Abstract
Phase transitions in soft matter systems reveal some of the interesting structural phenomena at the levels of individual entities constituting those systems. The relevant energy scales in soft matter systems are comparable to thermal energy (kBT ∼ 10−21 J). This permits one to observe interesting structural dynamics even at ambient conditions. However, at the nanoscale most experimental probes currently being used to study these systems have been either plagued by low sensitivity or are invasive at molecular scales. Nitrogen-vacancy (NV) centers in diamond is emerging as a robust quantum probe for precision metrology of physical quantities (e.g. magnetic field, electric field, temperature, and stress). Here, we demonstrate by using NV sensors to probe spin-fluctuations and temperature simultaneously to obtain information about controlled phase changes in a soft matter material as a function of temperature. The soft matter system chosen for the study is a standard liquid crystalline (LC) material which shows distinct phases close to room temperature. Individual NV centers at depths of a few nm are used as a probe to detect magnetic signals emanating from a few molecular layers of sample on the surface of the diamond. The organization and collective dynamics of LC molecules in nanoscopic volumes are discussed. Our study aims to extend the areas of application of quantum sensing using NV centers to probe the soft matter systems, particularly those exhibiting mesophases and interesting interfacial properties.
Highlights
Some of the fascinating aspects of nature are manifested when the states of matter undergo phase transitions
While the bulk properties of liquid crystalline (LC) are well known, at the nanoscale it remains a major challenge to non-invasively study structure and dynamics, nature of molecular ordering, and other properties which could be markedly distinct from their bulk counterparts
The 8CB molecule has a length of about 2 nm [16] and its structure is shown in figure 1(a). 8CB LC offers a simple model system for understanding phase transitions in two dimensions [17]
Summary
Some of the fascinating aspects of nature are manifested when the states of matter undergo phase transitions. Understanding the micro- and nanoscopic origins of macroscopic (bulk) phase transitions has profound impact in studying superconductivity, magnetic ordering, topological materials, ferroelectricity, superfluidity, rigidity, and fluidity of biological cell membranes etc [1,2,3,4]. Among them liquid crystals, owing to shape anisotropy of the constituent molecules exhibit ordering similar to crystalline solids, while still being able to flow like ordinary liquids [6] They are used as simple model systems to understand more complex phenomena in condensed matter [5, 7], self assembly of molecules in life sciences [8, 9], and even in cosmology [10, 11]. While the bulk properties of liquid crystalline (LC) are well known, at the nanoscale it remains a major challenge to non-invasively study structure and dynamics, nature of molecular ordering, and other properties which could be markedly distinct from their bulk counterparts
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