robust means to harness quantum sensing for such applications. We have developed an instrument to measure the magnetic stray field of a sample with nanometre resolution from 2 K to 300 K and that accepts samples without additional preparation, especially the need to prepare a microwave line on the sample. The instrument features a software-interface for controlling and synchronising all included optical, mechanical and electronic devices and which analyses the acquired information in real time. We present the key features and measurement results achieved with atomic force microscopy (AFM) tips hosting an NV centre and a fully remote controllable microscope platform. The magnetometer is commercially available, and a first demonstrator has been installed in a research facility. We show micro Tesla per square root Hertz sensitivity, low noise AFM tip control and optically detected resonance scans in a closed-cycle cryostat.

quant-ph↗

arXiv2022

Multimode-polariton superradiance via Floquet engineering

We consider an ensemble of ultracold bosonic atoms within a near-planar cavity, driven by a far detuned laser whose phase is modulated at a frequency comparable to the transverse cavity mode spacing. We show that a strong, dispersive atom-photon coupling can be reached for many transverse cavity modes at once. The resulting Floquet polaritons involve a superposition of a set of cavity modes with a density excitation of the atomic cloud. The mutual interactions between these modes lead to distinct avoided crossings between the polaritons. Increasing the laser drive intensity, a low-lying multimode Floquet polariton softens and eventually becomes undamped, corresponding to the transition to a superradiant, self-organized phase. We demonstrate the stability of the stationary state for a broad range of parameters.

quant-ph↗

arXiv2020

Measuring the dynamics of a first order structural phase transition between two configurations of a superradiant crystal

We observe a structural phase transition between two configurations of a superradiant crystal by coupling a Bose-Einstein condensate to an optical cavity and applying imbalanced transverse pump fields. We find that this first order phase transition is accompanied by transient dynamics of the order parameter which we measure in real-time. The phase transition and the excitation spectrum can be derived from a microscopic Hamiltonian in quantitative agreement with our experimental data.

cond-mat.quant-gas↗

arXiv2019

Two-mode Dicke model from non-degenerate polarization modes

We realize a non-degenerate two-mode Dicke model with competing interactions in a Bose-Einstein condensate (BEC) coupled to two orthogonal polarization modes of a single optical cavity. The BEC is coupled to the cavity modes via the scalar and vectorial part of the atomic polarizability. We can independently change these couplings and determine their effect on a self-organization phase transition. Measuring the phases of the system, we characterize a crossover from a single-mode to a two-mode Dicke model. This work provides perspectives for the realization of coupled phases of spin and density.

cond-mat.quant-gas↗

arXiv2017

Coupling two order parameters in a quantum gas

Controlling matter to simultaneously support multiple coupled properties is of fundamental and technological importance. For example, the simultaneous presence of magnetic and ferroelectric orders in multiferroic materials leads to enhanced functionalities. In high-temperature superconductors, intertwining between charge- and spin-order can form superconducting states at high transition temperatures. However, pinning down the microscopic mechanisms responsible for the simultaneous presence of different orders is difficult, making it hard to predict the phenomenology of a material or to experimentally modify its properties. Here we use a quantum gas to engineer an adjustable interaction at the microscopic level between two orders, and demonstrate scenarios of competition, coexistence and coupling between them. In the latter case, intriguingly, the presence of one order lowers the critical point of the other. Our system is realized by a Bose-Einstein condensate which can undergo self-organization phase transitions in two optical resonators, resulting in two distinct crystalline density orders. We characterize the intertwining between these orders by measuring the composite order parameter and the elementary excitations. We explain our results with a mean-field free energy model, which is derived from a microscopic Hamiltonian. Our system is ideally suited to explore properties of quantum tricritical points as recently realized in and can be extended to study the interplay of spin and density orders also as a function of temperature.

cond-mat.quant-gas↗

arXiv2017

Monitoring and manipulating Higgs and Goldstone modes in a supersolid quantum gas

Access to collective excitations lies at the heart of our understanding of quantum many-body systems. We study the Higgs and Goldstone modes in a supersolid quantum gas that is created by coupling a Bose-Einstein condensate symmetrically to two optical cavities. The cavity fields form a U(1)-symmetric order parameter that can be modulated and monitored along both quadratures in real time. This enables us to measure the excitation energies across the superfluid-supersolid phase transition, establish their amplitude and phase nature, as well as characterize their dynamics from an impulse response. Furthermore, we can give a tunable mass to the Goldstone mode at the crossover between continuous and discrete symmetry by changing the coupling of the quantum gas with either cavity.

cond-mat.quant-gas↗

arXiv2016

Supersolid formation in a quantum gas breaking continuous translational symmetry

The concept of a supersolid state is paradoxical. It combines the crystallization of a many-body system with dissipationless flow of the atoms it is built of. This quantum phase requires the breaking of two continuous symmetries, the phase invariance of a superfluid and the continuous translational invariance to form the crystal,. Proposed for helium almost 50 years ago, experimental verification of supersolidity remains elusive. A variant with only discrete translational symmetry breaking on a preimposed lattice structure, the `lattice supersolid', has been realized based on self-organization of a Bose-Einstein condensate (BEC). However, lattice supersolids do not feature the continuous ground state degeneracy that characterizes the supersolid state as originally proposed. Here we report the realization of a supersolid with continuous translational symmetry breaking. The continuous symmetry emerges from two discrete spatial ones by symmetrically coupling a BEC to the modes of two optical cavities. We establish the phase coherence of the supersolid and find a high ground-state degeneracy by measuring the crystal position over many realizations through the light fields leaking from the cavities. These light fields are also used to monitor the position fluctuations in real-time. Our concept provides a route to creating and studying glassy many-body systems with contrallably lifted ground-state degeneracies, such as supersolids in the presence of disorder.

cond-mat.quant-gas↗

arXiv2014

Optical transport and manipulation of an ultracold atomic cloud using focus-tunable lenses

We present an optical setup with focus-tunable lenses to dynamically control the waist and focus position of a laser beam, in which we transport a trapped ultracold cloud of 87-Rb over a distance of 28 cm. The scheme allows us to shift the focus position at constant waist, providing uniform trapping conditions over the full transport length. The fraction of atoms that are transported over the entire distance comes near to unity, while the heating of the cloud is in the range of a few microkelvin. We characterize the position stability of the focus and show that residual drift rates in focus position can be compensated for by counteracting with the tunable lenses. Beyond being a compact and robust scheme to transport ultracold atoms, the reported control of laser beams makes dynamic tailoring of trapping potentials possible. As an example, we steer the size of the atomic cloud by changing the waist size of the dipole beam.

cond-mat.quant-gas↗