Welcome to Android Central! Is this the Samsung Chromebook Pro? Click on the lower right to bring up the notification area and click the Lock icon, then close the lid. The Chromebook will be in a sleep mode, but when you open the lid again, all you have to do is enter your PIN. Otherwise, all Chromebooks will log you out when you power down. Logging back in should take a very short time -- are you most concerned about this delay?