Magnetic trapping of atomic chromium
Magnetic trapping of atomic chromium
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原子铬的磁捕获
DOI：
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发表时间：
1998
期刊：
影响因子：
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通讯作者：
J. Doyle
中科院分区：
文献类型：
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作者：
J. Weinstein;R. Decarvalho;Jinha Kim;D. Patterson;B. Friedrich;J. Doyle
最近，我们展示了中性物质的缓冲气体加载技术，通过使用它来加载铕原子@1,2#的磁阱。该实验中实现的原子密度（n;10 12 cm 23 ！与磁光陷阱中实现的最高密度相当，并且捕获的数量（N;10 12 ）比目前光力陷阱@3#获得的数量级高一个数量级。缓冲气体方法有望将磁捕获和蒸发冷却扩展到碱金属原子以外的物质，并且应该为生产 各种原子和分子种类的超冷致密样品。在此，我们报告了通过缓冲气体负载对原子铬进行磁捕获。铬具有许多令人感兴趣的特性。它是一种与铕不同的金属；捕获它进一步证明了缓冲气体加载的普遍性。铬的大磁矩为6 m B ~玻尔磁子！允许在升高的缓冲气体温度下捕获， 开启了用简单的泵送液氦低温恒温器捕获它的可能性。对铬的研究也源于其在原子光刻@4,5#中的应用。由于它具有四种天然同位素、三种玻色子和一种费米子，因此捕获可以为研究铬玻色爱因斯坦凝聚物或简并费米气体@6‐9#打开大门。缓冲气体加载原理及 我们设备的具体细节已在之前的论文@1,2# 中概述。下面我们进行简单的描述。我们的磁阱是由两个反亥姆霍兹线圈形成的线性四极场。捕获区域充满氦缓冲气体。缓冲气体通过稀释制冷机维持在低温。在这种情况下，感兴趣的物种〜原子铬！被引入到陷阱中，其中 它通过氦气扩散并通过弹性碰撞快速热化。处于弱寻场磁态的原子被磁场所包含，但随着原子的热分布，它们以由 h 确定的速率从陷阱中蒸发，h 是陷阱深度与原子温度的比率。对于足够大的 h，这种蒸发足够慢，以至于大部分原子可以保留很长时间 足够的时间~非磁性！氦气可以被冷冻泵出捕获区域。这留下了热隔离的、被困的样品。因为该技术仅依赖于与缓冲气体的弹性碰撞和物质的磁性状态，所以它应该适用于可以干净地引入低温环境的任何磁性物质。在我们的实验中，铬原子被产生、热化并被捕获在铜电池内。有 细胞底部的熔融石英窗口允许光学进入〜用于检测和消融！池顶部有一面镜子，用于回射探测光束，以对捕获的样品进行吸收光谱分析。电阻测温法用于确定电池温度。连接到电池的附加电阻用作加热器。该池充满 3 He 或 4 He 缓冲气体。存在足够量的 3 He ( 4 He) 以便在 温度高于 0.3 K ~0.9 K！密度约为10 17 cm 23 。低于这些温度时，密度由氦蒸气压决定。通过单脉冲激光烧蚀同位素纯 52 Cr 固体样品，使 Cr 原子进入气相。固体 52 Cr 位于电池内部捕获区域的边缘。双钇铝石榴石激光器 532 nm 处的典型消融脉冲能量为 25 mJ，用于消融。通过激光吸收光谱在 23 386 cm 21 处的 7 S3$z 7 P3 跃迁上检测到原子
Recently we demonstrated the technique of buffer-gas loading of neutral species by using it to load a magnetic trap with europium atoms @1,2#. The atom density achieved in that experiment ( n;10 12 cm 23 ! is comparable to the highest densities achieved in magneto-optical traps, and the number trapped ( N;10 12 ) is an order of magnitude higher than can currently be obtained with light-force traps @3#. The buffer-gas method promises the extension of magnetic trapping and evaporative cooling to species other than alkali atoms, and should open the way for producing ultracold, dense samples in a variety of atomic and molecular species. Herein, we report the magnetic trapping of atomic chromium via buffer-gas loading. Chromium has many properties of interest. It is a metal dissimilar to europium; trapping it further demonstrates the generality of buffer-gas loading. Chromium’s large magnetic moment of 6 m B ~Bohr magneton! allows for trapping at elevated buffer-gas temperatures, opening up the possibility to trap it with a simple pumped liquid helium cryostat. Study of chromium is also motivated by its applications in atom lithography @4,5#. As it has four naturally occurring isotopes, three bosons and one fermion, trapping could open the door to the study of chromium BoseEinstein condensates or degenerate Fermi gases @6‐9#. The principles of buffer-gas loading and the specifics of our apparatus are outlined in previous papers @1,2#. We give a brief description below. Our magnetic trap is a linear quadrupole field formed by two anti-Helmholtz coils. The trapping region is filled with helium buffer gas. The buffer gas is maintained at cryogenic temperatures by a dilution refrigerator. The species of interest ~atomic chromium in this case! is introduced into the trap, where it diffuses through the helium gas and quickly thermalizes with it via elastic collisions. The atoms in the weak-field-seeking magnetic states are contained by the magnetic fields, but as the atoms are thermally distributed, they evaporate from the trap at a rate determined by h, the ratio of the trap depth to the temperature of the atoms. For a sufficiently large h, this evaporation is slow enough that a large fraction of the atoms may be held for a long enough time that the ~nonmagnetic! helium gas can be ~cryo-!pumped out of the trapping region. This leaves a thermally isolated, trapped sample. Because this technique relies only on elastic collisions with the buffer gas and on the magnetic state of the species, it should be applicable to any magnetic species that can be cleanly introduced into the cryogenic environment. In our experiment, Cr atoms are produced, thermalized, and trapped within a copper cell. There is a fused silica window on the bottom of the cell to permit optical access ~for detection and ablation! and a mirror on the top of the cell to retroreflect the probe beam for absorption spectroscopy of the trapped sample. Resistance thermometry is used to determine the cell temperature. Additional resistors attached to the cell are used as heaters. The cell is filled with either 3 He or 4 He buffer gas. A sufficient amount of 3 He ( 4 He) is present so that at temperatures above 0.3 K ~0.9 K! the density is approximately 10 17 cm 23 . Below these temperatures, the density is determined by the helium vapor pressure. The Cr atoms are brought into the gas phase by single pulse laser ablation of a solid sample of isotopically pure 52 Cr. The solid 52 Cr is positioned at the edge of the trapping region inside the cell. A doubled yttrium-aluminum-garnet laser with typical ablation pulse energy of 25 mJ at 532 nm is used for ablation. The atoms are detected by laser absorption spectroscopy on the a 7 S3$z 7 P3 transition at 23 386 cm 21