Charge Variations in Cuprate Superconductors from Nuclear Magnetic Resonance

Charge inhomogeneities in the cuprates were reported early on and have been in the focus of much research recently. Nuclear magnetic resonance (NMR) is very sensitive to local charge symmetry through the electric quadrupole interaction that must detect any static charge density variation. Recent experiments in high magnetic fields that seem to induce charge density waves in some systems have rekindled the interest in static inhomogeneities. It has long been known that excessive NMR linewidths can be observed in all cuprates, but with the exception of a few materials. However, the relation of the quadrupolar linewidths with respect to variations of the charge density in the cuprates is not understood. Here, we investigate YBa₂Cu₃O₇ and we find even in a moderate magnetic field that below about 200 K, i.e., well above T _c, a temperature dependent NMR linewidth appears that must be related to incipient static charge density variations. We argue that this establishes field induced charge density variation as a more general phenomenon in the cuprates. In view of the very recent understanding of the relation between the hole distribution in the CuO₂ plane and T _c, it is argued that charge density variations are ubiquitous, but appear not related to the maximum T _c.     

SQUID-Detected Magnetic Resonance Imaging in Microtesla Magnetic Fields

We describe studies of nuclear magnetic resonance (NMR) spectroscopy and magnetic resonance imaging (MRI) of liquid samples at room temperature in microtesla magnetic fields. The nuclear spins are prepolarized in a strong transient field. The magnetic signals generated by the precessing spins, which range in frequency from tens of Hz to several kHz, are detected by a low-transition temperature dc SQUID (Superconducting QUantum Interference Device) coupled to an untuned, superconducting flux transformer configured as an axial gradiometer. The combination of prepolarization and frequency-independent detector sensitivity results in a high signal-to-noise ratio and high spectral resolution (～1 Hz) even in grossly inhomogeneous magnetic fields. In the NMR experiments, the high spectral resolution enables us to detect the 10-Hz splitting of the spectrum of protons due to their scalar coupling to a ³¹P nucleus. Furthermore, the broadband detection scheme combined with a non-resonant field-reversal spin echo allows the simultaneous observation of signals from protons and ³¹P nuclei, even though their NMR resonance frequencies differ by a factor of 2.5. We extend our methodology to MRI in microtesla fields, where the high spectral resolution translates into high spatial resolution. We demonstrate two-dimensional images of a mineral oil phantom and slices of peppers, with a spatial resolution of about 1 mm. We also image an intact pepper using slice selection, again with 1-mm resolution. In further experiments we demonstrate T₁-contrast imaging of a water phantom, some parts of which were doped with a paramagnetic salt to reduce the longitudinal relaxation time T₁. Possible applications of this MRI technique include screening for tumors and integration with existing multichannel SQUID systems for brain imaging.     

磁共振成像的3D头面部模型的标准化处理

通过磁共振成像技术获取人体头部的系列二维图像,分别提取其边界坐标数据,并采用傅里叶级数拟合各层的外形轮廓,建立了一种分层描述的3D头部参数化模型。然后根据人类面部主要特征定义了若干特征层,并基于面部特征相匹配的原则,对多个样本进行了平均处理,构造出3D标准头面部模型,能够反映特定人群的综合特点。作为实例,用558个样本计算构造了圆高标准头型,可作为头面部装备设计的依据。     

Magnetic Resonance Force Microscopy of a Permalloy Microstrip Array

The ferromagnetic resonance in an array of permalloy microstrips 3000 × 500 × 30 nm in size ordered on a rectangular grid 3.5 × 6 μm in size has been investigated by magnetic resonance force microscopy. The dependences of magnetic resonance force microscopy spectra of a sample on the probe–sample distance are analyzed. The possibility of detection of a ferromagnetic resonance spectrum of a single microstrip is demonstrated.     

Nuclear Magnetic Resonance and the BCS Theory

The author describes the inspiration for the experiment by Hebel and Slichter to measure the nuclear spin–lattice relaxation time in superconductors, the design considerations for the experiment, the surprising experimental results, their theoretical treatment using the Bardeen-Cooper–Schrieffer theory, and how comparing the nuclear relaxation results with those for ultrasound absorption confirmed the central idea of the BCS theory, the BCS pair wave function. An erratum to this article can be found at     

Features of a Silver-Beam Magnetic Resonance Apparatus as a Frequency Standard

Silver has two stable isotopes Ag107 and Ag109 with nearly equal populations (51.35 percent and 48.65 percent). A microwave magnetic resonance apparatus employing a silver beam and simultaneous magnetic resonance of both isotopes was built and a so-called superposition line was measured. The possibility of using such an apparatus as a frequency standard is discussed.     

磁共振成像系统图像均匀性的检测

医用磁共振成像(MRI)系统是大型医用影像诊断设备,如何对磁共振影像进行评价,图像均匀性是一项重要指标.本文根据医用磁共振成像(MRI)系统的技术特性,参考国际标准和国家标准,验证、推荐了一种图像均匀性检测方法,该方法科学性、适用性强,可作为计量检测、医院质量控制的技术依据.     

基于区域视角的单位内部安全防范标准研制探讨

本文介绍了区域性单位内部安全防范标准的研究背景。以广州市为例,探讨了基于区域视角的单位内部安全防范标准边界和概念内涵,分析标准研制过程的重点和难点,提出了区域性单位内部安全防范标准的基本框架和主要内容。     

人性化视角下的疫情防护用品设计

2019年12月,湖北省武汉市发生了新型冠状病毒肺炎疫情,随着疫情的蔓延,全国都进入了紧急防护阶段。同时,各医院医生护士也投入到紧张的疫情防护工作中。而在此期间,医用防护用品也暴露出了许多问题。为了改良当前疫情防护用品设计,使疫情防护产品更人性化,提高医护人员使用时的舒适性、安全性和有效性,在分析使用者的行为特征以及当前疫情防护用品的设计痛点后,从人机工程、CMF考虑,产品功能、造型设计、产品安全、用户心理等方面进行探讨,得到了从人性化视角考虑的疫情防护用品设计的指导措施。该设计研究可为人性化疫情医用防护产品设计提供理论依据和应用参考。     

Proton Magnetic Resonance Imaging of Specimens in Simulated Microgravity

In situ proton stray-field (STRAFI) nuclear magnetic resonance (NMR) imaging of diamagnetic samples in simulated microgravity is described. The magnetic field gradient at the edge of the superconductive coil in the magnet of a NMR spectrometer is exploited to levitate diamagnetic samples and, at the same time, obtain their corresponding one-dimensional NMR images. Proton T ₂ (spin-spin relaxation time)/ diffusion-weighted profiles of samples under simulated microgravity conditions are acquired with a solid-echo radiofrequency pulse sequence. Since water is a diamagnetic main constituent substance of living systems, the methodology described could be useful to monitor non-destructively changes in the NMR properties of water protons induced by simulated microgravity and magnetic forces. Furthermore, this approach may be used to observe other NMR-sensitive nuclei of interest, e.g., fluor-19, phosphorous-31, carbon-13, etc.     

Dielectric, Electron Paramagnetic Resonance, and Transport Properties of Spanish Moss

We present preliminary data on the electric resistivity, ac dielectric response, variable temperature electron paramagnetic resonance (EPR) and magnetization measurements on Spanish moss. We find that the four-probe dc resistivity is about 10 ohm-meter, the dc dielectric constant is 76 ± 1, close to that of water (80), and the ac dielectric measurements reveal well-defined peaks whose maxima depend on frequency and temperature, with an Arrhenius activation energy of ∼ 0.6 ± 0.1 eV. Magnetization measurements showed diamagnetism down to about 6K and then a Curie tail. The EPR measurements reveal a level of about 10 to 100 parts per million of isolated Mn²⁺ ions and a ten-fold higher level of Fe³⁺ cluster peaks, the later of which were strongly temperature dependent down to 5K. The origin of the Fe³⁺ structure remains unclear.     

Cyclotron Resonance in ZnO Heterostructures at High Magnetic Fields

The carrier-density dependence of the cyclotron resonance was studied in ZnMgO/ZnO heterostructures between n=5.4×10¹² cm^?2 and n=7.5×10¹² cm^?2. The effective mass was obtained as m ^?=0.32m ₀, and the significant carrier-density dependence of the effective mass was not observed in the present samples. This is partially due to the suppression of the resonant polaron effect in the dense carrier system. Oscillatory behavior appears in the cyclotron resonance spectra at the higher magnetic field side of the resonant field. The oscillatory period is perfectly coincident with the one of the Shubnikov-de Hass oscillation.     

Regional Cultural Differences Within European Countries: Evidence from Multi-Country Surveys

The purpose of this study is to investigate the patterns of regional cultural differences within European countries. This study extends our previous work (Kaasa et al. 2013) that used data from the European Social Survey (ESS), by using more recent data and combining the ESS and the European Values Study (EVS) as data sources. We aim to study how much attention should be paid to within-country differences and how they differ across countries. The indicators of cultural dimensions were created using confirmatory factor analysis based on the initial indicators selected relying on Hofstede’s cultural dimensions. The extent and character of within-country cultural differences were examined. Our results reveal a wide diversity of regional cultural variability among the observed countries. We identified countries where within-country cultural variability is larger (Spain, Portugal, France) and smaller (Finland, Sweden, Norway) than cross-national variability. Also, in many countries a more detailed regional division (more and smaller regions) brought out larger differences, although this cannot be taken as a rule. Patterns along regions differ across cultural dimensions, making grouping regions difficult. Studying regional cultural differences within countries is important. The extent of the optimal depth of regional division applied depends on the particular country, its regional cultural variability, and the needs and aims of the researcher.     

Coil and Magnet Design for Nuclear Magnetic Resonance in Inhomogeneous Field

High-resolution nuclear magnetic resonance (NMR) spectrometry is possible in nonhomogeneous magnetic fields-such as those in portable equipment-if the static and the radio-frequency (RF) magnetic fields are perpendicular and correlated in the measurement volume. From the easy-axis rotation theorem and the Amperian currents model, it is possible to exactly match two magnetic fields in two-dimensional systems. We derive a basic probe element that fulfills these conditions. Then we present a portable NMR probe design. The static and RF magnetic fields of the probe are matched on a large volume     

Designing Static Fields for Unilateral Magnetic Resonance by a Scalar Potential Approach

We present a method for designing single-sided magnets suitable for unilateral magnetic resonance (UMR) measurements. The method uses metal pole pieces to shape the field from permanent magnets in a target region. The pole pieces are shaped according to solutions to Laplace's equation, and can be designed using a combination of analytical methods and numerical optimization. The design leads to analytical expressions for the pole piece shapes and magnetic field. Here, we develop the method in Cartesian, polar, and spherical coordinates, and discuss the merits of each system. Finite magnet size has a substantial effect on the field quality in many cases, according to our simulations. We found that in order to achieve a compact magnet in which the static field closely matches that specified, a full 3-D design approach is necessary. A magnet designed by our method produces a static field with a constant gradient over a region 2 cm in diameter and 2 mm thick. This leads to a compact cylindrical magnet just over 11 cm in diameter, topped with a single metal pole piece. The design is validated through simulation. The simulated field is found to agree closely with that specified analytically through the design procedure     

Passive Shim Design and a Shimming Approach for Biplanar Permanent Magnetic Resonance Imaging Magnets

This paper presents a new passive shim design method and a novel shimming procedure to correct the magnetic field inhomogeneities generated by C-shape permanent biplanar magnetic resonance imaging magnets. The method expresses the shim distribution as a sum of orthogonal functions multiplied by unknown amplitudes. The oscillating modes of the shim magnetization-thickness function are normalized within a finite disk. By minimizing the shim set weight and constraining the magnetization-thickness function, the method produces a continuous map of the required shim contribution. The map defines the shim shape and a discrete process then determines the regions where no shim contributions are needed. With this methodology, passive shims capable of generating magnetic field harmonics with minimal impurities and ferro-shim pieces can be generated. The paper reports a study of magnetic coupling among the iron pieces and its influence over the magnetic field harmonics of linear and nonlinear iron, and demonstrates that the exclusion of the magnetic coupling in the shimming process produces an unacceptable error in the final shimmed field homogeneity. The proper selection and arrangement of individual shim sizes produces a better conditioned field source matrix and hence improves the design. A number of examples show that the new method can effectively cancel target impurity harmonics while controlling high-order harmonics.